Whats wrong with my diesel mileage?
 

Ive got a 2001 F-250 with the 7.3l Powerstroke, 3.73 rear end, 4" lift with 35" tires. I bought the truck with the lift and tires.

If you drive 55mph with the cruise on on flat highway, it will get 15.

If you drive 65mph on flat, it will get 12

When pulling my 27' baja (6k?) it gets 8

Is this normal. Is it the bigger tires bogging it down? Or is there something wrong I might need to check for. Pulls it great, runs great.

Big tires, small rear-end ratio certainly doesn't help.
It does seem excessively low even for that combo.

MPG
 

Remove the muffler and straight pipe it or if you don't like the sound then I reccomend installing a Magnaflow muffler. If you do this and install a K&N air filter you would gain almost 2 mpg. I have done dozens of these and all got 2mpg better and one 03 Dodge that got 3mpg better. :)

Could be the tires. I did 600 miles in the last three days , mostly flat and not towing , but at least 100 miles at 75/80 mph the rest 65/70mph and still averaged 19.9 overall. Does your truck also have a different chip. Mines an F350 x cab srw 4x4.

No chip, and its a srw f-250. I have not seen the day it gets over 16 :(

BR.............Do you have any idea what size tires came on the truck as "stock". My guess is the new final ratio could be down to 3.2someting with the 35's on it. (assuming the stocks were much smaller).

But yes, going to a larger tire diameter does decrease the final drive ratio.



Bob

Has the speedometer been recalibrated? On my diesels, if I don't keep the fuel filter changed and injectors clean, the mileage goes down.

Has the speedometer been recalibrated? On my diesels, if I don't keep the fuel filter changed and injectors clean, the mileage goes down.

Ive never changed the fuel filter. How do you clean the injectors? (ive only had it 10,000 miles and im sure it needs one)

Ive never changed the fuel filter. How do you clean the injectors? (ive only had it 10,000 miles and im sure it needs one)

You should also drain the water separator.

I have an '01 Dodge with d-box superchip and K&N filter. I haven't done the exhaust yet. I'm getting around 20mpg.

Not only is your gear ratio off but so is the odometer, this may make your calculations off. I am getting 16 in the city and 17-18 on the highway. I would recalibrate the spedo and see if there is a big change. There is also more drag from these tires and the 4" lift exposes more of the undercarriage so your wind resistance is up. Think of this also...if you took two tires, one a 35 and the other a 265/75/16 and rolled them the bigger tire would travel farther to make one rotation, your current spedo gear thinks it is a 265 (stock) and is all messed up from this. I bet at 70 you are off by 10 mph on the spedo.

Not only is your gear ratio off but so is the odometer, this may make your calculations off. I am getting 16 in the city and 17-18 on the highway. I would recalibrate the spedo and see if there is a big change. There is also more drag from these tires and the 4" lift exposes more of the undercarriage so your wind resistance is up. Think of this also...if you took two tires, one a 35 and the other a 265/75/16 and rolled them the bigger tire would travel farther to make one rotation, your current spedo gear thinks it is a 265 (stock) and is all messed up from this. I bet at 70 you are off by 10 mph on the spedo.

Not only is your gear ratio off but so is the odometer, this may make your calculations off. I am getting 16 in the city and 17-18 on the highway. I would recalibrate the spedo and see if there is a big change. There is also more drag from these tires and the 4" lift exposes more of the undercarriage so your wind resistance is up. Think of this also...if you took two tires, one a 35 and the other a 265/75/16 and rolled them the bigger tire would travel farther to make one rotation, your current spedo gear thinks it is a 265 (stock) and is all messed up from this. I bet at 70 you are off by 10 mph on the spedo.

I hear claims of fuel mileage all the time and I feel alot of the claims are like peoples boats speeds, exaggerated. I am not pointing fingers, I just have not ever achieved the kind of numbers others say. With that said, I have had 9 PSD Fords, and now have an 03 SWB CC 4X4 GMC. Most trucks and Excursions were lifted with 37" tires. The GMC is lifted 8" and runs 37" tires. My mileage towing my Eliminator(30 foot) is around 10mpg at 75 mph in the GMC. I run either the Edge box or Quadzilla box in the GMC, set at either 60 or 80 hp and the mileage is about the same. My Excursion, with 37" tires, stacked Superchip programmer and ATS 3 stage chip gets roughly the same. These are real figures, taken from known trip mileage, not vehicle computer or vehicle odometer. My experience with empty mpg. varies so much, depending on which setting of the chip, or programmer, how heavy my foot gets, I have a hard time tracking it. I would say the average would be around 15-17mpg. My experience with the diesels has not rewarded me with the high mileage that many others claim, but I wouldn't trade because they run sooooooo good.

Sounds right to me!!
 

I have a lifted 2000 PSD Excursion 35" tires, built in (behind the bumper) 12,000 lb winch, push bar, etc. (all kinds of wind resistance). 10-11 towing, 14-15 highway.

I have a 2000 f-250 7.3 did the chip, k&n filter,and the exhust 13 -15 city or highway. I also have 35" tires.

And I thought I was the only one not getting 20 mpg! I just bought a 2002 F 250 with 11k miles on it. I have done the mileage at different speeds all on flat interstate.

55 mph=24.5 mpg
60 mph=22.5 mpg
65 mph=18.5mpg
70 mph=16.9 mpg
75 mph=14.5 mpg

all these were with an empty truck

Two weeks ago, I pulled a 20 foot Donzi from Miami to Clearwater, driving 60 to 65 all the way on interstate and got around 13.5 mpg. I'm going to hate seeing what it is hauling my heavy azz 302 Formula.

A question, if you change the chip settings, will it show up on diagnostics if you change it back?

Change your air filter. I have a duramax & the more miles I get on it the better the mileage. When truck had 30k on was getting about 18, now at 70K it is up to 19. These are corrected numbers, I have bigger tires also & speedo is 7% off.

New Air Filter, New Fuel Filter, Drained The water separator.

Pull the boat this weekend, 8.2mpg

Drove on the highway, no traffic, 55mph, 14.5 with cruise on.

I dont know how some people are getting 19 and some 14. maybe its the oversized tires.

Also---My trailer was making noise this weekend. Checked the bearings, and they are really bad. I jacked up a couple of wheels and could move the tire 1/2" in and out, so the bearings are hosed.

Could this be why its pulling so hard, or do the tires bog it down that much?

I have a 2001- 7.3 PSD, F-350, DRW, 4x4, Crew Cab with 4.10 gears. This truck is box stock. Best mileage I’ve ever gotten is 15 in the NC Mountains. I get 12 mpg on the highway at 75 to 80 mph and 12 around town. With a 41’ (#16,500) or a 36’ (#10,500) in back I get 9. These are real numbers in level So. Florida, not overhead computer numbers.

I agree you need to check to see if your speedo is calibrated. Also the overhead computer is good to compare different conditions but not to get real numbers. My Dodge's computer was consistently off by 1 mpg. You may be getting better than you think.