Rear end ratios

Fireman · April 17, 2001

Just pondering. Did a lot of that during my career as a firefighter. In the days before medical calls, we had a lot of time to ponder... But, anyhow, I digress. I have a 98 Sierra with 235 75R15 tires and 3.42 rear end. A good friend just bought a used 99 Sierra with 265 75R16 tires and 3.42 rear end. I was always told, larger the tires (including upping the width) will decrease your gearing. Since the all the newer GM's have 16 inch wheels, and all the newer also have increased the tire width, how have the rear ratios been effected? JC

Shaners · April 17, 2001

Tire width will not change the gear ratio at all. It will just add rolling resistance.

Tire diameter on the other hand will change the final drive ratio of your truck.

For example: For every 1" you go larger in tire diameter, you change your gearing ratio by approx. .1.

If your tires are currently 29" in diameter and you put larger 31" tires on your truck, and you have a 3.42 gear in your rear end; then it would now act/drive/feel like your new ratio is 3.22. (2" taller = .2 change in gearing).

This would make your truck feel very sluggish off the line as compared to before with the stock tires. It would also throw your speedo off by several mph's at cruising speed, resulting in the need to get your speedo recalibrated (your speed will read for example 60, but your actul speed may be closer to 63 or 64). If this change isn't calculated into your mpg figures, then you will notice a significant drop in gas milage).

I hope this makes sense...

(the larger tires on the newer trucks have more than likely been compensated for with the final drive ratio in the transmission)

(Edited by Shaners at 3:28 pm on April 17, 2001)

GUMPA · April 19, 2001

Quote: from Fireman on 12:33 pm on April 17, 2001
Just pondering. Did a lot of that during my career as a firefighter. In the days before medical calls, we had a lot of time to ponder... But, anyhow, I digress. I have a 98 Sierra with 235 75R15 tires and 3.42 rear end. A good friend just bought a used 99 Sierra with 265 75R16 tires and 3.42 rear end. I was always told, larger the tires (including upping the width) will decrease your gearing. Since the all the newer GM's have 16 inch wheels, and all the newer also have increased the tire width, how have the rear ratios been effected? JC

Just a note : I had a 66 Ford P.U. with a 427 Police Interceptor Engine and 4:56 Posi axles.........it was awesome , even with 35" tires.