Changing Gears

[TSimpler]

Has ANYONE changed the gears on a Dmax Ally to a taller gearing. I am looking at changing to around a 2.8 or so. My math tells me that I should get around 5 MPG more.

I would like any imput anyone has. Even if its not from a Dmax

Thanks
Trent

[Colorado Kid]

Same post in multiple forums is a no no.

You've already got tires more than 2" taller than stock, and with 2.73 gears (which probably don't even exist for the AAM 1150 axel) you'd be turing less than 1500 RPM at 75 MPH. Trust me, the D-max doesn't make much power below 1500 RPM (mine had a 6 speed), so it would be aways shifting down to 4th, which would put you right back at 2100 RPM, where you (pretty much) are right now.

It takes a certain amount of HP to drive the truck through the air at a given speed. While it is true that the HP will be produced more efficiently at some combinations of torque and RPM than at others (that's why we have transmissions), 2100 RPM is not so far from the D-maxes "sweet spot" that a 25% improvement is possible while making the same HP. (25% improvement is assuming you're getting 20 MPG at 75 MPH now . . .which you probably arent'. If you're getting worse than 20 MPH then a 5 MPG improvement is even more than 25%).

The only reliable way to improve MPG by 25% is to slow down. The aerodynamic drag at 65 MPH is 75% of the drag at 75 MPH. If other friction and losses could be neglected (They can't ) and airspeed always equaled road speed that would result in a 25% improvement in fuel economy. Unfortunately you'll probably have to slow all the way down to about 55 MPH to achieve a 25% improvement in MPG in the real world.

[More Power]

TS,

CK is pretty well right on....

I'd suggest gearing (or driving) any diesel pickup to produce an engine rpm near/at the engine's torque peak, at the speed you wish to drive - for best fuel economy.

MP

[TSimpler]

So why does gearing make such a big difference on the older 6.2s?

I am just looking to gain some milage

Thanks
Trent