I apologize in advance if this is a repost. I recently swapped wheels and tires from moto rims and 315/70/18 cepek fcII to xd rims with 295/70/18 trail grapplers. The day I swapped I took it to my hometown on a 250 mile roadtrip. On the way I lost my rear wheel speed/abs sensor (strange coincidence I hope). Since I swapped these tires I've dropped from 18-18.5 mpg to a now terrible 14 on back to back 500 mile round trips. I know it's still got the factory 3.73's. No mods have been done to this truck other than cosmetic and a magnaflow 4" turbo back. This is killing me and I just can't handle the mpg's. That's hand calculated and no speedo recalibration but it wasn't with the old tires either and like I said I got a consistent 18 mpg. It doesn't seem to be down on power or smoke excessively either. Any help would be greatly appreciated, like I said I'm dying here. Derek

 

When is the last time you changed the fuel filter? Do you have a gauge monitoring your fuel pressure, maybe your pump is going bad.

 

Just changed the fuel filter last monday and I don't have fuel pressure gauge. I thought I remembered reading somewhere that the IAT or MAP sensors could cause similiar symptoms when they got dirty but that might have been the 6.7 thread. I'm about to send this POS down the river with only 80,000 miles. The couple years I've had it have been, well let's say rocky at best. My 99 never gave me a fit unless it was self induced This one is turning into drive it once work on it once. Anyway, enough of my rant, just looking for some ideas on making this better again. Thanks again, Derek

 

I also wanna know the problem too. My truck is having a similar problem too.

 

Buy my Smarty and then see! Rolling resistance on those tirs are horrible! Any mud type tire will eat fuel up!
USMC0311 I THINK THAT COW BELL HAS SOMETHING TO DO WITH IT!

 

You know I figured I would lose a little economy with the M/T's but 4 mpg??? Like I said I actually went down in size from the Cepek's and shaved about 8 lbs total weight per corner. I just would never have imagined a 25% loss with the trail grapplers. If it's all truly related to just tires I'll just deal with it because these things are monsters offroad. Took it on two hunting trips now and there isn't much they haven't eaten up and smiled afterwards yet. Haven't had them in the snow but this weekend should be good. Going back for my last trip and it's supposed to dump some powder this week. They're just as quiet as my bfg A/T's and Cepek FCII's on the highway at 70mph. If they wear well, I'll just choke down the mileage and deal. Thanks for the replies, Derek

 

Oversize tires will make for an rpm drop, so you can get a little more speed out of overdrive (be like swapping the 3.73's for 3.42's). BUT, your also making more resistance as mentioned already. Need to think of adding a Smarty JR to help regain the mileage.