So I've decided to move down to 265/76-16 tires (since they're cheaper than the 285's) and have already re-geared the truck to 3.92s; so the speedometer and odometer are going to be even more wrong.

So, how do I adjust the speedo?
Thanks

 

with 3.92s and 265's it shouldnt be off much.....i have 4.10s with 265/75/16's and my speedo is off 5 mph...GPS will say 60 mph and my speedo reads 65

 

Well, with the 285's my speedo reads 5% higher than my gps and mile markers

 

According to my calculations using this site, You are going to end up with a speedometer error of approximately 1% and some change......

 

That calculator doesn't seem to consider gear ratios...

 

You aren't changing the gear ratio. You are only changing tire size, so, it would factor itself out of the equation anyway.

Currently, you read a about 9% slower than what you are actually going. Going to smaller diameter tires is going to reduce the amount off error, by about 5%. (ok, my initial guess was a little off.....) So, at an indicated 65, you will actually be doing right around 62-63 MPH.

If you want to get it dead nuts on, give these guys a call, tell 'em what ya got, and they should be able set you up.

 

You are not read any of my posts. I HAVE regeared to 3.92 and my speedometer is currently reading 5% higher than my actual speed (as already stated).

 

Call the guys in the link. Tell them what you have, and they can sell you a speedometer gear that will get you back to accurate. Or, at least, closer than what you are now.

Your speedo gets its cue from the VSS in the back of the tranny. (output shaft on t-case? one of the two..... think its the trans....) That is driven by a gear, that is replaceable. Change the gear, it changes the speedometer calibration. They will want to know what gears you had in there before, what the stock tire size is, and probably what color gear is in there now.

 

Stupid question; if stock size is 225/70r16 and currents are 265/75r16, the speed would be around 9 or 10% off the actual reading. Would it change the odometer reading dramatically?

 

By 9 or 10%. If the speedo is off, the odo will be off by the same margin.