Rear axle ratio
 

1 own at 2010 ford f150 and installed 33'' tires and lost about 3-4 mpg.....any one know what axle ratio i need to run to compinsate for the tire size...oem rear axle gear is
355 oem tire size were 265/60/r18 new tire size is 275/70r18
all i am looking for is to gain back the mpg i lost thanks...

Just a thought, but when you drive are you still using the speedo as it reads to judge your speed? When you put larger tires on, you need to drive so the speedo reads slower that the speed you want to go. If you are still driving using the speedo you are pobably going about 5-10mph faster than you think, which means you are using fuel faster, which means poorer gas milage. Also, your odometer isn't reading true with the change either, so using it at face value is going to give you a lower mpg.

Your new tires are 8.1% larger in diameter. This means that if you use your odometer, you have to add 8.1% to the milage you traveled before you divide by the gallons used.

Typically, with putting on a larger tire and adjusting your speed that you travel on your speedo (in your case it should read 8.1% lower, so if you want to go 65mph you need to travel with your speedo reading 59-60mph) you will be running at a lower rpm and therefore use less fuel. So your mpg should have actually increased with the change.

Ford Dealers have a tool that can re-calibrate the PCM to the new tire size. Either that or go back to the original size.