My 97 Expy now has 265-75-R16's on it up from the stock 255-70-R16's. I put a 1 tooth smaller gear in the VSS but it actually seems to be more sluggish. Did I go the wrong way in gear size? Was I supposed to put a bigger gear in? Thanks.

 

You need a smaller tooth gear when going up in size since you need to spin the gear faster to compensate. Since you're running 265s best thing I can say would be to stop at a Ford dealer. 265/70/17s where a factory size tire and is the same overall size as what you have. They will be able to tell you exactly what gear it takes to make it read right.

 

Thanks for the confirmation on that. Thats what I did and it was 1 tooth smaller. I guess the tire size maybe just made that much of a difference. It changed my 3.73's to 3.54's.

 

is that difficult to change the speedo gear? I'm not even sure where it is. (on a '97 F-150)

 

The VSS gear is a small gear clipped on the end of the transmission/transfer case (4x2/4x4) VSS sensor. The sensor is held in by 1 bolt. Pull that and the clip pops off the gear. Total time to change the gear is about 5 min, most of which is actually getting the tools out. The sensor is on the tailshaft on the drivers side. You will see a large wire going to it and is very easy to spot.

 

Thanks! sounds easy enough.
now to figure out what gear to change to. I've got 33" tires with 4.10 gears. the speedo is approximately 4%+/- faster than actual (I think) I'll check with the dealer to see if they have any type of 'table' to reference.

 

If I remember right 1 tooth change is good for 4 mph.

 

If your dealer is anything like the ones around here their tables only cover stock setups. Since the F150 was never offered with 4.10 gears or 33s then they might not be much help. I'm trying to remember the way to figure out what tooth you need after a gear and tire change, but its escaping me at the moment. Without the gears its a somewhat simple calculation, but right now I just can't recall how to work the gear ratio in there.