i have a 08 1500 with a 5.3 cammed with converter and a lot of extras. i have stock 20s on it with stock tires. i want to change gears from 342 to 373. how many rpm will i gain at 70. currently at 70 im running 1800 rpm. it seems to get better mileage when i drive it 75-78. who has done this swap??? what gears do u recommend

 

I went to 4.10 's and am at 2200 rpm's at 70mph. if you are going to change gears go to at least 3.9 to make it worth while.

 

Chevy Silverado 1500 with stock 31.5" diameter tires –
The RPM's shown in the charts below reflect driving at the highway speeds noted in overdrive.
Gear Ratio 4L80E @ 60 mph 6L80E @ 60 mph 4L80E @ 70 mph 6L80E @ 70 mph
3.08 1478 1321 1725 1541
3.42 1642 1466 1915 1711
3.73 1790 1600 2089 1866
4.10 1968 1758 2296 2051
4.56 2188 1955 2554 2281

 

If you tow, your capacity will increase.
I have a spreadsheet to compute RPMs based on transmission and differential gear ratios. Send me a PM with your e-mail address if you want to work the numbers.

 

completly agree on going more then the 3.73 its just not a big jump and not worth is for the money.. got to at least a 4.10