what would be the impact of putting a 3.55 lim-slip rear end from a 5.4 into my 4.2l with the 3.08 diff... just wondering if my fuel economy... what little i have will go out the window, or what? help would be much appreciated

 

your will lose the majority of your mpg on the highway, because you will be cruising at a higher RPM than you do now.

city may see a slight increase due to you getting up to speed quicker, with less effort, but it's really debatable on how much you will see.

But get NEW gears, not old. Less room for failure.

Why are you wanting to swap to a 3.55 LS, if I may ask?

 

wellll the reason i want to swap them is because i want the LS, instead of just the standard in my 2wd 98 f150.

 

ok. that's understandable.

but as far as the parts to use, i'd look for a new set of 3.55 LS instead of used.

 

well at my price range... u can see my thoughts ? lol :P the truck was kept in amazing shape, how much of an effect do you think it would make on my mpg

 

I would think little to know change in mpg. My truck (also a 4.2 v6) came stock with the H9 3.55LS. It will be a nice upgrade for you. Keep in mind the 5.4 came with a 9.75 rear, where our trucks come with the 8.8. Dunno if that will cause any issues when you go to bolt it in.

 

mine has the 3.08 rear in it right now and its a 98 xlt stx, there is an 02 that i took the whole front end off of at the local wreckers with the fogs and newer style front end ( love it) and i found out it has the H9 3.55 LS in it... do you think that the 02 3.55 would fit in where my 3.08 is in my 98?

 

You will likely have to get your drive shaft shortened. The 5.4 got a 9.75 rear end (Twelve bolts hold the cover on) and 4.6 and 4.2 got an 8.8 (ten bolts hold the cover on). The 9.75 is longer front to back than the 8.8 meaning the drive shaft length is slightly shorter for the 9.75

 

http://cgi.ebay.ca/Traction-loc-31-s...item4156cc7655 so if i buy this... it will fit right into my factory 3.08 rear end?

 

it should i think, but let someone else chime in who knows a little more about the number of slpines are in what axel. i know my old rear end had a posi unit in it ant the bigest mistake i made was not removing it before i junked that axel... i could have put it on a shelf and then had it installed in my new axel. you will absolutely LOVE having posi. i know i did... it made for some wicked burnouts and really helped when i needed to put power down when going around a wet turn.

 

You will not lose any real world fuel economy with that swap.

 

haha very true... ive always known these trucks whether its a 4.2 v6 to a 5.4 v8 they are very peppy trucks. touchem and they r goneee lol... i know i used to do burnouts no prob in this one... its a 4.2 and i cant wait to get a posi i know i will love it! ahah

 

what is your price range? if youre going to swap gears, i would go with 373's or either 4.10's

 

i may just leave the 3.08 in it and get a posi for that

 

i have 3.08 gears... my rearend is an 8.8, my friend has a carrier with 2.73 gears in it that he is giving me for free... then when he takes the 3.31 gears out of his mustang... he is giving me those also... which would be better for the truck? i want decent gas mileage... he said that the 3.31 is hot off the line but is also good highway and city on mileage.