I SPEED

I've got 315s on the truck now and the speedo is off by 5mph. I went to the dealer today and the tech hooked up the DRB and entered the rev per mile for tire size. Now the speedo is off by 15mph.

The truck originally had 265s and 3:55 gears. Now with the 315s I've swapped up to 4:10s. I'm no math genius...how can I figure out this mess?

CSAGrey1

Problem is that he most likely adjusted it as if it was running 315's on the stock 3:55's and not your 4:10's. It would be much easier and accurate to adjust using a GPS.

I SPEED

Can the PCM be programmed for 4:10s instead of the OE 3:55s?

CSAGrey1

I think the ABS/pinion ratio can be set however you want it.

J BODY

I've found the best way is to take the drb and my gps on a road test and jack the numbers around until they are correct. I have also found on some that the rev per mile number doesn't always get you in the ballpark. The gears will have nothing to do with it as the reading is taken off a tone ring mounted behind the ring gear. I had 4.10 and swapped to 3.55 and no program change is required.

I SPEED

Thanks for the input guys...back to the dealer on Monday.