I had a set of 410s, and changed to 342s.

Cost out the door-@2000$(4X4). Your CPM needs to be reprogrammed to recalibrate your speedometer, and reset the ABS sensors.

It was worth it to me, because I do a lot of highway miles, and don't carry a lot of extreme loads.

Doubles my around-town mileage,(calculated by only having to go to the gas station half as often, and therefore spend half as much on gas) and added 3 MPG to my highway mileage.

So, if(like me) you don't have very heavy loads to haul or tow, and you do a lot of highway driving, it may be worth it for you.

Bear in mind I also drive with a very light foot on the gas pedal.

 

I have a 04 2500 6.0 running 35's. Everything is stock as far as I can tell (except the 6" of lift). I am getting around 10-11 MPG. I'm thinking. Just going of the display. I just got the truck and don't know what the previous owner did to her. With the truck running 35" tires does the computer have to reprogramed for a larger tire to set the MPH correctly? Also would this affect the MPG reading if the MPH and everythiing else is incorrect? Also I was thinking of adding a K&N and 40 series dual in dual out Flowmasters. Will this help at all in MPG gain? I know more air in and more air out helps with performance but not sure with MPG. Also if I do the mods do I need to get a tuner?

 

When making modifications such as lifts tires and gearing, a PCM tune will help in terms of speedo ABS and such. now, to really mes with your fuel mileage, you gotta get into a very very deep tuning software to mess with the fuels and timing curve. IMO. these trucks beig the fuel thirsty beasts as they are, any gain will be somewhat noticable but very well appreciated.

So at this point, id say that you have many options available, choose your path and let us know what you find so we can use it to help others in the future.

Hope to hear from you soon!

Eric.

 

Yeah, the truck needs to be reprogrammed when you step up to a different diameter tire from stock. The best way to find out if the previous owner did so is to get a handheld GPS and compare your speedo to your actual speed. If he didn't reprogram the computer the true speed of the truck (from the GPS) will be 5-10 mph faster than your speedo. This is because the distance your wheels are moving is farther than your computer thinks (because your tire diameter is larger than the computer thinks). I would bet that your truck was adjusted though, because I lifted my '06 with 410 gears and added 35" tires and after adjusting my computer I get the same 10-11 mpg as you. If your truck hasn't been adjusted your mpg would probably be a lot worst.

 

Sorry to say, but with all that lift and the larger tires, you need the 410 gears, at least.

BassinProf is right. I got a GPS to calculate my speed after my gear change, and they're surprisingly accurate, once you adjust to the signal transmission time. What a programmer can do for you is properly adjust your tire size in your truck's CPM, which should give you your speedometer accuracy back. Or you could pay a dealer or shop to do the same thing.

All things considered, that seems like pretty good MPG to me.