Inaccurate Spedometer....got pulled over

[Highway_101]

I was cruising up Highway 101 today in my 1963 Riviera with the needle right a 55mph (the posted speed limit). I got pulled over by a state trooper for doing 48 in a 55.......so that's why there were so many people stacked up behind me. Anyway, the trooper was very nice....didn't write me a ticket, and I was glad to know the inaccuracy in my speedo. So, could improper tire size cause a 7 mph difference? I am running P225/70/R15. Can anyone tell me the stock tire size for my car?

Thanks,

[Buicks Rule]

Mike,

Original tire size was 8.45 x 15. That size is not available, even in reproduction. 225 70 R 15 is most likely smaller in overall diameter than the original tire size. Thus, more tire revolutions per mile along the road, leading to a speedo that reads a higher speed than the actual. GM speedos in this era used to read a bit high anyway.

It is possible that a different speedometer gear in the tranny could bring the speedo closer to actual. Another choice would be to find a mechanic/speedometer shop with a rear wheel dynomometer. On a dyno you can then cross reference speedometer inidicated speeds with actual 55, 65, 70, etc.

Be aware that sometimes speedometer error is not a constant thing so you might be 7 mph low at indicated 55 but error could be less at lower speeds.

Good luck

[Idle Swede]

Check this out. http: //www.miata.net/garage/tirecalc.htm

[NTX5467]

The old 8.45x15 size became 8.55x15 and then basically changed to the H78-15. The P225/75x15 size actually measures (from the tire dimension charts in www.tirerack.com and other similar websites) to the 8.85-15 or J78-15 sizes, but those are the generally accepted substitutes for the earlier H78-15 sizing. The tire size page on the Miata website is really neat! Only thing is that it deals in absolute sizes that leave no room for production variations between tire brands, which is why the "Revs/mile" information in the charts for the different tires at tirerack.com come in handy.

Now, if you really want to spend some time and see just where your speedometer is, accuracy wise--speed and distance--find about a 10 mile stretch of reasonably straight and flat interstate with mile markers. Five miles might work too. 60 mph = 60 seconds/mile By keeping a steady speed through several of the mile markers (why a straight and level road comes in handy!) you can see where the odometer calibration is (it's driven directly off of the speedometer cable). Getting the odometer calibrated first is important as then you'll know which way to go with the speedometer gear/adapter, if needed.

The speed cup of the speedometer, that moves the speedometer needle against a clock spring, is run by the spinning magnet that turns inside the speed cup. The calibration here is done by "zapping" the magnet momentarily (with the appropriate tool at the speedometer shop) to make the magnetism stronger or weaker so that the indicated speed is accurate (within specs, which usually are +1 fast to -4 slow from the actual speed (from what I saw in an ACDelco speedometer and gauge catalog one time). Therefore, getting the cable speed/odometer calibration correct FIRST is important to getting things to closer tolerances.

From the Miata tire webpage, you can get a "Revs/mile" figure for that "perfect tire" and then use the similar numbers in the tirerack.com charts to see where what you have might fall. The P225/70R-15 size is the old G70-15 size so it could be possible it is "shorter" than what the vehicle was set up for from the factory. Perhaps the next size up P235/70R-15 size (rear wheel drive Roadmaster size and also on many late model S-10 4-dr Blazers) might be a better alternative (also available in white sidewall too, if that's a consideration)?

After doing the driving calibrations (as mentioned above), then you can figure what "indicated" speed you need to run to do the "actual" posted speed. It might take a little time to do it, but then you'll know where everything's "at" until you might purchase new tires or get the speedometer calibration customized for what you have.

Enjoy!

NTX5467

[BigFella]

I have always run a wider tire on my vehicles than comes stock from the dealer. Usually one or two size difference...wider and a little shorter. The factory usually gives you a size or two for interpolation but it does change the speedo...as for this I've always used those traffic side radar display things that tell you how fast your going as they seem to be all over the place now. I will run by it as a couple speeds and note the difference ... not very scientific but it's close enough :-)

[gungeey]

Michael, Just be aware that 225/75R15's on the first generation Rivieras DO RUB THE FRAME before the end travel of the steering gear because of the radials geometry. Other than that the ride quality of the radial vs. bias ply is quite substantial. With bias plys a relaxing ride becomes a lot more work. I have 8.55x15's on my'65 ( which is only one-tenth of an inch wider than original) and they too slightly touch the frame. I wouldn't be overly critical about the rub, you just need to be aware of it and learn your new amount of travel. Steve

[Reatta Man]

Highway 101,

Your post started to sound like an old Beach Boys song....

" I was crusing in my Stingray late one night, when an XKE pulled up on the right...."

As for the speedo question, the EASIEST way to see what speed you are doing is to get someone in a late model car, both of you with cell phones, on a fairly isolated part of road and track your readings while parallel to the other car. Late model cars tend to have VERY accurate speedos because the measurements are all electronic now vs. mechanical units in the older cars, and they must be accurate in order for the computer to make the right adjustments for the engine.

