hey guys, i recently got some new tires for my duramax. I had the stock, 245/75/16's on it and decided to go bigger. So I put some 285/75/16 Mickey Thompson MTZ's mud tires. OK, I know that I not only went bigger but with an aggressive tread pattern compared to stock. Now my truck gets like 15-16 mpg highway. It's a big truck so its never going to get great fuel economy I understand. Where did I go wrong with this upgrade? Was it the bigger size or the aggressive tread pattern that killed my fuel economy? Or both? If I go with a 265/75/16 on a regular highway tire will this fix my milage problem somewhat? Any suggestions fellas?

thanks, chris

I am sure that the aggressive tread is causing you the most loss in MPG. My hunting buddy put a 4'' lift and Pro Comp mud tires on his DMAX, I think they are 265 70 16 size. He dropped almost 4 MPG. Then put his all terain tires back on (due to a faulty tire) and his mileage came back up, and his truck rides and handles MUCH better.

One thing to remember is that if you didn't recalibrate your speedo, then you are actually going more miles than your speedo says, so if you are figuring your mileage from that, you are shorting yourself a little. However, I don't think by that much. A mud tire is not only very course in tread, but much heavier in weight than the stock tires. When you hang unsprung weight out on the ends of the axles, it takes more power to turn them and to stop them. Also, when you have a huge isolated lugs that run against the the direction of travel, you are going to get serious resistance to forward motion. You should be able to run the larger tire in a smoother tread like the stock tires, and should get better fuel economy. The weight issue will still hurt you, but the coarseness of the tread won't. don't forget the speedo change.

That makes sense. I did re-callibrate the speedometer. I like having the biger tires but the economy is starting to become an issue. So you believe thats its the tread thats causing the issue and not the jump in tire size?

I can easily get 18-19mpg (empty and highway 60mph)if I behave with my H2 takeoffs. If I drive in town a lot my mileage drops like a rock. I generally get in the 17-18 range. My improvement came after a trip to Kennedy diesel for his EFILIVE custom tunes (80 hp tune is the best in my truck for mpg). I had the edge/attitude and boost stick prior and got about 15-16 mpg empty on the highway @ 60mph.

I have tires and rims that weigh in at around 300 lbs a peice which if I knew that probably would of made me not buy them but they are also alot larger then stock 255/70/22.5 (a little over 36" tall) but they have a high way tread for semi's my milage in the city dropped quite a bit but if I take it easy it's not to bad but the milage on the free way stayed the same. it lowered my rpms at 70 mph I'm only running at about 1750 rpms so I run my engine slower and go farther. just went on 3000 mile trip and averaged whole trip city and freeway (90% freeway) 18 - 20. doing another 3000 mile trip in two weeks and I see how it does then to.

I'm thinking of going to a 265 A/T tire because I now have a much longer commute to work (about 100 miles a day).

Rotational weight is a big issue not only for economy but handling and speed. The lighter the rim and tires, less energy is required to rotate or steer it.

Yea, i'd definately agree with you there. I've noticed a diffrence in ride quality and handling since i've upgraded to these tires. I believe that i might go a bit smaller (265) and a less aggressive tread pattern. It sounds like that could solve my problems.

Each ounce of weight reduction on the rim is equal to about 24 pounds of weight at 100MPH! The further that weight is from from the center the more its effect.

Correct me if I am wrong please, but wouldn't keeping the overall wheel diameter the same but increasing the size of a rim from say 16 to an 18 and going with a say 65 series tire reduce overall weight and rotational weight as well? Also assuming that a lighter rim is used.