"Bias-ply G-78" = radial tire size for 71-76 GM full-size cars?

[Guest Shaffer]

I am trying to figure out what tire size I need for my 73 full-size Pontiac. The placard inside the glove compartment states either G78 15 or H78 15, but these are the old bias-ply sizes. The current replacement radial tires are P205's and are much too small for this car. Does anyone know the proper radial size tire for the 1971-1976 GM full-size cars? I used to have access to a conversion chart for bias-ply vs. radials, but cannot find it now. I am guessing that the sizes were the same for all of the 1971-1976 full-size cars. I am guessing that around 75-76 that radials may have become standard on all of the GM full-size cars. Thanks in advance.

[WQ59B]

Good question. Is your concern tire height or width (or both)? It's easier to find height differences across the different size systems but not so easy to determine width.

I ran a '64 Catalina daily a number of years ago. Stock tire for it was 8.00x14. 'Mathmatically' the equivalent should be about 205/78x14, which is 26.5" tall. I ran 225/60-14 radials on it for 2 years and they fit beautifully (also 26.5" tall) and looked great.

However, the 8.00" is a <span style="font-style: italic">sectional</span> measurement and the 205 is a <span style="font-style: italic">tread</span> measurement... not exactly 'apples to apples'. I suspect based on careful consideration that a 225 is a closer equivalent to a 8.00 than the 205 is.

According to a vintage Firestone pull-out ad booklet I keep handy, a G78-15 is equivalent to a 8.25 x 15, which I believe should be equivalent to a 205 (an H78 is supposed to equate to a 215). Another modern Goodyear tire booklet lists the old "ST" 225/70-15 whiteletter tire as having a 8.78" sectional width, a 6.7" tread width and a height of 27.4".

So where are we? For a radial replacement for a G78-15 I'd start with a 225 or maybe even a 235/70-15 (9.25 sw, 7.0" tw, 28" h). I am confident the 225/70 should fit well and look appropriate, if not slightly better (beefier). Good luck!

[Guest Shaffer]

Thanks. My main concern is speedometer accuracy, so I suppose that height would be the main concern. Someone on the Pontiac site said 205's, but the car has 205's now and as you can see (attachment), they are much too small. I think they are P205 65R 15s. If I recall correctly, my old 1972 Buick Electra 225 Custom sedan had P225's, but I cannot say for sure. Thanks again!

[Guest my3buicks]

my 69 Electra had 225/75/15's on it when I got it - they rubbed on the front when I turned the wheel full to turn not bad but you could see a "clean" area on the front frame. It just looked like to much tire under the car also (could have just been the particular brand) - I put 215/75R/15's on it now and have driven several thousand miles on them and have been very happy with them.

[WQ59B]

OK; height is the concern.

I believe your '73 originally had about 28.5 - 28.75" tall tires. My '59 originally ran G7<span style="font-weight: bold">8</span>-15s, and the one I have left measures just about 29.5" (tho that was without car weight on it).

The 205/65 in your pic is only 25.9" tall! I once tried a 225/60-15 on my '59 and I just about laughed when I stepped back to look at it (25.6"). It may be a beefy tire for a '90s mustang GT (which it came off of), but it's just about a space saver spare on a full-size Buick.

Since a 205 or even a 215 in a 75 series tire is really hard to find these days, you'll probably need to move over to a 225/70 ( 27.4"). After reading Keith's post, I would be wary of a 235/70 (28") rubbing (his 225/75 is roughly 1/2" taller than a 225/70). Again: good luck!

[55PackardGuy]

<div class="ubbcode-block"><div class="ubbcode-header">Quote:</div><div class="ubbcode-body">My main concern is speedometer accuracy</div></div>

Here's a quick and dirty way to check your speedometer:

Keep track of mileage markers for 5 miles or so on the road, making a note of your odometer reading at the beginning and end. If it has registered over 5 miles, your speedometer is reading "high" and your tire diameter is too small. (Of course, your speedometer gear could be wrong for the differential ratio, too, but that's unlikely in your case.)

