Modern Tire Size Conversion

[buicknewbee]

Hello All,

I have a 57 Buick Special that I'm trying to figure out what the modern tire size should be. The speedometer has never showed the actual speed since I bought the car. The speedo shows 65mph when I think I'm doing 55mph. I suspect that my tire size is incorrect, but I'm not sure. I don't have an original tire to compare them to, but the service manual states that it should have 7.10-15 or the optional 7.60-15. I assume the 7.60 is larger and would give me better gas milage, but not sure if it would throw off my speedo. Currently it has P235/70 R15 on it.

This link Tire Size Cross Referencing Chart | Diamond Back Classic White Wall Tires - Vintage Tires - Redline & Redline Radial Tires for Antique and Classic Cars if I'm reading it right states that I already have a tire size that is 1" taller than the original tires. The originals were 28" or 28.6" tall and the current tires are 29"tall. The only difference that I see between the tire sizes are the 70 to 75. I'm not sure what this is yet but I'm looking it up.

What am I missing?:confused:

Darren

[Guest Jim_Edwards]

The original factory specified bias ply tires would have been much narrower than the tires you now have on the car and thus might have been a bit more fuel efficient. The height diameter really won't mean squat with respect to what your speedometer may show, nor will it have any significant affect on fuel economy. Analog speedometers are subject to being 3 to 5 percent off due to gearing that can vary in two places in the scheme of things. Typically an analog speedometer indicating a speed of sixty mph will be showing three or four miles per hour over what actual speed may be, but some may be a bit more accurate.

[buicknewbee]

I just got back from a drive with a buddy of mine in the buick. He has a tomtom gps that shows mph. I was actually showing 60mph, but only doing 46mph. Meaning a 14mph difference. That is a little more than a 3% to 4% discreptancy. Either my tires are undersized or the speedo is junk!

I have to disagree with you that the diameter of a tire will not affect the cars speed. A larger tire diameter like a 29" will skoot you down the road much quicker than a tire with a 25" diameter. The engine will have to rev much faster to spin a smaller tire.

Darren

[NTX5467]

First thing, FORGET the diameters of the tires per se! (Sorry, my first reaction)

In looking at the Coker Tire website, they do have modern sizes with overall diameter specs. ONLY thing is that I was looking at the BFG section, which has radials in addition to the repro tires. One thing I found out in trying to get some radials for my '70 Dodge Monaco was that if you use OD as the main criteria, you'll not allow for the compression of the tires by the car's weight when everything's on the ground . . . by about 1/2 ", in my case with the older "H78-15" size. Bias ply or bias-belted tires don't have quite so much compression as their sidewalls aren't designed to bulge as radials are . . . which is where the diameter loss will come from.

In order to keep the speedometer's speed AND odometer working right (as accurate as they were designed to be back then), take the spec'd diameter and turn that into a "Revs/mile" figure to accurately compare the older tires to the more modern radials. TireRack has these Revs/Mile specs for each of the tires they sell on their website, in the "Specs" section for each tire.

The "analog" method of determining a tire's "rolling radius" is to first measure its rolling circumference. An old method (mentioned in an old Hot Rod Magazine Engine Annual, from the earlier 1960s) was to chalk mark the tire's tread, then drive the car far enough to get TWO chalk marks on the pavement. That would be the circumference measurement, in a day when slide rules were high-tech computational devices, with all due respect. This measurement would account for all of the flex built into the tire, too. Take that measurement (converted to feet from inches) and divide it into "5280" (the feet in one mile) to determine a "Revs/Mile" figure. Then look in the BFG section of TireRack, considering more consumer-type tires rather than high performance, or even the BFGTires website itself, and see which tire comes closest to the computed Revs/Mile figure.

In the older times, there was no governmental oversight of tires and their sizes. Each manufacturer was "on their own" in this area. One brand of 7.10-15 tire could be a little wider or a little taller, for example, whereas all of the tires since 1968 have had to meet their stated sizing specs.

When I got the tires for the Monaco, which were originally H78-15, it was already easy to drag the rear factory tail pipes on some approaches, so I wanted something a little taller, so that would have been the J78-15 size back then. Worked great!

And then there's the cosmetic issues to deal with! If you do as was initially recommended when the P-metric designations first came out, to use "Tire Capacity" to determine the correct size, you'd end up with tires physically smaller than the ones that came off of the vehicle . . . as the P-metrics were spec'd at 35psi rather than 32psi . . . not cosmetically good.

Consider, too, that we are so accustomed to seeing wider tires now that when we see an older car with the correct repro tire size, they strike us as VERY skinny by comparison.

