In a way its too bad that you truely do have 3:55s, cause now the dealer won't be helping you change to 4:10s under warranty (assuming that he would have anyway).

 

Isn't the the human (gearhead) mind a wonderful thing?

I have a 3.55. But now, I can't wait to upgrade to 4.10's....

I told my wife ( who was counting the driveshaft rev's in 30 deg F temps as I rotated the rear wheels) that I was happy Ford shipped me the right rear end, but I was bummed I couldn't get a free upgrade to 4.10's.

 

Why, thank you very much. It is kind of you to notice, although I do try to be modest whenever possible.

 

1st gear in these cars with the 3.55's is almost worthless as you find yourself shifting to 2nd halfways through an intersection when driving normally. I can't imagine what 4.10's would be like for anything other than a simple drag-race...

 

Perhaps the most important aspect of going to a 4.10:1 gear ratio in a GT is where 3rd gear is placed. If you are passing on a two lane highway, a rapid pass probably will be done in 3rd gear. With that in mind and with the stock 3.55:1 gear ratio, peak horsepower comes on at 99 MPH, as is calculated here:

Tire Diameter (stock) = (23.5 cm)(1/2.54 in/cm)(0.55)(2) + 17" = 27.18 in
Circumference = (3.14159)(27.18 in)(1/12 ft/in)(1/5,280 mile/ft) = 0.0013477 mile/rev
Speed = (5,750 rev/min)(60 min/hr)(1/1.32 3rd gear)(1/3.55 axle ratio)(0.0013477 mile/rev) = 99 MPH

and with 4.10:1 gears, 3rd gear peak horsepower occurs at 86 MPH.
On a typical two-lane road, most passes will be complete well before you reach 86 MPH. That is why a 4.10:1 isn't a bad idea. The real drawback is you probably will loose 1 MPG from the switch.

 

I thought it was because I was a noob.

 

so a gt automatic is suppose to have 3.31s? If that's the case then i'm definately getting some 3.73's or 4.10's

 

Gearhead - I appreciate the time you took to figure this out and explain it here, but for that odd time that I would need to pass another car at highway speeds, I could drop into 2nd if need be - but I personally think that my GT has enough nuts to get the job done in 3rd gear right now.

It would be good to work up a spreadsheet that shows the gear spacing / RPM / MPH so people could make a rational choice in regard to gearing. Frankly - I think that anything deeper than 3.73's for the GT is a waste of time. Also by increasing the torque multiplication at the rear end, you are passing that additional load up the driveline. The transmission (TR3650) is a relatively strong piece, but it's not as strong as the TR3550 or the TR3550-TKO. I'm just not sure the tranny would be in the game for the long haul with deeper gears, drag-strip starts and tires that actually hooked up..