Does anyone have the formula for computing RPM/MPH relative to tire size....I live in the mountains of NC rarely ever see 60 MPH...currently running true 33s on my 2010 JKU Rubi AT.....thinking of regearing to 5.13,s ....do not plan to ever go to 35....RPM charts on this forum show the RPM for 70 MPH....like to know what it will be at 60 MPH????any help or comments appreciated.

 

Rpms from the chart, divide by 70 multiply by 60.

If you don't intend to go over 60mph, you drastically reduce your power requirements. It takes over 20% more power to cruise at 70mph than it does at 60mph.

The question really becomes what are your objectives? Do you want the power from being in the 90% of peak torque power band, or are you looking for efficiency at your cruise speeds? Better off road performance, or what makes it roll best on road?

 

Thanks, since I drive in the mountains exclusively and mild wheelin out west, Moab/Ouray, i am looking for better performance in the RPM range that I operate it in....now, with the 4.10s, I rarely ever get out of 3rd gear,and when I do, it is constantly hunting back and forth between 3rd and 4th....most of the time I simply shut the OD off, maybe this is just the best solution, as you know regearing is expensive....I was just hoping that by regearing to 5.13s, I would be able to use the AT more effectively, FWIW I am already using a Flashpac Tuner with the "tow tune", this has helped but still leaves a lot to be desired in the speed range I use it.

 

OK, that helps.

You could certainly do 5.38s for a more performance oriented gear set. With 33s, durability shouldn't be an issue and at 60mph you would still be hitting 90% of peak torque.

On the Superchips side of the equation, you sound like a good candidate for the advanced trans tune. You can get the gears to hold longer when accelerating and not feel like you need to get out and push before it downshifts. There's another member with a similar set up that's getting things pretty dialed in with a similar rig. Let me know if I can help.

 

After doing some figuring maybe going to the 5.38 vs. the 5.13 would make more sense......even at 70 mph (which I never see up here) the 5.13s would turn 2522 rpm vs, the 5.38 @ 2645 rpm, which compares very close to a stock 6spd Rubi with 4.10s and 32s.... while i understand the effect of the different gear ratio set ups on the final drive, I would assume the regearing will also have a net positive effect on the performance in the 1 thru 3 gears up to the final OD....can anyone comment on this ???? thanks

 

Yes, it will definitely have an effect through out all the gears. It's still going to short shift as it does from the factory and it still won't downshift until you've reached a point of no return.

 

Thanks, since I already have the Superchips tuner I will look into the "advanced trans. tune" option....can you tell me how it basically works??? I know nothing about it.

 

The Superchips Advance Trans Tune allows for changing the shift points by changing the mph at a given throttle position percentage. What you can do is make the transmission shift at a point that it doesn't bog in the higher gear and have better response in downshifting when you need to pass someone or climb a grade.

Here's a video of a Dodge Ram using the Trans Tune package.

http://www.youtube.com/watch?feature=player_embedded&v=8jdWYU7VrEg Here's an example of what a tuned shift schedule looks like as well as the stock schedule.