Figure out the percentage of difference between your reading and the actual one. That's the percentage you need to add to your tires' diameter.

Say its 10% off. Use the following equations to find a tire 10% "taller" than what you have now:

205 x 65% = 133.25

225 x 65% = 146.25

Comparing the numbers on the right side of the equation, you get about 9.7% increase in diameter by going to a 225 65 tire.

This formula lets you plug in numbers to your heart's content to find out how diameters compare for different series of tires.

A 225 70 tire as mentioned in another post would compare to your present tire like this:

205 x 65% = 133.25

225 x 70% = 157.75

That's over 18% difference in diameter. Unless your speedometer is presently reading way off, it might be more diameter than you need.

As you probably know, the middle number is the percent of the tire height (from the tread to the wheel) to its width. So, if that number gets higher and the width remains the same, the tire will have a greater overall diameter. Likewise, if the first number gets higher and the second number stays the same, the diameter will increase--but so will the width. You can get greater width ("beefiness") with the same diameter by increasing the first number and decreasing the second one by the appropriate amount. Naturally, your wheel well size will limit how wide you can go with an appropriate diameter tire.

BTW, I don't even know if the number on the right side of the equation has any intrinsic meaning. It doesn't really have to, because knowing the percentage difference is the important thing.

[Guest Shaffer]

Thanks. If I recall, I think my 72' Electra had P225's on it and they were about perfect. I think my 69' Caprice has P225's as well and they seem fine too. I guess I may go with 225's. Thanks again.

[WQ59B]

There's a chart at the bottom of this page:

http://www.musclecarclub.com/library/tech/tires.shtml

Here's some heights that have been mentioned here:

G78-15 : 27.7"

H78-15 : 28.4"

225/70-15 : 27.4"

235/70-15 : 28.0"

225/75-15 : 28.3"

[Buicks Rule]

Do the following.

Check the owner's manual for the correct tire size.

Go to Coker Tire--www.coker.com. Look up the actual dimensions of their repop size.

The "height" referred to above are actually over-all diameters. Split the number in half for tread to spindle center distance.

Once you have the dimensions you need, go to the website for your preferred brand of modern tire. Ex: www.goodyear.com.

At some point each will give you the actual dimensions of the modern tire size for each of their tire lines. You may have to play around a bit on each site to get the hang of the way they've organized their tire info. Needless to say, choose select tire by size. If you do it by vehicle you'll get either no information or possibly miss something you would prefer to buy.

There is actually some leeway in the P metric sizes, so each tire manufacturer might be slightly different in actual dimensions even for the same size tires.

You should be able to find something very close in over all diameter. Remember divide the difference by 2 to check difference in actual spindle to tread height. It may be possible to get a difference of around 1/10 of an inch--which isn't going to be visible.

[Guest Shaffer]

Thanks. It looks like I will go with the P225 75 15s. I do not remember- but is the higher the number on the "center" number- means narrower tire? ex: 75s wider or narrower than 70s, or other way around? I am going to look at the tires on my 69 Caprice tomorrow and see what size they are. I think they are P225 75 15s, and I think they are the perfect size. Thanks again.

BTW- I have no idea where I got "G-78", but it actually stated H-78 and I think as well as J-78 in the manual. G-78 was for the midsized cars ( Pontiac Tempest/LeMans/Luxury LeMans, Buick Special/Skylark/Skylark Custom, Chevrolet Chevelle/Malibu, Oldsmobile F-85/Cutlass/Cutlass Supreme).

[55PackardGuy]

The thing to remember is the second number is a Percentage only--it has no meaning except in relation to the first number. It only tells you what percentage the height of the tire, (from the tread to the wheel) is compared with the width.

That's why a higher middle number with the SAME first number results in a greater overall diameter tire. 75% of 225 is more than 70% of 225, for instance.

If you want a "beefier" tire (same diameter but wider tread) you DO go to a lower "middle number" but you have to, at the same time, INCREASE the first number. That way, you have a wider tire with the same overall diameter.

