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Lake O Hunter 08-05-2013 11:10 PM

towing vs. highway questions
04 st hemisphere 4wd 35" tire
superchips performance tune and other minor upgrades

I stock gears but do not know the ratio; I am turning about 2k rpms at 70 mph

I tow a boat weekly that weighs approximately 7000# but also drive a lot of highway miles. Ithink I need to re-gear for better towing performance and off highway performance but I am not sure what ratio?

Also will my superchips tuner adjust for gear ratio?

I am open to suggestions and let me know if I need to provide more info. Thanks in advance.

mb power wagon 08-06-2013 01:02 AM

have you thought about going to the gears in the power wagons? they are 4:56 gears and with the 35" tires but it might raise your rpm's a bit more than they are now. but you will have alot better power towing. and the engine wont work as hard when pulling. best thing to do is see what you have in the truck now before considering a gear change.

Lake O Hunter 08-06-2013 05:26 AM

Is there a way to figure it out without pulling the differential cover; I am certainly not opposed to going that low to improve pulling ability and just general acceleration.

snrusnak 08-06-2013 07:52 AM

I'm not positive but I assume the 04 rams have the 545rfe, not the 45rfe. If that is correct, then based on the math:

If you had 3.55's with a 35" tire, at 70mph you'd be turning 1598 rpms in the 6th gear
If you had 3.92's with a 35" tire, at 70mph you'd be turning 1765 rpms in the 6th gear

The superchips is able to correct for gear ratio. Do you have it corrected for tire size currently?

You can also request your build sheet from chrysler and it will have your differential ratio.

Lake O Hunter 08-06-2013 04:25 PM

Requested build sheet and checked my rpms today; apparently I drive a little faster than I thought; I was @1600 rpms at 70mph.

Yes I have corrected for tire size using the programmer and truthed it against multiple gps and vehicles.

I would love to see the formula you are using, so I can estimate what it would be with 4.56 or other gear ratios.

Thanks for your help.

snrusnak 08-06-2013 06:00 PM

RPM = (MPH * Final Gear Ratio * 336) / Tire Diameter

so, in the 6th gear(ratio is 0.67:1):

RPM = (70 * (0.67*4.56) * 336) / 35
RPM = (70 * 3.0552 * 336 / 35
RPM = 2053

Lake O Hunter 08-06-2013 06:18 PM

Thanks again. That doesn't seem unreasonable for highway speed? Plus it would improve towing and city driving aka using less throttle.

With factory tires I would be around 1800 @70mph, correct?

I just substituted tire size of 31 in your formula

snrusnak 08-06-2013 07:12 PM

With factory tires (31" diameter), and 4.56's, you'd be at about 2318 rpm at 70mph in the 6th gear.

snrusnak 08-06-2013 07:13 PM

2000 rpms isn't too bad with 4.56's and 35 in tires. I'd probably go that route if you are keeping 35in tires for the life of the truck. If you plan to go back to a smaller tire I'd get 4.10's.

Lake O Hunter 08-07-2013 06:54 AM

Sorry I meant factory tires and factory gears; i was trying to figure out what the rpms were stock; I don't see me going any higher or lower with my truck; the leveling kit and 35s is the perfect to me, so I think 456 it is...

I greatly appreciate the information.

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