How to: Calculate modern tire size and speedometer / odometer error

[showbizkid]

Hi all!

Recently I was puzzling over my Stude's lackluster (ok, who am I kidding, abysmal) gas mileage, and decided to calculate the speedometer error to get a real number. So, for anyone else who needs to do this, I'm posting the resources and formulae here.

First, consult your owner's manual for the size of the tires that your Stude was equipped with at the factory. You'll need to translate this old tire nomenclature into a modern tire equivalent, so head over to TurbineCar.com's tire size cross-reference. My Stude's original tires were 6.50x15, which translates to P175/75/R15 in modern tire size. Tires mounted now are P225/70/R15.

Next, you'll need an online speedometer error calculator. I use Palo Alto Speedometer's online calc. Input the OEM tire size, and the size of the tires now mounted, and you'll get a readout of the circumference of both the original tire and the current tire.

Now for a little math

To calculate the speedometer error percentage, divide the diameter of your OEM tire size diameter by that of the current tire diameter. If the resulting number is less than 1, your speedometer is indicating lower than the actual speed your car is traveling; if greater than 1, indicated speed is higher than speed traveled.

To calculate your actual speed and mileage, invert the product of your calculations above (divide 1 by the number you obtained), and multiply your road speed / mileage driven by the product of that number.

For instance:

According to turbinecar.com, my original tire diameter was 25.197". My current tires are 27.4". 25.197 divided by 27.4 = 0.92, meaning my speedo is indicating slow.

Inverting 0.92 = 1.087, which represents the number of turns the tires my car was originally equipped with would be turning for each turn of my current tires. So, to find out how fast my car is traveling when my speedometer says
65 MPH, I multiply 65 x 1.087. The answer: 70.7 MPH!

It works the same way with the odometer. Instead of getting 120 miles to a tankful of gas, as the odometer says,
I'm actually getting 130.4 - enough to bump my actual gas mileage up by a full 1.5 MPG.

Hope this helps someone else who needs to do the same thing!

[okc63avanti]

Thanks Clark .... but I've learned a much simplier way. I just drive with a Garmin Novi GPS and it has a speedometer on the display that calculates your speed while in motion by using the rate at wihich the GPS is recalculating your global position via satelites. It is extremely accurate (have checked it with several modern vehicles) and is usually dead on or within 1 MPH of your car's speedomter. The rate at which it calculates your speed is almost instanteous by the way.

[VAG88]

Hi: I believe there is something wrong with the math in your tire size calculation. A 650x15 tire has an outside diameter of about 28.6 inches,a175 -75-15 radial tire would have an outside diameter of only about 25.5 inches,even a 195-75-15 would only have an outside diameter of about 26.6 inches. A 225x70x15 radial will have an outside diameter of about 27.6 inches,still about 1 inch smaller than the 650x15 original tire size, according to my calculations. I'm sure some one will correct me if I'm wrong. Art

[ivorydan]

Clark, I think your formula will work if you correct the original tire diameter. It looks like you concluded the original tire diameter is the same as the suggested modern replacement tire in the charts. That's not the case. Art's size may be on target; I'm not familiar with the 6.50s but I do know original Firestone 6.70s were app 28.58, and Goodyears of the same size were less. I replaced the originals with P215/75R-15s , which are shorter than the originals at 27.7 inches. After doing calculations I find I'm actuall going a mile per hour slower than what my speedo shows up to about 60mph, and app 2 miles per hour slower at faster speeds. And remember that two different brands of the same size tire may have different diameters. Dan

[Laemmle]

Clark, I think your formula will work if you correct the original tire diameter. It looks like you concluded the original tire diameter is the same as the suggested modern replacement tire in the charts. That's not the case. Art's size may be on target; I'm not familiar with the 6.50s but I do know original Firestone 6.70s were app 28.58, and Goodyears of the same size were less. I replaced the originals with P215/75R-15s , which are shorter than the originals at 27.7 inches. After doing calculations I find I'm actuall going a mile per hour slower than what my speedo shows up to about 60mph, and app 2 miles per hour slower at faster speeds. And remember that two different brands of the same size tire may have different diameters. Dan

So if my Avanti is running 205X75XR15 what is the actual speed I am traveling.........considering the OEM tire was 6.70X15.

[HookedonStudies]

Different brands and possibly even different "models" within a brand can have different diameters. (and therefore different circumferences). Tire inflation can, and does also affect the circumference. Then there is treadwear.

I'd say go with the GPS reading!

[jackb]

Laemme....you're actually travelling slower by a maybe couple mph and you're odometer is reading higher (looking like you've travelled farther) giving you a false reading on gas mileage. You're actually getting less mpg in your curernt tire set up.

[50Champ]

I think this may be what you're looking for....

http://www.1010tires.com/TireSizeCalculator.asp

[tyaughton]

I'll second that. I have no idea what our truck has but I know they're smaller than stock (someone replaced our Tcab fender with a Lark fender so stock wouldn't fit the wheel well on turns), and we have a 700R4 transmission so the gearing is off there as well. The speedometer and odometer both work, but they're off. I didn't do any fancy calculations, but used road signage, odometer checkpoints and mapquest distances on several occasions and determined that the truck was running at about 95% of actual. We discovered the Garmin 'dashboard' on our last trek and now use that as our speedometer (especially at night because the stock gauges are so hard to read) and I was right on my original estimates, the speedometer and odometer are 5% shy of actual speed and miles driven.