I'm a newbie to engines and stuff but I'm trying to learn. My GT has a 3:31 Rear Axle Ratio. I have seen options for a 3:55 and 3:73. Several shops have said I could put a 3:73 Gear ratio in it and it would run faster. My 1st question is, what do these numbers mean? My 2nd question is, would a 3:73 work in a 06 GT Convertible? I have been on Ford's website and configured a couple of new GTs. The convertibles are always ONLY available with a 3:3I, even with a Shelby.
Gear Ratio Question
- Thread starter black06pony
- Start date
SpartaPerformance
10 Year Member
The gear ratio is how many revolutions the rear wheel have to the drive shaft, so 3.31 turns compared to 3.73 turns. The vehicle also accelerates quicker because torque to wheels is multiplied by a larger number. For example
300lb ft of torque * 3.31 = 993
300lb-ft of torque * 3.73 = 1119
300lb ft of torque * 3.31 = 993
300lb-ft of torque * 3.73 = 1119
mbuckcoyote
Member
While that's an interesting take on gear ratios, the more common explaination is: how many teeth on the pinion gear there are as opposed to the ring gear. for example: 3.73= 11 teeth on pinion to 41 teeth on ring. 3.31 would be something like 10 teeth on the pinion to 33 on the ring. That's not completely correct but, close on this last one. The higher the number on the ratio (3.73, 4.11, etc.) the quicker the acceleration. But, will cost in top speed. The inverse is true for lower gears. 
j_lutter31
Member
Another viewpoint.....
Say you have your 3.31 gears in your car. Maybe you have thought about upgrading to 3.73's or 4.10's. Well the higher you go with your ratio, the higher your rpm will be at any given speed.
For example, now at 70 mph you might be turning 2000 rpm, with the 4.10's it might be closer to 2400. Which will slightly hurt your fuel economy on the highway. Something to think about, some people will say it doesn't affect it enough to matter, some say otherwise....
Say you have your 3.31 gears in your car. Maybe you have thought about upgrading to 3.73's or 4.10's. Well the higher you go with your ratio, the higher your rpm will be at any given speed.
For example, now at 70 mph you might be turning 2000 rpm, with the 4.10's it might be closer to 2400. Which will slightly hurt your fuel economy on the highway. Something to think about, some people will say it doesn't affect it enough to matter, some say otherwise....
I think we should know if you are 5sp or auto?
4.10's for auto and 3.73's for 5sp is generally what people are running unless you plan on going SC, then the general consensus is to stay @ 3.73 for an auto.
My auto 06 vert GT has Ford Racing 4.10's and is 2500 @ 70mph fyi.
Gas mileage is down by about 15-20 miles per tank, but the fun factor is up 1000%
Especially with a custom tune and CAI, that all you really need in these cars.
4.10's for auto and 3.73's for 5sp is generally what people are running unless you plan on going SC, then the general consensus is to stay @ 3.73 for an auto.
My auto 06 vert GT has Ford Racing 4.10's and is 2500 @ 70mph fyi.
Gas mileage is down by about 15-20 miles per tank, but the fun factor is up 1000%
Especially with a custom tune and CAI, that all you really need in these cars.
I have an automatic but it's convertible. I don't know why a convertible should matter, but I've noticed that Ford doesn't put anything except a 3.31 in the convertibles. You can't even order them with anything else. Looks like they do it that way for the Shelby convertible too. I can't figure out why.
They probably assume that performance is a lower priority to people buying convertibles. They might have been trying to keep the CAFE number up a little by keeping the convertibles' mileage up with the 3.31. There is no difference in the rear end between a coupe and convertible, both use the 8.8" rear. Only the V6 had the weaker rear end. You can use any ratio made for the GT.
Related Question
I would like to piggyback on this question. I bought my '08 GT used from a dealer and there were some mods done to it which I'm still trying to figure out. One that I suspect is the gears. Currently when I'm going 60 mph per the speedo, my GPS (two different ones) showed that I was going 70 mph. During a visit for scheduled MX, I asked the dealer to check on it. They said the wheels/tires are the original diameter. They said to find out for sure, they would have to take the diff apart. Question, with this info, do you think the gears were changed and what ratio do I have now? Looks like there is about a 10% difference with the speedo registering slower than the actual speed.
I would like to piggyback on this question. I bought my '08 GT used from a dealer and there were some mods done to it which I'm still trying to figure out. One that I suspect is the gears. Currently when I'm going 60 mph per the speedo, my GPS (two different ones) showed that I was going 70 mph. During a visit for scheduled MX, I asked the dealer to check on it. They said the wheels/tires are the original diameter. They said to find out for sure, they would have to take the diff apart. Question, with this info, do you think the gears were changed and what ratio do I have now? Looks like there is about a 10% difference with the speedo registering slower than the actual speed.
Assuming you have a manual transmission and the speedometer is correct, it sounds like you have a 3.73 according to this calculator. JawsGear.com
You have a .68 transmission ratio in 5th. If the GPS is correct, then you might have a 4.10 rear end.
You have a .68 transmission ratio in 5th. If the GPS is correct, then you might have a 4.10 rear end.
