I have a 98 f150 4x2 xlt, with automatic transmission. I'm not sure what gear ratio I have. where can I look on my truck or in a manual to find out what gears I got. Which is better on gas 3.55 or 4.10? Do you have any other suggestions to boost my gas mileage? right now I average 15 mpg. I added a tonnoeau cover to cut wind resistence but that is it. Is switching gears expensive? how much to put a limited slip in the rear? I live in southern NH. Some help would be great.

 

The "Vehicle Certification Label" is on the drivers side door jam.

Rear axle ratio codes:
18 - 3.08
19 - 3.55
26 - 3.73
27 - 3.31
H9 - 3.55 Limited Slip
B6 - 3.73 Limited Slip

 

thanks for the info