how can i tell what rearend i have?

 

what kind of truck, year etc.....

 

its a 2004 ford f 150 supercrew 4x4 lariat.

 

4.6 = 8.8
5.4 = 9.75

Phil

 

Im sorry i wanted to find out what gear i have 3.55 or what ever.it has the 5.4

 

on the rear end there is a tag. it will say 3 55 or 4 10 or 3L55 for a locking rear etc....

 

thanks it said 3L73 is that good with 315/70r17

 

You have 3.73 gears with a limited slip.
Should be fine with your tires depending on your usage. You'll lose some power (towing would be affected a bit for sure) but lots of people run 35's on 3.73's, and some on 3.55's.
If you really pull a lot of weight around or you are concerned more about losing power, go with a 285 or a 33x12.50r17.

I always plan to re-gear when I buy new tires though. You can get away without, but it is so nice to get the engine back in the correct RPM range and gain the power back, and then some. Makes me feel better about the automatic transmission too. It's spendy though, around $1100-1500 depending on the shop. I'm putting 35's on with 3.73's but I will eventually re-gear when my wallet permits.