I have a '55 ford truck with a mildly hopped up 292 from a '62 car (.30 over mild cam ecz G heads, 4 bbl carb headders). I'm going to keep the original three speed at least for a while so I can get it on the road.
A freind gave me a 9" rear with 3.25 ratio from an '86 F150. But since that won't directly bolt up,I bought a 9" from a '71 F100 with a 3.70 ratio.
I'd like to be able to cruise highways with this truck with no problem.

If you were me would you go to the trouble to swap the 3.25 3rd member from the '86 into the 71 housing or would you just stick the 3.70 ratio rear in there and be done with it? Will the .45 difference in ratio make that much of a difference?

By the way I'm going to stick with the 15" rims and a fairly tall tire because I might go with large whitewalls.

 

That's a good bit of difference in ratio and I'd prefer the 3.25 over the 3.70, but the truth is either will do a good job for you. You can only interchange them if both use the same spline on the axle 'cause the longer axle shafts from the '86 won't work in the '71 housing. Pull'm and count either 28 or 31.

Also make sure they use the same bearing - later ones use a tapered bearing and race and earlier used a square shouldered bearing and race. These don't interchange either, but there isn't a distinct cutoff year.

 

I've run mostly all of the gearing ratios and flip flopped gears for different reasons. The 3.25 is a nice setup which allows you to still drive at highway speeds without taching out the engine. I've had 3.70 and there is a big difference in RPM and torque especially in city driving from the 3.25. If you do a lot of highway miles, I'd go with the 3.25's, if you do more city cruising and want to come of the line faster, go with the 3.70's. Personally, my favorite gearing for all around was the 3.50 gears. I had the best of both worlds and really enjoyed that gearing ratio. Friday night street races, the 4.57's went in.

 

Ditto to what imlowr2 (Ed) said.

I run a 3.50 9-inch and think it's pretty perfect. I had a 3.0 in it before. That was great on the freeway, but the 3.50 around town is just too much fun!