So here's the deal. Your truck is a four wheel drive, and it weighs in at over 5500lbs, plus you said it has bigger tires. Lastly, it is almost 10 years old (fuel economy was not as much of a concern 10 years ago as it is today). So ya, 13 mpg is about the norm. That's not to say that you might not be able to squeeze another 2-3 mpg out of it, but that would probably be about it. But, there will be cost to do so, and is 15 mpg compared to 13 mpg really going to make that much of a difference to you if money is really an issue? Probably not. So you probably need to consider selling it for something else that gets better mileage, or live with what you got (it isn't really going to get any better then that). Sorry.

 

what silver said...

my truck gets about 14, but i'm looking forward to warm weather testing with the fixed plenum and new tires (higher pressure, 65 compared to 44) and seeing what my mileage does

 

It's all about the math. seriously. you need to figure out the best final drive ratio for the truck. Ive only got 2wd (a whole lot less weight) but ive got a quad cab and a tool box that had over 300lbs of tools in it. ive got dual exhaust and a CAI, 3.55 gears and 275/70/17 tires. 1998 318 with 150k+ miles on it. i get around 22 highway (cruise control at 65 no faster and i draft behind semi's as much as possible) I get close to 16 in the city and 13 on bad gas (closer to 10% ethanol) i have a friend with a 1998 quad cab 318 70,000 miles 4X4 rear gears 3.26 stock tires. he was getting close to 10 in town. hasnt changed anything but the gas station he fills up at and he gets 14 now in town. same roads to and from work just different gas (lower ethanol content).the numercally lower final drive ratio the better mileage but less power and eventually you get too low and the truck has to work to hard to tur the tires. numerically higher ratio better low end but terrible mileage. other tips would be dont use the A/C and it gets better gas mileage anythign over 45 just crack the windows instead of all the way down ( less resistance more aerodynamic) get an electric fan and a different thermostat i dont remember if its hotter or cooler that gets better mileage. even with all of these things you probably won't get much more than 16-20 average while going easy on the gas pedal too, which is hard especially when you have duals.

 

You really want MPGs? Park it, either walk or buy a moped.

You're driving a truck whose motor's design is over 40 years old. It's in efficient and outdated.

The following might not have been a productive post, but it has been the truth.

 

Bud, don't worry about it.Sounds like you Dad's going to be buying the gas for it anyway.

 

myth busters busted the myth about ac using more gas....they discovered you lost just as much gas mileage with the window down due to added resistance... Where are all the guys getting 17-18 mpgs I know their are lots of them as they usually chime in on mpg threads... I get 13 with 35" mudders and 4.56 gears...on the highway

 

I got 18 mpg for 1 fillup on a trip from NY to NC with a 35 mph tail wind, lol.I check mine every fill up and average about 15 mpg hwy.I crunched some numbers and at an average of 14 mpg there has been about 31,857 gallons of fuel put in my truck.......I've got entirely to much time on my hands! LOL!!!

 

i think that the truck being 10 years old and the engine design being 40 years old have nothing to do with the fuel economy, even though MPG wasnt a concern then look at a sticker on a new ram, or any truck for that matter its 13/18, we took my in laws 09 sierra z71 to NC from NJ over the holidays and on a 800mi round trip, almost all highway, with it dropping for to a v4 at cruising speeds we still only got 15.5mpg

 

What mpg does GMC "claim" it gets on that model ?

 

EPA estimates are 21 for a 5.3. Same as the new Rams.