Peeps with 37's and 4.56's, get in here!

Ok, got some 37's put on with the stock 3.73's and it's a little too sluggish on hills for me. Even slight hills...with the 2" PW suspension and 37's, it's just too much hog to keep moving up minor hills without dropping a gear all the time. I lost about 2MPG going to the PW suspension and 37's, but I bet I could get one of those back going with some 4.56's. What kind of RPM's do you guys get with your 37's and 4.56 gears? How fast do you have to go to hit 2k RPM's? With stock tires and 3.73's, I wouldn't hit 2k til about 81mph.

Just plug in the numbers to find out the rpm at a certain speed.


RPM = (Axle Ratio x Vehicle Speed x Transmission Ratio x 336.13) / Tire Diameter

Or you can plug in the numbers here and it will do it for you...

Engine RPM Calculator

66RFE Trans gear ratios.
1st 3.231:1
2nd 1.837:1
3rd 1.410:1
4th 1.000:1
5th 0.816:1
6th 0.625:1

As SouthTexan stated, playing with a gear ratio calculator is going to be your best bet.

I run 4.56's in my truck with 37's and a Cummins. With mixed city and highway driving I actually lost about 0.5 mpg doing the swap because of the higher RPM. A lot of guys think you gain by doing it but from my experience and discussing it with others that's rarely the case. Most people will end up staying pretty much the same or within 1 mpg (plus or minus). However that drop was totally worth it for how much better the truck drives with the 4.56's. Night and day difference over the 3.73's without a doubt.
The 6th gear ratio in a 68rfe and 66rfe are the same. I turn 2000 rpm at 120 km/hr (75 mph) so you would be about in that ballpark as well.

brandonjansen said:

I turn 2000 rpm at 120 km/hr (75 mph) so you would be about in that ballpark as well.

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Yeah I was playing around with that calculator and it was saying the same. 2k rpm at 74-75mph is just fine with me, pretty close to what I had with stock equipment. Right now, according to the same calc, I'd have to go about 93mph to get 2k rpms.

I don't really feel like paying $2k+ for the swap though :/

A little off subject but since you mentioned it. Did the Power Wagon front springs level you out up front?

RAM_Designs said:

Yeah I was playing around with that calculator and it was saying the same. 2k rpm at 74-75mph is just fine with me, pretty close to what I had with stock equipment. Right now, according to the same calc, I'd have to go about 93mph to get 2k rpms.

I don't really feel like paying $2k+ for the swap though :/

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If you're on the hwy and doing 70, drop into 5th. See how the truck feels. That should give you pretty close to what the truck will feel like in 6th with 4.56s. It's pretty close to the same RPM.

I've been downshifting to 5th on the hwy to see how my 6.4 with 3.73s and 35s does. It seems to like being in the 1900 RPM range. It holds speed better and still shows pretty decent fuel economy on the evic. I just need to save up for it.

I hear ya about the $$. Looks like it's about $1400 for the complete kit with gears, install kits, oil, etc. Labor in my area is about $450 for the front and $350 for the rear. Hopefully this summer... unless uncle sam bends me over again this year and I end up owing... again...

danh63 said:

A little off subject but since you mentioned it. Did the Power Wagon front springs level you out up front?

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I put the PW suspension all around, new shocks and springs and new track bar also. I'm sitting about 1.25-1.5" taller in the back. Looks just the way I wanted.

RAM_Designs said:

I put the PW suspension all around, new shocks and springs and new track bar also. I'm sitting about 1.25-1.5" taller in the back. Looks just the way I wanted.

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OK, thanks! I want my front leveled up and I hear the PW front springs would do that