Here's what I came up with from the Coker website:

Repro

6.70-15" = 28.00" diameter 7.10-15" = 27.98" diameter 7.60-15" = 28.66" diameter

which leads to . . . 6.70-15 = 14.00" radius 7.10-15" = 13.99" radius 7.60-15" = 14.33" radius

Current

P205/75R-15 = 27.10" diameter/13.55" radius P215/75R-15 = 27.20" diameter/13.60" radius

P225/75R-15 = 28.30" diameter/14.15" radius P235/75R-15 = 28.90" diameter/14.45" radius

P225/70R-15 = 27.40" diameter/13.70" radius

From the Cooper Tires website, for their Trendsetter SE tire

For many reasons, obviously market-based, where you can still find specs on all many formerly-common tire sizes. Without having to get into the "repro" tire criteria . . .

us.coopertire.com/Tires/Passenger/TRENDSETTER-SE.aspx

You can see how the P205/75R-15, P215/75R-15, P225/75R-15, P225/70R-15, and P235/75R-15 sizes relate to each other and the "diameter" situation.

I believe that you'll see that the modern P225/75R-15 size will be close enough to the earlier 7.10-15 and 6.70-15 sizes to be just about right without having to spend $200+ per tire and still have a reasonably period-correct look. Narrower tread widths, included.

Using that tire size as a baseline, you could then progress to BFG Radial T/As in P225/70R-15 or P235/70R-15 sizes. They'll let the car be a little closer to the ground and be wider, too.

As you're already at the P235/70R-15 size, TireRack's specs for a BFG Radial T/A in that size indicate a 28.0" diameter and 743 revs/mile. As your car is a Special, I suspect the 6.70-15 size would have been the base size, with the 7.10 being for cars with factory a/c or station wagons . . . just a hunch.

Now . . . get the tires aired up to about 30psi front/28psi rear, put some fuel in the tank, and head to some straight and reasonably level Interstate highways where you can maintain a steady speed for about 10 miles or so. What you're going to do is use the milemarkers on the side of the road to check the accuracy of the odometer. Whether you run 60mph or 65mph isn't really important for this, just that it's a stable speed. ALSO, take a stop watch with you (of some sort) to measure time. 60 seconds = 60mph . . . or "A mile a minute". This way, it will not matter how correct the tire size might be for what speedometer gears are in the transmission, you'll be able to see how they relate to actuality. To see if your speed/feel perceptions are accurate.

The reason for checking the odometer FIRST is that it's a geared connection to the speedometer's distance measuring section. It needs to be accurate so that as the speedometer cable spins the "speed cup's magnet", which then, in turn moves the speedometer speed indicator appropriately . . . via magnetism rather than a geared connection . . . can be inaccurate even with an accurate distance measurement. In any event, the odometer's reading needs to be accurate before the speed reading can be accurate.

Once you get the odometer check completed, if you kept time on the total odometer check distance, you can also figure an average speed. Then you can relate "actual vs indicated" to generate a correction factor. Distance traveled = 10.0 miles, distance indicated is 11.0 miles, for example. Once you get this done, then you'll know where your vehicle's calibrations are and how much they'll need to change for the tire size/rear axle ratio in the car.

In the transmission, usually on the tailshaft, there's a speedometer "drive" gear which slips onto the tailshaft/output shaft of the trans. Where the speedometer cable goes into the tailshaft, if you remove the retention bolt to remove the housing, it should have a "driven" gear in it, which physically is what the speedometer cable shaft indexes with to spin the cable. These two gears have a particular ratio between them which is what determines how many times the speedometer cable shaft turns/mile. Now that you know what the correction factor for the odometer reading is, you can then get with a trans shop to determine which driven gear needs to be with the existing drive gear to move that correction factor closer to "zero", for more accurate speedometer readings.

Sorry for the length.

Take care,

NTX5467

Edited March 4, 2012 by NTX5467 (see edit history)

[NTX5467]

Over the years, when we did speedometer "recalibrates" by changing the driven gears, many of the "more vintage" techs tried to get a relationship between how many teeth needed to be on the driven gear, compared to what they had, to get the SPEED reading where they wanted it to be. Only thing was that this would directly affect the odometer reading, which would then affect ANY fuel economy readings the owner might make in later times. Of course, the tech doing the work was oblivious to that side issue, his job was to get the speedometer speed reading correct so the owner didn't get any tickets.

This worked reasonably well, but when I got involved in these things, I didn't want the side issue of vehicle fuel economy suddenly changing to surface, so that's why I developed the process I described above, in a time when a satellite was for exploration or other military uses. When GM started using speedo gears with more teeth, their shade tree method was ruined anyway.

