Hi all!

Recently I was puzzling over my Stude's lackluster (ok, who am I kidding,

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!

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!

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