Recently I switched to 33x10.5r15 and I seem to be getting better gas mielage than before when I had 235x75r15 which is technically a 29. Is this possible? Also I seem to coast better as well.

 

nope chances are your numbers are off since your speedometer and odometer now read less mileage than you actually travel, get them calibrated properly and then see what you think

 

how do you calibrate the speedo for tire size changes?

 

If it's reading lower rotating miles then wouldn't it show less mpg??

 

+1 it would, My speedo is 10 mph off no matter what speed I am going so I have no clue what my actual mpg would be because of this...But are you going by hand or by the overhead computer?

 

im not sure which way it will affect the mileage calculation and its too late for me to think it through, but you can readjust the speedo by either buying a program (hypertech or superchips, i have heard that one of them can do it, but i dont remember which one) or i would imagine you could pay the stealer to do it for you also.

 

My speedo doesn't work so my odo doesn't either. I'm just going off what I'm used to paying for gas

 

Not sure how it can be 10mph off at any speed...most likely it's a certain percent off (15%, whatever) at any speed, but you're noticing the 10 mph difference because that's how far it's off at the speed you normally drive.

My work truck reads 20% low at any speed. That means if my speedo says I'm doing 45mph, I'm actually going 54. If it reads 60, I'm going 72 mph. Shows 30, I'm going 36. Yours probably does much the same. If you can figure out exactly how far off it reads it'll be easier to deal with.

 

No brhodes, one of my speed sensors is off, so my dash isn't calibrated right...so if I am going 25 mph my dash says I am going 15, if I am going 35 my dash says 25...If I am going 60, its at 50 etc....

 

I see, so you have the right progression, just the wrong starting point. Makes sense, with an electronically controlled system. I was thinking in terms of older vehicles, I guess.