Basic Question: What should be turning lower RPM's at the same given speed, say 70mph, a 3.55 rear end or a 3.92 rear end?

My dad's 3.55 turns about 100 lower RPM's at 70 than my 3.92. He has a 03 quad cab 2wd and I have a 03 reg. cab 4wd. Both with the 4.7.

 

The 3.55's will turn a lower RPM.

Mitch

 

Thanks. I had thought this was how it was, and that I understood Gear ratios correctly.

*New Question*

Assuming the driving was exactly the same, what "should" get better gas mileage? My REG. Cab 3.92 4x4 or his QUAD cab 3.55 2wd?
and by how much?

 

Assuming all things even, the reg cag with 3.92's will have less weight and more "power" transferred to the road thus having to take less gas to accelerate. The Quad cab is heavier and has to work a little harder to get to the same speed. By contrast though, once speed is reached the Quad cab should get better mileage since the rpm's of the engine will be a little lower than the reg cab. So mileage is relative but it shouldn't be much difference.

 

It depends on the overall weight as stated, but I'd imagine it would be pretty close if you have 4wd and he has 2wd. It really doesn't matter though, because if you include the weight of a full tank vs the weight of an almost empty tank, driver weight, tire size/inflation,road coefficient of friction, blah blah blah, they'd probably amount to about the same or near enough to not have a statistical significance. Ideally as StumpBreaker said, once up to speed the one with the lower rpm will likely get better mileage. In the real world it depends on the driver...drive like a blue hair and you get better gas mileage than the 16yo that just got his license

 

You will get worse gas mileage, reguardless. If you're like me you'll have your foot in it way more often than your old man.

 

Hey, hey... all old men DON'T drive like ninnies!

 

This is a "basic" chart that shows the relationship between axle ratio and tire size and RPMs.

http://www.summitracing.com/landing/...converter2.htm

I have a new 06 1500, 4.7L, 5 speed auto, 3.55, 30" diameter tires, regular cab 2WD and at 75 MPH I am running about 2000 RPM.