I have 3.55 LS now on my 97 F150 4x2. I do some towing and was wondering what gear ratio did you go with? I was thinking either 3.73 or 3.90 gear ratio. How much more do they raise you rpm at 65mph cruising speed?

Thanks
Kurt

 

Since these trucks have a ~ 30" tall tire stock,the RPM change is minimal when swapping from 3.55 to 3.73-it's barely noticeable,and I'd say that 3.90's wouldn't be bad either.
Assuming a 30" tire @ 60 mph in OD-here's what you'll see with a 4R70W:
3.55=1670 rpms
3.73=1750rpms
3.90=1835rpms
4.10=1925rpms
JL

 

I've got 31.5-inch street tires onmy Expy, with 4.10s.

Second-best mod I ever did.

Here's a pic of my speedometer & tachometer at various speeds.
https://www.ford-trucks.com/forums/5...with-pics.html