hi I have a 1997 f 350 powerstroke and I was wondering what the optimal rpm operating range would be for this truck right now the truck at 60mph turns 1400 rpm and to me that seems to low it almost feels like the truck is lugging only get 13.5 mpg right now wondering what the best rpm is for this engine thanks luke

 

Hey Luke, That operating range is a good one for mileage, but not optimal for power. The way a diesel engine works, the faster it runs the more fuel it takes (duh, right?). These engines make peak power around 2500 RPM in stock form and the turbo doesn't really get spooled until 1700-2k RPM. That's probably why you feel like the truck is lugging a little...it kinda is. However, for running around more or less empty keeping the turbo spooled is more or less unnecessary and just makes the engine use more fuel.

It sounds like you have 3.55 gears in your truck and if the engine is running well and everything is well maintained, you should be able to get better than that for mileage. My truck has 3.55 gears in it (and a 5 speed) and I have hit a high of 18 MPG on the highway. Average for mixed driving to and from work on county roads I see 12-14 MPG. I think you're in the range if that is just running around. If that is highway I think you can do better, but if you have the auto you're probably not going to see 18-20 regardless of what you do. My old truck (CCLB with 3.55s and the slush-o-matic transmission) never would get me anything better than 16 MPG on the highway and averaged about 12 around town. I have a lot of short (10 mile or less) trips so it doesn't really get good and warmed up until I'm about ready to shut it down. I think you can do better for mileage, but I think what you're seeing is, unfortunately, somewhat typical.

 

Thank You DIY mechanic for your reply I guess I should have provided more info about the truck yes I has the E4OD automatic trans in but also it has BFG 35s tires on it that's why I thought I should possibly bump the gear up to a 3:73 to better match the engine to the bigger tires because I do also haul with the truck nothing real heavy just a car but it seems to struggle a lot with that and yes I have never got better than 13.5 even on the high way

 

Wit the E4OD and 35s, I would swap to 4.10 gears. The only two ratios offered in the OBS trucks were 3.55 or 4.10. That's not saying you can't do 3.73s, but I think the 4.10s would be much better for what you have.

 

ok so where does that but my engine rpm at like 60 mph with 4 10s and 35 inch tires I just seem to think that the 1400 rpm is making the eninge wroke harder and use moore fuel because its not in the optimal power range IDK for shure tho thanks every one!

 

With the E4OD and 35" tires, I would guess that 4.10 gears would put you in the 1800-2000 rpm range at 60 mph. Right around in there should be the sweet spot for cruising.

 

I'd guess 1700-1800. I don't remember the formula. Mine is closer to 2000 with 31 inch tires.

 

Also, I ran those gears with 33's, pretty much a stock truck. I could get over 19 at 60. I got 17 at 75. I think you have some other issues you need to find before spending money on a gear swap. I know you have a 350, but there isn't that much difference.

Are you sure you don't have any fuel leaks? Rear brakes not dragging? Wheel bearings all good and greased well? Tires fully inflated or do you run them a little low? Bigger ones hurt a little, but I had 305/70's. Little things can add up to hurting mileage.