My gas mileage has been very erratic, and a lot lower than I expected with this truck. Its a 2001 Quad Cab Long Bed 4x4 with a 6 speed. Its got 4.10 gears and stock size tires (245/75R16 which is 30.5" tall IIRC). I utilize the old cave-man method of topping it off, and then recording mileage and gallons and dividing. Oh, and before someone points it out, I always make sure to let the foam dissipate so I make sure its actually topped off.

This is what Ive got so far since I bought the truck:
15.9mpg on first tank (50/50 highway and city, truck had 4" exhaust)
16.8mpg next 2 tanks normal daily use (about 25/75 highway and city, started using additive (4oz FPPF and 8oz 2 Stroke oil)
17.8mpg driving to Maryland doing 85-90mph the whole way and about 50 miles of city driving in Maryland
14.8mpg returning from Maryland (let the girlfriend drive, I blame her for this one)
15.8mpg normal use (installed AEM intake and M3 chip halfway through this tank, also was when I made a short trip of 30 miles with the M3 disconnected and 4200lbs of concrete for half that)
15.6mpg normal use, different fuel additive (8oz PowerService DieselKleen and 8oz 2 Stroke oil)
16.4mpg about 100 miles of city use, then driving to Maryland doing 75-80mph the whole way
15.7mpg about 50 miles driving around Maryland, then doing 75-80mph the whole way on the return trip

I really just don't get it. I will admit that I have somewhat of a lead foot, but I'm generally a consistent driver. I mean, I like to get into it some, but I'm not redlining it in all my gears and trying to race with it. I also thought that mods like the Exhaust, Intake and M3 chip were supposed to help my gas mileage. I have seen basically no improvement! My roommate has a 1999 2500 2wd auto that is bone stock and averages pretty much 20mpg all the time, and he drives it like a wild man. Hes always doing burnouts, drifting, launches it from every stop sign and traffic light and does plenty of other stupid stuff. I know my 4wd is a bit taller and heavier, but does it make that much of a difference? We both have 4.10 rears and stock size tires, but shouldn't my 6spd be a little more efficient? I'm beginning to think something is wrong with my truck. Any advice?

 

I notice a big difference on how the truck is parked during refuel
( nose low vs nose high ) Nose low seems to get more fuel in tank..

For local driving, I fill at the same pump same spot ( left front tire against
cement pad at fuel pumps. Milage stays consistent 20-21 with very
carefull driving...High of 22-23 at 55 mph constant, no Cruise.no AC

Also , Northern fuel ( summer blend, 45 cetane ) seems to get about
1.2 to 2 MPG better than 40 cetane southern fuel
And ,Northern fuel is now winter blend , Less MPG

SO: lot's of variables to consider

 

My truck has the 4.10's too and your mileage actually looks better than mine has. I noticed that you said on the highway you are running 75-80 mph so I have to think that your RPM are really high and that is driving your mpg way down. Try driving at 2000 rpm or less and see how it changes and I bet you will be pleasantly suprised.

I am actually swapping my 4.10's for 3.54's this weekend in an effort to squeeze more mpg out of this truck. My average over the 8000 miles that I have owned it have been just over 15 - not what I was expecting at all. I am hoping for 18+ but I guess I will have to see what happens.

 

If I keep my RPMs at 2000, Im barely doing 60mph. Im looking at doing the Flex-lite electric fans, which is supposed to make a huge difference. I also want to go to 35" street tires (i.e. some 265/70R20's) on newer rims. I did the math and that is basically the equivalent of switching to 3.73 gears and would drop my cruising RPMs at 60mph to about 1650rpms.

Im also going to disconnect the batteries, and let it sit for a while, then reconnect them to reset the factory ECU. It never mentioned that in the M3 instructions, so I never did it. I wonder if that would help?