Speedometer calibration with no math involved

**Northwoods** · October 3, 2010

After swapping out gears and tire sizes I made the mistake of trusting the stealership to get the right speedometer gear for me based on their charts. $80 later the speedometer went from too slow to too fast. :banghead:I tried to do the math to figure out exactly what I needed but I got caught in the paradox of 4.11/3.55=115.8% versus 3.55/4.11=86.4%. Did I need to change the gearing 15.8% or 13.6%? :banghead: In retrospect, I'm pretty sure that this is what caused the dealer to give me the wrong part and it makes me question the accuracy of all the online conversion tables.After a few beers and remembering some algebra from 20 years ago, I figured out how to make the truck tell me the answer. :thumbup2:Here's what you do:First of all, you need to know what gear you're currently running, so pull the gear and read the stamp which tells you the number of teeth.Second, reset your trip meter and gps (or pull up even with a mile marker out on the highway)Third, drive the same number of miles (measured by gps or mile marker) as the number of teeth on your gearWhen you get to that point, look at the trip odometer, and it will tell you exactly which gear you need (round up or down to the nearest number).Here's how it works if you're actually interested in the math:In my case, I was running a 29 tooth gear, and drove 29 miles. Taking those first two known values I had 29 teeth/29 miles for a 1:1 ratio. On the other side of the equation I had 30.3 miles on the trip odometer. Since the first side of the equation was already set to 1, I knew I needed a 30 tooth gear to balance the second side. After ordering the right gear and putting it in, I double checked the odometer against the gps and I'm less than a mile off after a hundred miles of driving.If you need the gear, its $80 an up at the dealer with some of them going for over $100. I ordered mine through wermopar.com for $70. Later on I found out that there are aftermarket ones on ebay for $40. The dodge/jeep part #is 520676xx with the last two numbers being the number of teeth you need. I've got a spare 29 and a 36 if anybody needs them.

**DieselD24** · January 13, 2011

Sorry, but what is he talking about when he says "gear"? Differential, tranmission? I know it's been a while but, dang... that's a lot of math.I never was good at math problem solving. Rather better with upfront numbers, no reading, too confusing.But, interesting. I'd like to figure this out on my truck as well, as it has larger wheels.

**stodg73** · January 13, 2011

What he is talking about is the speedometer gear in the transmission. On certain older models, in order to change the speedometer to the correct speed, they would just change out this gear. On our trucks, this is done by recalibrating the speedometer through an update to the computer, either a dealer or if you have a SMARTY, you can do this.

There are some aftermarket boxes that you can put on your truck to get the correct readings. I have oversize tires and I just use a GPS for the correct speed.