I just drove through a 5 mile stretch of measured mileage. My odometer is reading a bit higher than the actual miles measured.

I programmed Superchips with a 34-inch diameter for my tires, but my odometer reads 1.0 mile early, about 600 feet before passing the actual measured mile sign.

Over the course of 4.0 measured miles, my odometer shows I have gone about 4.1 miles.

So, is my 34" tire diameter too big or too small? Why?

 

You should adjust your superchips to the actual tire height, go break out the tape measure and do it. It's too big, so go adjust it down.

 

Try using 33.15 ish inches for the tire size in your Superchips. Based on your error it looks like you are off by 025%.....if my math is right....

 

Actual measurement was 34 inches. Maybe now closer 33.9 with some tire wear?

 

To get mine to read correctly i had to use a lower measurement in my Bullydog then the actual measurement, it took a few tries to get it spot on...

 

Okay, will reprogram to a lower number at next rest stop!

 

Do you have a gps?

That was a big help in finally getting my speedometer to match the gps and then the actual distance traveled.

matthew

 

(4.0/4.1)(34.0) = 33.17 Based on your measurement, 33.2" should work. The tire diameter from specs is inflated on the bench, but when it is supporting a load it flattens out a bit. When it flattens out, the radius is reduced, and there's less travel per revolution.

I used an Aeroforce scan gauge to set mine. It uses the amount of millimeters traveled per revolution instead of rolling diameter -- but when converted, mine are set similarly less than the bench inflated spec as well.

 

I reprogrammed with SuperChips using 33.5" diameter. It reduced the error by about 50%. The next available step is 33.25 inches which correlates with your suggestion. Thanks!