I would not recommend anything lower than 3.55, esecially if you will be doing a little highway driving. I have a C4 w/ 3.55's and luckily, Ido not do much highway driving. I could not imagine going lower, It would be extremely uncomfortable (already doing3k @ 65)

 

as to the tryres, they do have an affect.
i'm not talking 14x7, 15x8 or different. I'm talking the rolling diameter, which can change to stock.
Example
BFG RADIAL T/A 195/60-15 are 24.2" tire diameter
BFG RADIAL T/A 225/70-15 are 27.4" tire diameter

they'll both fit on 15x7 rims.

So from this, 60mph on the same car with 1:1 trans (manual no overdrive) and 3.55 gearing
will give those results:
195/60-15: 2700 engine rpm at 60mph
225/70-15: 3000 engine rpm at 60mph

that's why we need to know the engine, the trans and the tyre dia (measure from center to top and double that)

Kalli

 

The reason you use higher gearing in autos is that the trans gearing islower in the auto itself. Nothing to do with weight. So you generally use a higher gearto get you out of the hole easier.The power loss on a auto is 20-25% and on a manual is 15-17%. Now the C4 is the most efficient auto Ford has but the AOD surpasses it with the OD. Ford would put 3:24 in autos and 3:55 in manuals for factory upgrades. Now, if you have a C4 I would look at the 3:55 but if you have an AOD then 3:73 or 4:11, the RPM difference in OD is small. 4:11 is what I run and love them.

 

.....

 

i think i'm going to go with the 3.73's since i only paid 75.00 for em and i'll be doing in town driving like i said with a few sprints of 20-25 miles. so if anyone wants a 3.08 gear..

 

youll be very happy with them, id start saving money for new tires cause your gonna need em!