If a 225 75 is the correct diameter for your car and you want to go to a wider 235, you need to go to a 70 series tire to get close to the same diameter:

225 x 75% = 168.75

235 x 70% = 164.5

The 235 70 is about 2.5% smaller diameter, which is not going to affect speedometer accuracy appreciably. It is a little wider than the 225 and will probably look a little more racy.

Be aware, though, that the 235 tire may give clearance problems, especially when turning. Full steering lock going over a bump is the most likely time that you may notice this.

[NTX5467]

Basically, there were several different tire sizes back then that could have been on the larger cars. The federally-mandated tire information decals in the door frames or on the glove box lid were usually oriented toward "minimum" tire sizes to prevent anybody from putting smaller than needed tires on cars when it came time for replacement. In the earlier times, the OEMs sometimes skimped on tire sizes for various reasons.

As things have played out with the many variations of tire nomenclatures, the old G78-15 (prior 8.15-15 size) equivalent P-metric tire is now a P215/75R-15 with the G70-15 being P225/70R-15. H78-15 (prior 8.45-15) is now replaced by P225/75R-15, but if you check the section width dimensions, the current P225/75R-15 is really closer to the old J78-15 size than the H78-15--probably about in the middle between them with respect to section width and circumference measurements. 8.85 x 25.4mm/inch = 224.79mm so that explains the P225/75R-15 size being more "J" than "H".

But the reason the P225/75-15 works in place of the H78-15 is that with the added sidewall deflection of a radial compared to a bias ply (at proper inflation pressure) results in the same ground clearance as the H78-15 would have. Just my observations. For example, I have a '70 full size car that came from the factory with dual-stripe Firestone bias-belted whitewall tires in H78-15 sizing (or at least that's what's in the trunk for a spare and it had Firestone tire warranty information in the glove box). After I bought it, I noticed that it tended to scrape the factory dual exhaust pipes in places that other similar cars we had did not. So I upsized to a JR78-15 radial and stopped that problem.

The production life of the J78-15 was somewhat short-lived, taking a back seat to the L78-15 size. The L78-15 (prior 9.15x15) equates to the current P235/75R-15 size.

Somewhere in my archives, I have a copy of "CARS" magazine from about 1970. It has a chart of ALL of the tire sizes in it at that time and their related dimensions. This was when "metric" was really "metric" and not "P-metric".

In those earlier times, a 195-14 radial would equate to a F78-14 or the earlier 7.75-14 or the still earlier 7.50-14 size. From what I've noticed when the P-metrics came out that 195-14 size would become a P205/75R-14 size. Not sure why it worked that way, but it seemed to in many cases.

Looking at the section width specs in the various tire manufacturer (or places like TireRack with many brands and their respective tire specs, which CAN vary from brand to brand a little for a given size spec), if you match section width and aspect ratio, you'll probably end up with tires that might look a little small, width wise, compared to what we're used to seeing these days, but with comparable circumference (relates to the "Revs/Mile" spec in the charts) dimensions. I ALSO need to note that I've found a current tire to replace the somewhat obsolete H78-14 size in P225/75R-14 sizing of a Cooper Trendsetter SE whitewall tire, for anybody that might need something in that particular size.

In another article on tire sizes in a circa 1968 issue of CAR LIFE, they stated that there were "now" sizing specs for the letter series tire designations. Specs that were "government specs" which allowed for a 7% variation and the letter designation not only related to a sizing spec, but also to load capacity. This was all in the name of "consumer protection" and to help make tire buying easier for the general public. All of this worked pretty well as long as just normal 75 aspect ratio tires were being talked about. When the aspect ratios started getting in to the 70s and 60s for wider-tread performance oriented tires, it got more complicated. Like a F78-14 would generally have the same circumference and loaded radius as a G60-14.

