Gear question

[maddawg89]

Hi.
My 99 is supposed to have 3.73 stock. When I bought the truck, I thought that's what it had. It had 315/75/R16s on it then (isn't that 35s?) and now I have 305/70R16s. When I switched I dropped from 15mpg to 13.6mpg. My speedometer is now 4-5mph out (slow) and it now runs 2100rpm at 70mph instead of 2000rpm. My question is, because of huge tires, could the gearing have been changed out? Maybe 4.10 or 4.30? Thanks!

[JSchuricht]

Yes it's possible that the gears were changed. 35's aren't that big but I am guessing 4.10 but it could be 4.30. To figure out, jack the rear up and spin the drive shaft by hand, count the number of revolutions it takes to make the tire go one revolution. A chalk mark will help you to keep track of where things are. Your speedometer is not affected by gears, just the tire size. It would be a good investment to purchase a autoenginuity scanner, it can reprogram your tire size. Your odometer is tied to your speedometer so with the speedometer off, your MPG calculation is also off.

[BIG Laing]

Originally Posted by JSchuricht

Yes it's possible that the gears were changed. 35's aren't that big but I am guessing 4.10 but it could be 4.30. To figure out, jack the rear up and spin the drive shaft by hand, count the number of revolutions it takes to make the tire go one revolution. A chalk mark will help you to keep track of where things are. Your speedometer is not affected by gears, just the tire size. It would be a good investment to purchase a autoenginuity scanner, it can reprogram your tire size. Your odometer is tied to your speedometer so with the speedometer off, your MPG calculation is also off.

Or if you have help you can do it in a parking lot. Chalk line on wheel and drive shaft. That's what I did to confirm my gears.

[maddawg89]

Ok. I jacked 'er up and It was around 3 and 3/4 turns on the driveshaft for 1 turn on the wheel. Yep... 3.73:1. Thanks y'all!