So i have 265/70r17 gt2 tires with the oem 3.55 gears and as you can imagine going from a 245 hyw tread tire to the 265 off road tire with the same gears my gas mpg has dropped about 3 mpg cruising. so since i leveled out my truck i want to go to 265/70r18 a 33 in tire. i know i have to go with a 3.91 ratio at a minimum to best match up the oem rpm while cruising, but since the tires have a bigger fulcrom would i get a better chance with more mds activation time if i use 4.11's? what do you think? thanks guys

 

get the 4.10 and call it a day..... Did you reprogram your speedo when you changed tire sizes? If not that might be why you're seeing bad fuel economy b/c the trip meter might not be correct.

 

If you're trying to save money by spending it on new gears, you better be driving a lot of miles. BTW, slightly larger tires won't affect your mileage that much in either direction.

 

The 245 and 265 numbers do not denote tire height, but how many mm's wide the tire is, and if the tire height is the same, should not affect mpg's. Since you are running 70's now were you running something lower numerically before you changed tires, such as 55 or 45. If so then you would see a decrease in mileage, because your truck now thinks it has a higher numerically axle ratio, which is lugging your truck. With 33 inch tires 3.92 is what the factory should have installed. My tires are 275-60-20 and are 32.5 inches tall and my truck came with 3.92's, which seem to be a perfect match. Good luck

 

31.6 tall tires, what gears? 3.55's suck.

 

Well if you drove the stock 245's and 3.55 hemi combo, the gearing isn't that bad for highway driving if running empty. Did it for a few years and had no complaints and had great mileage. That tire's only 30.5" so if going to 33", you'd want at least 3.92 to be close to matching your original final drive ratio. Now with it off the ground a bit more and the bigger / agressive tires, I would go straight to 4.1. It would likely end up having the same performance you had orginally and have a chance of keeping the MDS on longer. Your new combo will always cost you at least a few mpg with any gearing you go with.