Tire size/Injectors/Fuel mileage??

[Zapper]

I'm stumped, I own a 02 CC short bed with about 45,000 miles. My in-town mileage has consistantly been 17-18mpg, with highway around 18-20mpg (unloaded). About 3 months ago I put a set of 255/85/16 Cooper Discover LT tires on and really like them, but it seems almost immediately my mileage dropped roughly 3 mpg. To put another way I would always get 375-400 miles in between fill-ups, now I'm lucky to get 330 and this number seems to be dropping weekly. My delima is I would have thought the 1 1/2" added hieghth of the new tires over stock would have increased my mileage or at least not lowered it. I have not noticed any of the symptoms GM mentioned in the SB on the injectors, but after driving a friends '03 I get the feeling my truck is way down on power and am wondering if it has just slowly dropped power without me realizing it. It really seems to drop off at higher RPMs and I do occasionally get a strong smell of fuel, but have never been able to find a reason. I put a new fuel filter on about 2000 miles ago and also replaced the air filter, also made sure the sticker covering the hole in the fender well had not come off. I guess my question is could I have an injector issue without any of the symptoms mentioned in the SB? I am really having a tough time believing it is tire size related, but could it be? Sorry about rambling but I had a hard time constructing the best way to ask these questions. Thanks for any input.

[Jim Brzozowski]

According to my calculations,245 x .75 vs 255 x .85 turns out to be a 15 percent change in tire height from the rim increase. That means you are traveling approximately the same distance, but with the taller tire your odometer is telling you its less. Less miles divided by same number of gallons looks like you are getting worse fuel mileage, when you may not actually be. You are recording roughly the same ratio of miles as the change in the tires. I changed from stock tires on my old dully to 85 series tire and it did the same thing, but I didn't buy any more gas per trip than I did before. It took a while to figure out.

[More Power]

To check for fuel in oil, do as others have suggested here in this BB. Place a drop of engine oil off the dip stick on a clean white paper towel. Fuel in oil will produce two concentric rings - a darker inner ring and a more clear outer ring. If there's no fuel in the oil, you'll have just one ring.

Fuel mileage.... Get yourself a programmer (http://www.thedieselpage.com/reviews/programmers.htm) to correct for tire size.

MP

[DarylB]

I went with 255/85/16's too, according to one computer I used, I came up with an 8.53% difference in stock vs the new size. Just figure up the % of difference between stock vs you're new tire height then calculte. Add that increased percentage to your mileage and you should be pretty close. My mileage seemed to drop about 1mpg, but I did go from the stock tread to a mud grip.