Gear Ratio Calculator and Tire Size Calculator!

[N7RMK]

The whole point is this is planning for tire size after lift. I don't forsee the budget to regear anytime in the near future and don't want to burn up a transmission.

[HARDTRAILZ]

Manufacturer specs are highly inaccurate. Plus they don't account for varying wheel width or psi or vehicle weight. Bet you are not running the tires at an effective 30.5 diameter.

[v7guy]

There's so many possibilities for a 96 rpm discrepancy. First of which is most likely is that the needle is simply slightly off. But as others have mentioned, tire pressure, wear on driveline components, torque converter lock up etc. all come in to play. For the purposes of that chart, it's pretty accurate, and 96 rpm is not going to be the difference between burning up your transmission or not. If you're stuck with 3.42s stay with a tire at 32" or smaller and toss on a trans cooler.

[Shdwdrgn]

Checking my dash this morning, in 3rd gear I just hit 2000rpm at 46mph. Going by the chart I should have been closer to 2070rpm. Am I going to burn up my transmission from lugging it down by 70rpm? Not in this lifetime.

The RPM changes between gears aren't quite as significant as you might think. Do a comparison between stock 29.5" tires with 3.42 gears at 65mph (1762rpm), and 30.5" tires on 4.10's (2043rpm)... With a small increase in tire size the difference in RPMs is less than 300, not enough to make a difference in the operating temperature, but it's enough difference to put me more firmly into the power range. As mentioned, if you're getting into the red zones then add a transmission cooler for safety and forget about it.

[N7RMK]

Thank you everyone for your input. I was making the small difference into a big problem needlessly. Can't wait to get past the holidays and finally get started on this project.

[Trail X]

Actually - Jeff, I assume you didn't account for the 100 rpm slip that is built into the torque converter (and in my opinion, you shouldn't in your calculations).

Even when the TCC locks up, its programmed as stock to allow a 100 rpm slip to make the engagement and power delivery smoother. So, in reality it looks just about right.

So if you add about 100 rpms to the table is likely a bit more accurate unless you have replaced the PWM valve with a one piece isolator converter valve that has 0 slip.

The whole point is this is planning for tire size after lift. I don't forsee the budget to regear anytime in the near future and don't want to burn up a transmission.

Just keep in mind that there are no hard-stop rpm limits. The red portion of the gear ratio graph is faded in for a reason - it depends on a vast number of factors. 33s on 3.42s may work for someone, while 31s on 3.42s may overheat for someone else. Its hard to tell exactly how it will affect your truck, so we have these loose guidelines that we've created after seeing others setups that have had issues.

[maricard]

Nice tools thank...