In the old AC-Delco speedometer manuals, the speed indication specs were usually something like -2 (slow) to +4 (fast) when using a constant-speed speedometer drive checker. It didn't check odometer accuracy, though, as the speed reading was adjustable with a magnetic "zap" gun. It was "cut and try" method as to how the zaps affected things, but you could eventually get the speed to be pretty accurate. Anybody that did speedometers had one of the bench test drives and a zap gun . . . back then.

Genuine "police" speedometers had a different clockspring that the speed needle worked against. Their specs were + or - 1mph, indicated, between 32 degrees F and 90 degrees F, for the full range of the scale.

So, a cable spins a "speed cup" with a bar magnet in it, which spins another "speed cup" that's attached to the speedometer needle (or similar), which moves against a clockspring, to indicate vehicle speed. Yep, several things in there that might vary with age, that might need some readjustment as time progresses.

I have a '67 Chrysler. When I put the P245/70R-15 BFG Advantage T/As on it years ago (which are a correct rev/mile match for an H70-14 tire size), it made the odometer almost dead-on accurate, but the speed reads too high, by a factor of 10%. Considering what it takes to get that instrument cluster out of it, I'll live with that. The other Chryslers and such that we have, the odometer is almost perfectly aligned with the speed readings, though.

Regards,

NTX5467

Edited March 4, 2012 by NTX5467 (see edit history)

[buicknewbee]

O thank you! That info make total sense to me and some of which I did not account for. Earlier today I have already checked to see if the odometer was reading wrong with the speedo. It had read about 3 miles more than I had actually driven at a given time. So I'm very sure it has either got to be the tires or the gears in the trans as you have mentioned.

I'm going to experiment with some different tire sizes on the rear end when I can get a matching set. Thanks so much for your time and ideas!

Darren

[Guest Jim_Edwards]

I just got back from a drive with a buddy of mine in the buick. He has a tomtom gps that shows mph. I was actually showing 60mph, but only doing 46mph. Meaning a 14mph difference. That is a little more than a 3% to 4% discreptancy. Either my tires are undersized or the speedo is junk!

I have to disagree with you that the diameter of a tire will not affect the cars speed. A larger tire diameter like a 29" will skoot you down the road much quicker than a tire with a 25" diameter. The engine will have to rev much faster to spin a smaller tire.

Darren

You won't get drastic a change in diameter without going to a different diameter wheel. I don't think you are going to be putting 12" wheel on a car needing a larger wheel, unless you might be one of the idiot tuner crowd people. Even if you put a 22" wheel on a car where a 15" wheel has been the lower profile of the tire will offset there being any major difference. Thus if you use the correct wheel for a given vehicle there is no possible way to get a tire that has a diameter great enough to affect speed to any significant degree.

In the case of there being a 14 mph discrepancy I would suggest someone in times past has swapped transmissions resulting in a gear change on the transmission end of the speedometer cable or possibly put a different speedometer in the dash than is supposed to be there. This is one of those things where you're going to need to get into the shop maintenance manual for the year of the car and begin comparing numbers. Basically, there is no way you can have that much discrepancy without an incorrect speedometer and/or transmission for the year model of the car being present. Of course a change of the rear axle could cause that so you need to check the tag number on the axle for correctness for the car.

Edited March 4, 2012 by Jim_Edwards (see edit history)

[Matt Harwood]

I'll bet you another cheeseburger, Darren, that someone has swapped the rear end (or gears) on your Buick...

[buicknewbee]

I'll bet you another cheeseburger, Darren, that someone has swapped the rear end (or gears) on your Buick...

I suppose it is possible. I'm going to be looking into it. I just found a couple of tires that may be closer to the correct tires that it once turned. Going to mount them on the original rims soon!

Darren

[john2dameron]

I suspect the problem is in the rear axle gearing. If Buick had an option for a low-speed rearend (higher numerically than normal) your car might have been ordered that way and they didn't always provide the proper speedometer gearing for the optional axles. Mr brother sold me a '65 Chevrolet pickup up he ordered with a 4.11 rear end to run on the drag strip. That thing was way off. I got a ticket one day for going through radar at 57 mph and I know just before I spotted the trooper the speedometer was showing 70. That explained why my brother claimed he could do 90 mph in a quarter mile and my '65 Impala SS would outrun it by several car lengths and only be doing just over 90.

[Buicknutty]

The speedo on my '56 Buick was very inaccurate (15-20 %) for quite some time, till it broke, and the shop that did the repair promised me that it would be accurate, and it is now reasonably so, and seems to have stayed that way.

I think that your issue is some of both, but I suspect that the biggest culprit is wear and tear in the speedo.

Keith