I did a before and after test of my rpm's at 5 mile increments from 50 to 85 when I got my 3.73's in, but unfortunatly I don't have it on my laptop, and I won't be home till Thanksgiving. I might have a bookmark to a website where they have gear comparisons. I think it is also in a thread here. I'll look.
"The following information is for a stock 2004 GT with stock size rims and tires (245/45/17) and the TR-3650 tranny."
I copy pasted that from the link I put in an earlier post.
I'm about to get off, but I'll leave you(or your friend) with this; if I were to be doing gears again, I would be getting 4.10's. I was afraid of getting crappy milage, but with the mods I have, the MPG's I get are the same (although sometimes better, sometimes worse) as stock, it varies depending on how you drive.
My buddy put 373 in an auto and had the lock up in the trans removed. He said he should have gotten the 4.10. I was in it a few times, it was sweet but not a kick in the pants. (this was in a 90's fox vert)
Well I have a 92 GT manual with vortech super charger and some other little mods and I have 4:10's in mine. But the car was a auto and I swapped it for a stick. I believe the tires are 245/55/17 but dont hold me to that. Should I switch gears?
The one that will get you closest to your shifting rpm in 4th gear at the end of the 1/4 mile. If the 1/4 is not the goal, its your own opinion based on the balance between cruising rpm, gas mileage, and acceleration. I still have 2.73 in one of my foxes .