Thinking about upgrading to a 285/70/17 from a 265 on my stock 06 megacab. I do not have a leveling kit and still running stock 8 lug wheels. First will 285's rub? Read bunch of posts in search but all seem to be 4x4 second my hwy mileage is about 15.5 at 75 will it help or hurt mileage? and third How far will my speedo be off?

Thanks,
Mike

 

Don't know how you're determining 15.5 at 75, but it isn't likely. The Mega is actually a 2500, so it probably has 3.73 gears. 285s will not likely rub, nor will they seriously affect mileage in either direction. The increase in diameter is only about an inch. The speedo/odo won't be too far off. Revs/mile won't decrease much.

My 285s produced a 4.3% error (low) when new. Now the error is about 4.2%. The mileage at 75-80 is a bit over 14. Best ever was 16.6 at 55ish (two-lane road, light traffic)

Your GPS device will help you determine the exact error. It'll likely be about the same (4.3% low). When I fill up, I add 4.2% to miles driven and do the math. I DON'T rely on the computer. They're notoriously optimistic.

 

15.5 was actually at 72 went back and looked, was running back and forth from AL to NC and figured mileage off gal / miles driven. Comp seems pretty close in my truck at least telling me when empty is LOL wife has been at 0 a couple times in it but made it to station LOLOL My GPS is on my phone so not sure it tells mph? Now towing anything mine drops to 12 or less

 

When I bought my '07 Mega Cab 2500 diesel, one of the 3 trucks I test drove was a half ton (1500) mega cab.

OP- you should be fine with the tires, however they will have an impact, it may be slight.

This gearing calculator is a great one to figure changes with since you can input tire sizes etc
http://www.grimmjeeper.com/gears.html