Did a comparison between 235/40/18 and 245/40/18 with a tire size calculator (see below). Do the revs/mile translate into an incorrect odometer reading? I don't know how the odometers on our cars work. Thanks.


Stock Tire - 235/40R18 >Search Tires Tire 1 - 245/40R18
Section Width: 9.25 in 235 mm
Section Width: 9.64 in 245 mm

Rim Diameter: 18 in 457.2 mm
Rim Diameter: 18 in 457.2 mm

Rim Width Range: 8 - 9.5 in
Rim Width Range: 8 - 9.5 in

Overall Diameter: 25.40 in 645.16 mm
Overall Diameter: 25.71 in 653.03 mm

Sidewall Height: 3.70 in 93.98 mm
Sidewall Height: 3.85 in 97.79 mm

Radius: 12.70 in 322.58 mm
Radius: 12.85 in 326.39 mm

Circumference: 79.79 in 2026.6 mm
Circumference: 80.77 in 2051.5 mm

Revs per Mile: 818.8
Revs per Mile: 809.0

Actual Speed: 60 mph 100 km/h
Speedometer1: 59.2 mph 98.8 km/h

Speedometer Difference: - Speedometer Difference: 1.211% too slow
Diameter Difference: - Diameter Difference: 1.21%

 

Bueler?

 

I would think that 1.21% is the number you are looking at. So calculate that based on whatever speed, RPM, or mileage. The computer would continue working with a 1.21% variance on everything it registers.

Am i just being a little too simple minded on that one?

 

Circumference = 2 * 3.141 * Radius or

C = pi X D

so i agree that the percentage you are looking at translates to the odo difference as well.

 

Math-wise, the speed is calculated from rev/mile. But the rev/mile number is calculated from circumference, which is calculated from diameter (c = pi x d, like PeterUbers said above).

The speedo computer calculates how many full rotations the front hubs (or a certain gear within the gearbox or transfer case -- 4x4 --, etc) in 1 minute or 1 second. From that number and a variable that is set based on the tire size the car uses, it calculates the actual speed and displays it on the dash.

Heh, I hope it makes sense.....