The speedometer on my truck reads 4-5 mph too high. When it reads 55, I'm really going 50. The stock tire on the door tag is LT235/85R16E. The tires on the truck now are LT285/75R16 (ie taller). Differential is 3.55. So with taller tires the speedometer should read low, not high. Isn't 3.55 normally the lowest ratio used? What could account for the inaccuracy?

 

what year truck? what trans? the gear ratio could have been changed, or the speedometer drive gear could have been changed if truck is older than 92. if 92 or newer, the PSOM could have been reprogrammed for different tires.

 

1987

 

You need to change out the speedometer gear which should be located wherever your VSS is. On my 1990 4x4 the VSS is on the driver side of the t-case. Remove the VSS, 1 bolt, and there will be a plastic gear on the end of it. You need to buy another gear with a different tooth count. https://shop.broncograveyard.com/Spe.../products/853/

 

first thing i would do is check the speedometer against a gps device.
then figure out what tooth speedometer gear is on the cable and determine what tooth gear is needed to bring it down.
i had to drop 2 teeth to get my speedometer in the 88 correct.

 

So reducing the number of teeth reduces the speed shown by the speedometer?

 

yea, pretty sure that was the ticket....but it was also almost 40 years ago, so i could be wrong and added two teeth.

 

Well sure you can do it that way. Just keep driving and ol' Johnny Law will tell you how fast you're going.

 

The lazy man's solution is to note the difference between speedometer and the GPS which I have found is remarkably accurate then do the math. Add or subtract and then match the answer to your speed. Requires 1st grade math.

 

If your speedo is reading high add teeth to slow it. Subtract teeth to speed it up...seems right to me

 

I did THAT math. As I said in my post, my speedometer reads 5 mph high. I based that on GPS which I corroborated with mile markers on a flat, straight stretch of interstate. Now, do you know the math to tell me how many teeth to add? That's what I need.

 

Thanks. It's counter-intuitive so I wanted to make sure. Is there some rule or formula to calculate the correct number of teeth to add? I could add teeth and do it by trial and error, but I'd rather not have to go to that trouble.

 

https://www.tciauto.com/speedometer-gear-calculator

 

^^^ according to that charts calculator^^^, i need a 44 tooth driven gear in my 88.

 

Right so if it reads 5 high, subtract 5. So 35 on the speedo means 30 in true speed. I hope the link posted answers your question about tooth number. Report what you find!