Once you have this set up, here is what you do. Friend/wife is in lane one, you parallel in the other lane, with both cars doing an indicated 60 MPH according to the newer car. As you talk, the other driver tells you when the speedo is dead-on 60, and then you look down at yours. It may say 58, 68 or 48 as you already found out. Then, if local laws and conditions allow, do the same thing at 65, 70, etc and make a note of the differences.

Chances are at highway speeds your margin of error will be about the same. In other words, if your old mechanical speedo is off by 6 MPH at 60, it will probably be off by 6-7 MPH at 70 and 5-6 MPH at 50.

If you just HAVE to have it read accurately, you can go through some of the steps outlined in the other posts. Otherwise, this will give you an idea of your indicated vs. actual speed. For cars used infrequently on cruise nights or to club meetings and car shows, this will probably suffice.

By the way, if you are doing 70 in a 45, an inaccurate speedo probably won't impress the cops and avoid a ticket. BUT, if you are in a NASCAR town, you could always tell him you were just trying to keep your tires hot the way Dale Earnhardt Jr. does it on TV every week when under a yellow flag......

Joe

[NTX5467]

Perhaps the best way to see what your most accurate speed is would be to use a G-TECH device. Not only will they read lateral Gs, but also time/distance and speed (why you can use them to do timed acceleration tests outside of a "trap" set up at the drag strip).

Modern electronic speedometers CAN be much more accurate than the earlier cable-driven ones, for obvious reasons, but there's still a tolerance level and basic calibration spec just as in any other electronic meter device. There are still parameters within their software for certain tire/vehicle combinations which do the same thing as the older driven/drive gears in the transmission. Analog or digital, the base calibration has to be accurate for the vehicle specs.

In an article on the then-new letter designations for tire size in the late 1960s, CAR LIFE magazine listed the "standard" sizing specs for each tire size, from A to L letters. As the first batch of "letter" tires were the also then-new 70 series tires, that's what they had listed. The "standard" size for each letter of the 70 series tires were listed and it was also noted that government specs would allow 7% deviation from those sizes. Nothing was said about the additional deviation for tires which had gone from 11/32" tread when new to 2/32" when totally worn (at the tread wear indicators), nor high speed "expansion" for the non-belted tires of that era.

So, considering all of these factors, speedometer accuracy can be something of a "crapshoot" in some cases. Knowing that how you arrived at your "corrected" speed, either by pacing and factoring the indicated speed into actual speed or getting things done by trial and error and road timing, is reasonably accurate can give you some extra confidence should your observed speed be questioned.

As mentioned earlier, the tire charts in the manufacturer's respective websites (section width when mounted on a particular width rim, rev/mile, etc.) can be a very good source for seeing what size you might need. Downside is that many of us have gotten so used to seeing wider tires on all vehicles, that putting something of the correct section width on an older car makes them look reallllyyyy skinny and undersized. For example, the P225/75R-15 tire on a 6" rim is usually listed as 8.8_" in section width, making it fit more of the old 8.85-15 or J78-15 sizing instead of the H78-15 sizing that it usually replaces (maybe due to the sidewall deflection at road load making the "taller" radial tire be the same "rolling" height as the older bias ply type tire?). If the 8.45-15 tire size is close on particular vehicles, as noted, then the P225/75R-15 will be closer still. Please note also that for each 1/2" of rim width variation, that results in a .2" difference in section width "as mounted". And then there's the "offset" spec that can affect tire/body/suspension clearance too.

When determining what tires your vintage vehicle might need, use the manufacturer's dimension charts to make sure the cosmetics stay reasonably the same. If the physical sizing is the same, the load carrying capacity should be in the ballpark too, usually higher with the newer P-metric tires and their 35psi+ max inflation ratings. You can size the tires by using the carrying capacity ratings alone, but you'll usually end up with tires with a smaller circumference and that will lead to road speed being a good bit slower than what the speedometer indicates.

Just some thoughts . . .

NTX5467

[Reatta Man]

Uh.....most people don't have access to or need a $250 G-tech device to check a speedometer.

My suggestion was intended to be quick and inexpensive.

[Idle Swede]

Get a $60.00 Garmin GPS & it will do the trick

[Highway_101]

It tooke me a while to get back to read all these responses. Thanks to all who responded to my post. There is lots of good info here for me to work with. I am disappointed that my current tire size (225/70/15) isn't too far off from original.....I was hoping that was the bulk of the problem.

Again...thanks,

[Highway_101]

Oh Yeah.....isn't "Dead Man's Curve" a 'Jan and Dean' song or did they borrow it from the Beach Boys

[Reatta Man]

Hmmm....

Looks like it is back to Google for some '60s flashback and research time for me....