[Jackpine]

Originally Posted by cody994x4

unleashedford, im not sure abt mpg's but i know if you were to re-gear you would have alot of restored power. the minimum reccommendation, as far as ive ever heard is at least 3.73's with 35's, 4.10 or 4.56 would be better. you will be able to get going alot easier and put your rpms back around the powerband range. im curious to what your rpms are at say 70mph with the 35ers

interesting how this plays into the mpg equation though, your tires are lowering your rpms thus saving gas...

for economy you might wanna stay where you are but im pretty sure your asking alot out of a smaller ring/pinion.

you might wanna research restored power vs economy if you were to re-gear.

I've "bolded" the part of your response that is most significant. The lower RPM's CAN save gas, but you've got to get there from a standing start. In his case, the bigger feet make his engine work a lot harder getting it up to speed, due to several causes:

1. A reduced "mechanical advantage" (lower effective gear ratio) due to the larger radius tires.
2. More mass to get spinning from a standstill - Newton's third law still applies.
3. More wind resistance (a minor effect, but still there).

Once at highway speeds, you're still not home free because:

1. The bigger feet weigh more and have more wind resistance - so, more fuel needed to keep them rolling.
2. They probably have more "rolling resistance" (skin friction effect with the road) so it takes more power to keep them moving.
3. Even though the engine RPMs are lower, the torque output of the engine has to be higher due to the reduced mechanical advantage. That torque has to come from somewhere, and it can only be additional fuel per cylinder charge.

So, the whole concept of power vs economy is a very complicated tradeoff. Ford's engineers actually tried to get a good compromise at the plant. It CAN be improved, but I suspect it can much more easily be made worse.

- Jack