bought a 99 f250 7.3 4x4 sometime ago, runs great, yet just noticed that rpms are 2000 at 70mph and gives me bad mileage about 12 highway/city. can that be related to 410 gears? and if so how hard is it to change to lower gears? will that improve mileage?

 

Your rpm's at 70 and the mileage you are seeing are normal for your truck. Swapping to 3.73's could be done, but would be costly. Even if you do it, in my opinion your mileage probably wouldnt improve enough, if any at all to make it worth it. If you tow with your truck it would not be advisable to go to 3.73's in the 7.3 as they would hurt your performance there. Have you considered adding an aftermarket tuner to your truck? Many here have claimed to see 3-4 mpg increases by adding them. This would be cheaper than changing the gears and if the tuner results are as claimed, would also be better than you would achieve with a gear swap. Good luck

 

2000 rpm's is the magic number on these 7.3L engines. Stay just below 2 grand and your fuel mileage goes up.

Is your truck DRW? Just wondering if you actually have 4.10's, since you are turning 2K rpm's at 70mph. With stock sized tires, you should be at higher rpm's @ 70. So either you have oversized tires, or you have 3.73's. Some more info from your truck will give us a better estimate of why your mileage is low. Turning 2K rpm's on the highway should give you better than 12 mpg.

Last, how are you calculating mileage? The fuel tanks do not fill up all the way when the pump automatically clicks off. Depending on how much foam you have, you can still add 3-8 more gallons of diesel in the tank if you slowly dribble it in. This can throw off your calculations by quite a bit. The "Harpoon" mod takes care of that issue by dropping the tank and cutting off the vent tube inside. This allows you to fill with diesel all the way to the neck, instead of filling up with a bunch of foam.

 

I agree with Pocket, if your truck is all stock it does sound like you are running a 3.73 gear.

How I calculate fuel economy: I fill up with pump on auto fill slow mode (to prevent splashing fuel from shutting off pump) until it clicks off. Dont add anymore. Reset trip meter/odom. Run vehicle fuel most of the way down. Return back to the exact same station and pump. Fuel using auto fill slow mode until it clicks off. Dont add anymore. Write down the gallons on pump (or get a receipt) and the exact miles on trip odom. Take the total number of miles traveled and divide by the gallons you put in the 2nd time. Keep in mind any mods that have been done to truck like bigger tires and a gear swap will put your calculations in error.

 

My truck, with 3.73s, runs 2000 RPM at 70 MPH.

 

It can't be a DRW(stock anyway)...In the new 250's I don't think you can't get 4.10 gears with a diesel...probably the same with the 7.3.

Tim

 

since you guys mentioned the tire size might be a factor, theyr'e 305/70/16 and I also installed a hypertech tuner and a cold air intake, forgto to mention that.

 

If you haven't re-calibrated your speedo, your mileage calculations are off. Larger tires equate to less rotations per mile, meaning that your speedo and odometer are both slow. So when you calculate fuel mileage, you are actually calculating for less mileage than you really traveled.

Stock tires are 31" tall, yours are 33". You're looking at a speedo that is at least 6% slow compared to what it used to be.

 

I’ve been looking around the forum for fuel mileage info and haven’t really found what I was looking for, so here goes:

I bought a 99 F-350 Lariat long bed, crew cab, super duty, 7.3 diesel 4x4 with a chip in it (no paperwork but it has a plain round **** that rotates to 4 different positions and I think it’s a bully-dog), an open after-market cone air filter (looks like a K&N), the exhaust appears to be aftermarket from the turbo back, and 35" tires with no lift kit (makes nice rubbing noises on the springs & inner fender). It also has what looks like plumbing for propane, but nothing’s hooked up. I’m out of town and can’t remember exactly, but the front axle had something like 3.63 printed on it (gearing?). The tranny was rebuilt, and it ate a couple crank angle sensors before I got it. The truck indicates about 150K miles. Oh, and it’s an automatic. That’s about all I know for now. I haven’t de-coded any numbers to find gearing, fuel capacity or the like yet.

I bought this thing because I read that diesels get 17 MPG even when you hot-rod them (Diesel Power magazine). In the 6 or so months I’ve had it, my best was 12 using the stevenn1 method and my worst was about 6 towing an enclosed 24' trailer weighing about 2500# loaded. I don’t drive like an idiot (no burn-outs, no more than 10% over any speed limit, smooth acceleration & I usually get left behind at traffic lights), so what the hell? Granted, I doubt the speedo was re-calibrated, but I have passed several speed traps (school zones etc.) with the big digital speed display and the speedo doesn’t appear to be off. Maybe he did re-calibrate it, maybe he didn't, the math would only make a couple miles difference at most and that still doesn’t turn my average 11.5 into 17 (+10% would only be about 12.6 MPG). Where’s all my fuel going? If I can’t do any better I’m going to sell it and go back to either my 56 or my 61 uni which both have 460's, get 13-15 MPG, look cool, drink cheaper fuel, and are cheaper to insure and maintain.

Are there any glaring problems with these trucks that will cause large difference in fuel economy? Are the guys at Diesel Power using voodoo?

Thanks,

Irwnn

ps. The position of the chip’s **** makes a big difference in smoke and acceleration but no real difference in economy (I’ve tried complete multiple tanks in all settings). Interestingly enough 2 vs 4 wheel drive does nothing to economy either.