me and my dad are building a 351w and are putting it into a 91 coupe. my question is should i put an 8.8" or just go ahead and get the 9" just to be safe. i have ot buy one either way because i only have a 7.5" in it now, so which should i go with. i will prolly be putting down about 300-320hp.
thanks,
todd

 

8.8!!!!!!

 

+1... 8.8" bolts up straight forward and will be plenty, IMO!

 

thanks, thats what i wanted to hear that will save me alot of money.

also do any of you know about what RPMs i would be running at 70mph if i had 4.10 gears in 5th gear. thanks

 

That depends on the tire size alot too.
but i would think about 3300 to 3500?
depending on what tire size you have might be less or more.
but you will be safe to drive it on the interstate for short runs.
i wouldnt go across country unless you have someone elses gas card though.
i used to drive mine to the track with 355,373, and 411., no problem with a 5 speed, and the track was 30 miles away.
actually with 355s it got better gas milleage, and it would pull out in 5th gear too, but with stock gears it wouldnt(with my old motor) pull out in 5th gear.
which i dont recomend anyone doing!!!!
but yeah i think you will be safe with 4,11s even for a daily driver, just take off in 2nd gear. cuzz you will get tired of the low 1st in it.
if its going to be a weekend warrior car, i would put 373s in it. thats an all around good gear, for a 5 speed.

 

depending oh hom much horse power u are going to make up grade to 31 or 33 spine axles too . I spent the extra cash and got the 33 splines

 

Here's a calculator. Just download it, works great!
Good luck
Indy

http://links.vintage-mustang.com/download/RatioMph.exe

 

thats a cool calculator.. fun to mess around with...
remember that the gear ratio also is equal to a 27" tire
if that helps you out any too.
say if you have 373 and a 27 " tire, your gear ratio will be 373
if you have a 26" the gear ratio will be lower, maybe a 3.90(just guessing at it.) thats just from the rearend, and tires,not counting the transmission.
if you have a 28" tire, you will have a higher gear, maybe..350
so if you put a tallerslick on your car, you usually have to lower the gear to make it equal to the gear ratio before you changed tires out.
im gonna go play around with that calculator some more... see what my car does with some pimped out 20-fo spinner wheels and tires

 

Here is another calculator that I found you could try out http://www.midwestfords.org/analyzers.htm

 

Hey arrid,
Do you pimp a chain that hangs lo like yo 20-fo's to match! LMAO!

The info you gave on how the tire size effect the final ratio is great too!
Indy
Still LMAO!