I have an '03 FX4 SCrew w/ 3.73s. I'm thinking of moving from the stock 17s w/ 265s to 16s with 305s. I was wondering how much mpg and mph will be affected with this change. Anyone care to throw some insight? Thanks.

 

305/65R16 would be very close to the same size. 305/70 would be 1.2" in diameter larger, or 3.8% difference in your speedometer/odometer. So, if it says you are gong 50mph, you are actually going 52mph, in theory.

 

I'm running 305/70R16 on my 2002 FX4. Like APT said mph is very small you can't notice it or you can reprogram the computer for the correct tire size. Anytime you install the widder tire you will suffer a little on mpg. I'll say about 1mpg

 

The tires are very close in height so you won't see much difference. The stock 265/70-17's are essentially a 32x10.50-17 and the 305/70-16's are essentially a 33x12.50-16. I went from a stock 30 inch tire (255/70-16) to a 33 inch tire (285/75-16) and it threw my speedo off by 5 mph.

 

Thanks! I read somewhere that chaning the size more than 3% or so affects the ABS. I didn't get all the info regarding that..anyone heard of this or know anything about it?

 

A small change in tire size is not going to effect the ABS. There are a ton of people here and at www.fordtruckworld.com who are running 35's and 37's without any problems.