Well, I finished a lot of modifications to my truck and it runs great. It , according to the manufacturer, now has 650 ft lbs torque. I have a 6 sp,HO, 3.55 rear end. I do a lot of freeway miles and at 70 mph the engine sits at 2200 rpms. My question is, with all this torque, can I change the rear end to get the rpms down to about 1800 to increase fuel economy and lower the roar in the cabin? As it is now I keep reaching for a 7th gear that isn't there!!



2001 HO, FASS, All the Banks Stuff, 6 sp

 

That's strange. My dually (see signature) with LT235/85R-16E tires turns 2350 RPM @ 70 MPH with 4.10's. The same dually with 3.55 gears normally turns 2000 RPM @ 70 MPH. What tires are you running, and are you sure the speedometer is accurate?

Rusty

 

I have 265/R16's on it and I'm pretty sure of the speed because I have a Garmin GPS that's showing the same speed.

 

I have the 3.55 gear also, but at 70, I am turning 2050 rpms on 245/75/R16. Your numbers don't sound right.

 

grz, with your tires/gears/trans, 2200rpm should result in around 80 mph, not 70. Did you mean to type 2000? Are you sure you don't have 4.10's in the truck? 4.10's would back-up your Garmin.

But to answer your question about regearing it. I'm not sure how low of gears you can install in your Dana, but you probably won't see much of an increase in fuel efficiency. If you did, it would be so slight that the money you saved in fuel consumed would take over 100k miles to pay for the parts labor of doing the gear swap. I think that OD units fall under the same cost-benefit analysis results. If your truck is 4x4, double that part/labor cost.

 

Ya...numbers is wrong somewhere. 3.55's = 2k rpm @70 w/ 235 85's

To drop RPM's you can go bigger tires, (but it would take 38's to drop 20%), re-gear, or add an auxiliarry trans. Re-gear is the least desireable IMHO because you couldn't get back to where you started. Aux trans are pretty sweet because you get inbetween all upper gears to optimize rpm to the situation..but they're around $3K

 

Ok, maybe I'm off a bit on the rpms at 70, but I'm interested in the principal, is it realistic to expect to cruise at 1800 rpms? The Aux Trans sounds kewl and what the heck, I got a ton of money in this thing already, got any links to a site that does this work?

 

I'm sitting right around 1800rpms at 70, maybe a wee bit higher.

35 inch tires and 3.55 rear end.
Auto tranny though.

It doesn't really help with the fuel mileage though.

After I pass by 65, it starts dropping like a rock.

According to the overhead, I'm getting 20mpg at 65mph.
16-17mpg at 70.
12-13mpg at 75. (2000 rpms)

These trucks have the wind resistance of a very large brick.
Anything faster 65 is pushing too much air to see decent mpg at any rpm.


phox