Hello all. I haven't posted on here for awhile but I have a question about gearing. I have a '05 1500 quad cab with the 4.7 and 20's.I've been wanting to gear it for awhile now. Finally I decided to get off my rump and do it. I thought that I had 3.93 or 3.73 for a gear ratio to start. Come to find out I have 3.55's.so my question is...

With 3.55's at 70 mph I’m turning around 1500 to 1600 rpms.Evey little hill it shifts down. and pulling any kind of trailer(I don't do that often),I gotta work it pretty good.
So if I was to get say 3.93's what would the rpms be at 70 mph, or even 3.73's
Any help would be appreciated

 

3.92s are what you mean and those are what I have on 20s but I have 34" tires but I turn about 1750 rpms at 70. 3.73s are only for the 10.5 AAM rear end I'm pretty sure. You might want to look into 4.10s or 4.56s if you're towing. I get down shifts occasionally on the hills of florida hahaha and that's empty

 

yeah that's what I meant lol.my tires measure out to 33".I don't do alot of towing just now and then.1750 is about where I want it to be up near the power band but not as low as mine is now.I'm thinking 4.10's or even 4.56's might be a little to low.thanks for the info

 

stick or auto?

 

When thinking about re-gearing there are two important things you have to think about. First is the fact that you have an automatic transmission that essentially has two over-drive gears, so numerically over-gearing is pretty hard to do. With 4.56s & 35" tires I only tach just slightly over 2000 rpm at 70 mph and the Hemi engine doesn't really even begin to reach into it's power curve until about 2200 rpm.

Secondly, you have to think more about effective gearing than the actual gears. Stock 3.55 gears only act like 3.55 gears with 31.5" tires, so even 33" tires have your gears down in the neighborhood of 3.3:1.

If you plan on staying at 33" for your tires then 3.92s wouldn't be a bad idea BUT 4.10s would give just a bit more bang while not using any more gas on the interstate than 3.92s or would be so close it would be just about immeasurable.

IMO 4.10s would be a perfect balance of power increase/less transmission work and MPGs on the highway. Of course 4.56s would be my choice if I towed/hauled much or if most of my driving was confined to in-town applications OR if a loss of 1-2 MPGs on the interstate doesn't bother you. Obviously, since you would have to give it less throttle to achieve acceleration from a dead stop, numerically higher gears actually help in-town driving MPGs...

 

If you're carving the pumpkin anyway, I'd go 4.1 over 3.92 as well for the little extra kick and not much difference on mpg. 4.56 is nice for towing and hauling but it would leave you about 300rpm higher than the 4.1 and start into the mpg on the highway. If you mostly run empty, I'd stick with 4.1.

 

ok, I have an 03 with 5.7 hemi. I do about average all around, towing, highway, and in town driving. I just would like better acceleration without going overboard and over gearing my engine. I'm guessing the 4.1 would be good for that. But my big question is how would you adjust the speedometer to read right? This is the first vehicle I have owned with a working speedometer and I want it to stay accurate. I have 30.5 inch tires and a bullydog programmer where I can adjust the speedometer with by changing tire size.

 

speedo isn't affected by gears as the sensor is on the axle side of the gears. If you stick with your stock rubber there's nothing to adjust.

 

BTW I have the same size tires and think you would be pretty happy with the result. Although it does get off the line like a raped ape with the 4.56 and those little training wheels. Thankfully the .67 OD ratio brings it back to something more manageable on the highway.

 

I got a motor build going on. (spare motor) Maybe when I'm done with the build, I should get the 4.56 Gears and try to lift the front off the ground. Motor will have both top and lower end completely redone. Bored, stroked, ported, and blown. Are their larger gears than the 4.56 that could be used on the strip?