Why sudden bad mileage?

[peacekeeper]

I just moved up from my 01 Burban and got a 07 GMC Yukon XL 2500 w/6.0, I drove it from Huntsville AL to Chicago and was getting about 14-15 MPG, since getting it home, my mileage has dropped to 7-8 MPG. The only changes have been tire & rim upgrade (17" rims & BFG all terrains) and a brake controller for my camper. Why has the mileage dropped sooo badly???? Any suggestions on what is causing it??? I am being VERY easy on the pedal to see if the MPGs will increase, but no luck...HELP!

[dsfloyd]

Are the tires the same size. Speedo could be off but that still seems like a large drop.

[peacekeeper]

All tires are the same size, I dont know if I got a bad tank of gas or what????

[dsfloyd]

bad gas is possible but you would be running rough too. is the 7-8 all city. I only get 12-14 highway and about 8-9 in city, and I have the 6.0. your highway mileage should be more than mine but not surprised if city is around the same.

[Mean_Green_95]

If you switched from a ht tire to an at tire then thats going to be one of your biggest culprits.

[kevyn403]

I have a 08 silverado and I just put on 285/70/17 nitto terra grapplers and the first tank I burned with them on I got about the same gas mileage, but with the past 2 tanks I've been getting almost 90 kms a tank less is this normal.

[vncj96]

Going from a HT to a AT tire should be very minimal MPG drop, maybe a mile per gallon but surely not 5-7

[MWright936]

I would say run another tank of gas through it and see if it does the same thing. That would rule out bad gas. If it's running fine, it certainly seems like a weird problem...

[kjb13]

I just had a big drop in mileage and also just noticed my temp gauge is way low, about 150 to 160 degrees. Don't know if it's related or not. I'll be putting in a new thermostat this week to see if that fixes both problems. I read somewhere that if the engine is running too cool, the computer will dump extra gas just like it does when first started. I'll report back.

[Mean_Green_95]

You thermostat might be stuck open, which wouldn't allow your engine to run at optimal temp. That might cause a decrease, along with the change in tires. Run some GM TEC in the engine or some Seafoam to clean out any carbon buildup. Carbon build will rob you of gas big time.