Rear End Change Question

I have a 99 Peterbilt 379 with a 3406E CAT, 13 spd and 3:70 rears. I get just a hair over 5 mpg. Truck pulls great, but was thinking of swapping to 3:55 rears. How much, if any, would this change my fuel economy and power?? Thanks

 

What RPM are you normally running at? What size tires?

 

I’m running 1550 at 70mph. I run 11R24.5

 

3.36 may be your best bet with your engine.

 

Are you on low pros ? Because I was set up like that with a B model and 1550 rpms was a true 78 mph
I just switched to 3:36..haven’t tried it out yet.. but Iam on tall rubber

 

I’m on tall rubber. 11R24.5

 

Isn’t there quite a bit of difference between the 2? How much difference in pulling power would that be? I pull emulsions in summer on a lot of county roads. I’m in Tennessee in hill country, don’t want to hit a hill and have it fall on its face.

 

None is what it will change it. You can say lower rpm equals better mileage till hell comes home. You can say sweet spot this or that but from personal experience I can tell you it will not change.


Make sure your motor is performing at its best. Absolutely no boost leaks, no air in fuel system, over head properly adjusted etc. I get that you may be making x lbs of boost etc however that doesn’t indicate you don’t have problems.

I have been from 3.36 to 3.70 to 3.55 with the same truck. I pulled hills at same speed with all of them, fuel consumption never changed, only thing that changed was what gear I pulled x hill in. I get it on paper it looks good but in the real world it’s not worth the money. All that said I like my 3.55 best. I run 11R24.5 with a RTLO18913A trans. With them I can split down to 12th and still run 65 out on a 2 lane with a hard crosswind. The 3.70 I got a little gear bound in that situation at times. With the 3.36 high gear was just too high to use so I was turning my auxiliary all the time.

take it for what it worth

 

I concur, went from 3:55 to 3:70 l.p. 24.5. Thinking it would pull hills better? Nope just a different gear.

 

No.

Your motor is making the power.....not your rear gears.
If you swap your rears you will only change your start ability (1 gear, getting started), and your top speed at a given RPM. All other driving will be in a half gear lower (less than that actually, going from 3.70 to only 3.55). In other words hitting a hill you may pull it in 8th instead of 9th. Etc.......... Your power stays constant, and it will only fall on its face if you fail to use your gearbox for it's intended purpose.

What wore out said was correct. I just explained it a little differently . I think.

Also, you may go slightly faster in reverse.