I have been having the same issue with my speedo showing faster than my GPS. I read your post about measuring the tire size as I'm running the same tire as the OP 245/60/15's. I'm running 32psi in the tires and I took a measurement to the center of the wheel X2 and I come up with 25". So the chart shows I should run a 40 gear which I had in there and she read 5 mph fast, so should I go to a 42 tooth gear ? I went to a 38 as someone suggested and now I'm 10 mph fast on the speedometer to my GPS.
Very often the tape measure on the diameter does not work out. For greater accuracy , you need to measure the circumference. Many times the pinions will only get you within 3% of perfect; either over or under.
As an example if you measured your tire as you say, then the circumference is 2x12.5 x 3.1416=78.54
But if you were off by a mere quarter of an inch and the half-diameter was really 12.25, then 2x12.25x3.1416=76.97.
This is an error of 1.57 " or 2.0%
But the easiest method of all is to figure your exact error in percent, and to then change your pinion by that exact amount. As an example; if your out by 5 mph at 60 mph, that is an error of 5/60=8.33%, so you will need an 8.33% change in tooth count. If your speed-O is fast , you will want to slow it down. That takes more teeth , so if you had a 40 tooth pinion, you would add 40 plus 8.33% = 40x1.0833=43.3 teeth
But another question is; how do you know the GPS is correct, and what might be the error in it?
But the final question is this; if the GPS knows where you are and your direction of travel, and rate of speed, and knows who you are and where you live.......... who else has that information and what can they do with it? Just wait until you start getting speeding tickets in the mail that say something like; "your phone was recorded Northbound on hiway 66 on June 10, 2018, doing 68.5 mph. You have been assessed a fine of $148. Please send your payment within 10 business days, or we will send someone to get it, which will make it very expensive. You will not receive a second notice."
But, you say, I'll just turn it off. To that I reply; how do you know off really turns the GPS off?
How do we really know anything anymore?
But I digress,lol.
Anyway; look on the chart where 43.3 teeth falls, with a 25" tire.
The chart runs on the inch. So if you have a 24.5 or a 25.5 tire, you might think the same pinion will work for all? Not hardly. That is a variation of plus or minus 2%, and that the leads to fine tuning from 42 to 44 teeth, not including your possible error in diametrical measuring.
If you go to the line where you fit in, namely 4.10s, and follow along how the pinions change with each tire size. Notice that sometimes, with the smaller tires, the pinion counts change by 2, and at other times by just 1. That is because of rounding during the creation of the chart. Notice on either side of you, the chart jumps by 2 teeth. So depending on where your diameter exactly falls, you might require 1 tooth higher or lower. Isn't this fun?
But the percent method on a known error with a known pinion, is dead nuts accurate.