When the P-metrics came out, they also included the higher inflation pressure orientation to aid in fuel economy. With them, the old 32 psi limit became 35 psi for normal passenger vehicle tires. In the P235/75R-15 size, there were "Extra Load" (or 6 ply-rated) tires that were rated at 40psi.

These inflation pressure increases, with respect to "crossover" sizes, led to the orientation that you used max load capabilities at the higher max inflation pressure to determine what P-metric tire was a suitable replacement for the earlier size designations. This usually resulted in a tire of smaller circumference and narrower tread replacing a larger tire. Load capacity is important, but so is speedometer calibration. Now, I think, most of those crossover issues have been adjusted somewhat to reflect physical sizing rather than specifically load capacity.

Seems like the 69 Electras would have had J78-15s from the factory? J78-15s replaced the earlier 8.85x15 size, if I recall correctly. LeSabres would have had G78-15 (for the more base equipment models) and H78-15 (for the more equipment laden models, and Wildcats). But with respect to load capacity and physical sizing, I suspect that a P225/75R-15 might work for all of them, but be sure to check the speedometer calibration for good measure. I suspect the larger P235/75R-15 might be a little too large, circumference-wise, for proper cosmetics--but that would be an owner's judgment call.

One other thing to mention is that, generally, tread width of the tire should be within 1" of the rim's width. Plus that for each 1" of rim change, tire section width will change about .2" For example, a tire designed with a 6" tread width and is designed for a target rim width of 6", and a 8.55" section width in these situations, would measure 8.75" when mounted on a 7" wide rim and 8.35" when on a 5" rim. In other aspects of this illustration, the wider rim would tend to make the tire's sidewall more vertical (and less compliant on bumps) and tend to sharpen handling responses. Similarly, using the narrower rim width would put more arc in the sidewall and ride a little smoother and have a little softer steering response--all of this at the same inflation pressure too.

And then there's wheel "offset" and how it relates to "backspacing", which can relate to clearance issues of the tire to chassis/body parts . . . in similar ways at the tire's section width can.

Enjoy!

NTX5467

[NTX5467]

Radials became standard in '75 pretty much across the board on GM cars, along with catalytic converters and "retuned" engines for better response and economy.

Prior to that time, Pontiac had their "Radial Tuned Suspesion" that included different suspension bushings, a rear sway bar, and different shock absorber calibrations along with radial tires. This "new" suspension calibration/tire/body interface was designed to showcase the added capabilities of the radial tires compared to the bias-belted tires. In later times, everything would have superceded into the radial bushings, I suspect. Base Catalinas would have had G78-15s and Bonnevilles and station wagons would probably have had H78-15s, but I suspect that the radial tire package was just with the HR78-15s. If your Pontiac had the Radial Tuned Suspension package, there should be several nameplates announcing that inside AND outside the car as it was an "upscale" option back then.

The rear sway bar could also have been available as part of a "police" suspension option without the need for radial tires, but the rest of the vehicle would also relate to that equipment orientation too, I suspect, if that was the case. You might inquire to Pontiac Historical to find out how it came from the factory, if you desire. For a reasonable price, they can put together the information on your vehicle, if it fits their available range of year models, to detail just how it was built originally.

Just some thoughts,

NTX5467

[Guest Shaffer]

Thanks guys for the additional replies.

I looked at the size of the spare in trunk- which I believe to be the original and it is a "Uniroyal H-78 15".

My 69 Caprice has P235 75 15s and they look fine (no scrubbing), but it has a P225 70 15 in the trunk- which I bet would actually be a better size tire for it, as well as the Grand Ville. Attached is a photo of the rear tire on my 69' Caprice sedan. I do not think my Grand Ville came with the optional radials of the time, since the original spare in the trunk is a bias-ply tire. I really to not want a huge tire on this car, so I think I will go with the P225 70 15s. If I recall, I think this is what my 72' Electra 225 had on it and it also had the original "H-78" spare in the trunk and I think it too was a Uniroyal. I do not think my car has the "RTS" option, because it does not have any emblem in the car, but I am not 100% sure that they put that in the older full-size cars. I am familiar with that emblem however, because my uncle had several 70s Trans Am's through the years and all had that on the instrument panel. I am guessing anyway that radial ply tires were a factory option on the 19??-1974 full-size GM cars, before they became standard in 1975?

