I have a 89 f150 4x4 with a i6 and it has 31'' tires. I have 3.18 gears in it right now and I am thinking about switching over to 3.55s. I get 13mpg and sometimes 14mpg on a good day. How would switching the gears affect my gas mileage? What about the power? How would it affect the truck going 65 or 70 down the highway?

 

What transmission does the truck have?

 

All I know its a standard.

 

All I know its a standard

 

With overdrive or not?

 

Yes. Its a 5 speed

 

It will barely affect gas mileage at all. You might see an extra 1 or maybe 2 MPGs out of it. Power will be better, and RPMs going down the highway will be higher but not unreasonable. If you have the gears laying around and can do it for cheap then go for it, but I wouldn't spend any real money on it. Don't forget you have to change front and rear to match.

 

+1.. 3.55's are right in the sweet spot for that engine and tire size and will put 70mph cruising rpms at about 2000rpm. The reason this should improve milage is because it won't take as much throttle to maintain speed... lighter engine load equals less fuel injected, and because the increased mechanical advantage will allow the computer to stay in closed loop more. BTW.. your truck should now have 3.08 gears, there was no 3.18 option.

 

Ok thank you

 

Ok cool thank you. Yea I thought I put 3.08 but I was in a hurry this morning.

 

Thats a good gear choice for your setup.