i have a 97 f150 4.6 4x4 that had 3.55 gears and the rear end went out so i swapped it for one that i got out of a 97 f150 2wd with the 4.6 and now my spedometer is off 15 mph slower than im actually going i think the new rear end has 3.08 gears if someone can tell me if this is the fact then i will swap pinions if possible.

 

You didn't check to see what gearing was in the 2wd rearend before buying it?

There should be a tag on the rearend telling you what gearing.

 

\/ What he said.

 

how the hell do you buy a rear end for a 4wd without checking the gear ratio? good thing it wasnt a front because youd have put it in 4 one day and poof **** woulda hit the fan and youd b buying another one. a new ring and pinion set is not gonna run a good 2 or 300 bucks

 

if everything runs fine, why can't you calibrate the speedo?

 

The pcm uses the information from the trans for more than just the vehicle's speedometer. The rear gears are wrong, change them.

 

its 4wd. you cant have your wheels moving at two different speeds. if you were to have it in 4 catch total traction with both front and rear kiss your t case good bye

 

I actually seen this happen on a dodge dakota.

To the OP, you can also check the code on the door to see if it is similar. Doesn't matter if it is open diff or LS, just so the gears are correct. They have to be the same is the front and rear, no exceptions. I wouldn't go thru the work of changing gears, me personally, since I have changed the whole rearend before, i would just change the whole damn thing out. Labor and gears will cost a bundle, swapping your DIFF case and all will be cheaper.

 

He would have to check the door of the truck he took the rearend out. He needs to read the tag on the rearend dif to see what gear ratio it is..

 

BTW if you enjoy driving your truck at all DO NOT USE 4 wheel drive till this issue is taken care of.