YMMV???

I understand that larger tires throw off your speedometer but what about your odometer?

Right now I get 200-240 miles per my 15 gallon tank! Well I get 240 and my son get 200. I know that a larger tire turns more revolutions than the stock tire so you are in fact traveling faster than your unmodified speedo indicates. So in theory your actually traveling further than you unmodified odometer indicates?

Is there a formula to account for this?

I'm not looking to improve my mileage right now, just trying to get a true number!

96 4 door, D35/C8.25 (3.55 gears in front, tags are missing on the rear axle so 3.55???) AW4, RC 3in AAL, 31x10.50

In my experience, speedometers are off by a percentage.
2 easy ways that have served me. Since the gears didn't change, it should be reasonably simple.

The ratio of the 2 tire sizes x the odometer reading.
For example if the ratio 31/27 = 1.15 for example, multiply that by your odometer to get the correct reading.

The other is to get a GPS based speed reading, doing what your speedometer says is 55 ( wouldn't want to speed you know ) and do the same ratio thing.

-Ron

bugout said:

I understand that larger tires throw off your speedometer but what about your odometer?

Click to expand...

The odometer uses the same sensor to figure miles as does the speedometer. So, yes they will both be off.

Put the correct speedometer gear in it and be done. You'll be close enough that it won't matter.

The method I use to calibrate my Speedometer is to get the Odometer to be accurate. I zero set the trip odometer at a mile marker on the highway and then pound down 50 miles. However off the trip odometer is, is the correction that need to be made.

Now, I am using my F/IC to calibrate the thing but the same principle applies by changing the gear in the transfer case. Just figure the percentage of the error and apply that percentage to the tooth count of the gear. Pop in the nearest gear to correct and you will be close.

With my combination of P285/75R16 tires and 4.56 gears (AW4 transmission...) I had to alter the frequency of the VSS by -10.2%.