i've you've got an auto don't fear the 4.10's at all man. a lot of the guys got 3.73's for 5-spd and 4.10's for the auto. I've always assumed to compensate for some of the power robbing that takes place with the auto, but i honestly don't know for sure.
There's not much of a difference between the automatic and standard transmissions in these cars as far as power goes. Once you get a good tune in both the difference is even less: the automatic is no slouch. I think the approximated power loss is like 3% more for an automatic which calculates to something like 6.2hp...
In any case, 3.73's or 4.10's will do good things for you if you want more 'go.' I'm in the middle of getting my **** together to get mine done and i've decided on the 4.10's. Its my understanding that the gas mileage doesn't change that much and you'll feel a much bigger difference with the 4.10's than the 3.73's.
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Black 2005 Mustang v6, 5 speed, IUP, EUP and Leather.
Razzi 1/4 window scoops, Mustang GT Sway Bars, Flowmaster American Thunder Single Exhaust, Clear Front Turn Signal Lenses, Demolet CAI, X-cal2 91 tune, Hurst Comp/Plus Shifter, FRPP T-Lock, Ford 3.73 gears
if u do alot of highway driving the 4.10's will hurt ur gas mileage a bit more though. but if ur mostly doin around town driving i wouldn't worry bout it.
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Torch red 2007 4.0, in dash flip out Jensen touch screen dvd player, single Flowmaster American Thunder exhaust, Airaid CAI, 4.10 rear gears, Ford Racing T-lok, SCT XCal 3 tuner w a bama race tune, sequencial taillights. 2 Audiobahn 400w 12" subs w 2 pioneer amps, glowshift gauges, Pioneer navigation system, Shaftmasters 4" aluminum drive shaft, Zex wet nitrous kit 75 shot, with window switch, and dual outlet purge kit, hollowed out catalytic converters.
around town it could help because you can get into a higher gear sooner. Instead of going to 4th at 35, you could get it at 30 for example. Not exact numbers, I'm just exampling.
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2005 V6 Manual
K&N CAI
GT takeoff sway bars
Steeda Sport Shifter
Rear Blackout Panel
Flowmaster A/T axle-back
around town it could help because you can get into a higher gear sooner. Instead of going to 4th at 35, you could get it at 30 for example. Not exact numbers, I'm just exampling.
That is true... My city mpg actually improved slightly.
3:73 vs 4:10 fuel & power info? 
