Originally Posted by 4x4sporty
I am starting a 4x4 conversion on a ford van and I am going from the stock 245/75/16 to 285/75/17 tires do I need to go to 4.10 ratio to keep it about same rpm as it is with the 3.73 ratio it has in it now. It has a 5.4 engine so even a little lower wouldn't hurt my feelings
Standard 245/75R16 are 30.47 inches nominal in diameter.
Proposed 285/75R17 are 33.83 inches nominal in diameter.
To keep same RPM, 3.73 X 33.83/30.47 = 4.14
So it appears 4.10 gears are closes. If you want to get a little lower then 4.30 should be available.
Note that if you don't change gears, the equivalent would be:
3.73 X 30.47/33.83 = 3.36
Ford does run some vans and pickups with 3.31 gears (going from memory), but for a 4X4 heavy van that seems on the high side to me. Still, if fuel economy is the number one priority as you've stated, then leaving it with 3.73 might give you better MPG results.