About the speedometer calibration- I cannot check now, because the speedometer has decided it wanted to stop working. I think it is the cable. I want to replace it, but not sure how to go about it. I am guessing it is the same technique as the other 1971-1976 GM full-size cars. Someeone told me that you can change it simply be removing it from the transmission and pulling out the actual cable and insert a new one in its place, without having to touch anything under the dash. I have heard others say that is not so and that you have to actually replace the entire cable ("housing" too) and you have to remove it from the dash- which seems like a big job. Would a transmission shop be better for this task? I am just concerned that it is not the cable and it is the actual speedometer that has gone bad. Hopefully not. Thanks again for all of the replies.

[NTX5467]

Just as with "AntiLockBrakes" or "V8" or "V6" namaeplates in the '80s, the Catalina/Bonnevilles had "RadialTunedSuspension" nameplates on them in those 1973 era vehicles (and some afterward too). Back then, the radials were just normal radials and not specific "performance" radials, although the handling improvements were there anyway.

It used to be that you greased the speedometer cable at reasonably regular intervals, with either graphite or application-specific lube (GM/Delco has one that seems similar to DippityDoo in color and consistency, but is a lubricant instead of hair gel, Part Number ST800?). Usually, fresh lube would put things back quiet and smooth.

But if the inside of the cable housing has worn through to expose the wire metal sheathing inside the plastic housing's "wear surface", then you need to do the whole thing. Considering the age and such, it might not be a bad idea to replace the whole assembly (which comes pre-lubed and ready to install, at least the GM ones do) with none of the other labor operations to do (as the universal cables do which you cut to length and glue to tip onto).

AND, if the vehicle has a factory cruise control, it could be that the cruise transducer under the hood has locked up and caused the failure of the lower cable (and possibly the related speedometer gears too). Then you'll not only need the new lower cable, you'll possibly need other things too.

In some cases, the gears in the speedometer have just flat worn so far that they don't touch any more.

If there's a speedometer ratio adapater in the mix too, which screws onto the transmission and then the cable screws onto it, it might have locked up or worn out too. There will be some stamp numbers on it to indicate the "ratio" inside the adapater, which will need to be matched to keep the speedometer calibration package for the vehicle accurate for when it was produced. IF you change to a different tire size from what the vehicle came out with, it changes all of this.

There's a neat tire size comparison utility at the Miata Club website. Pretty neat as it not only has numbers but graphics relating to the "starting" size and the "ending" size of the two that are being compared. Even figures the speedometer error too. Only thing is that I believe that it's calibrated with metric sizing and not alphanumeric. Still, it's pretty good for comparisons with the more current sizes.

Enjoy!

NTX5467

[Guest my3buicks]

Radials available 1967-1968-1969 - Not in 1970-1971-1972 and I think back in 1973

[55PackardGuy]

Here's a link to a site similar to that mentioned by NTX. It even blips up a warning if you look at a tire that is more than 3% over or under sized, which it says can result in brake failure?? It also gives approximate rim widths that can be accomodated by various tire sizes, calculates speedometer readings etc. Have fun!

http://www.1010tires.com/TireSizeCalculator.asp?action=submit

[Gene Brink]

Have a Word doc showing most common old to new size comparisons and will e-mail to you directly as I have no idea how to attach on the Forum (if at all possible) and cutting a pasting does not work as the formatting does not hold.

[Guest Shaffer]

Thanks. I will look forward to that e-mail.

[Guest Shaffer]

Thanks for all of the replies. To update, I had the new tires installed today. I ended up putting P225 75 15s on it and it looks fine. Here is a photo taken a little while ago with the new tires. Looks much better than the 205 65 15s that were on it. Whitewalls helped a lot too. Thanks again!

[Guest Shaffer]

Another view: