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.

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.