Gears

[megax29]

Gears is the next thing on my list. What will my rpms be at when cruising at 70mph w/ 4.10s and 3.73s? I cruise at 2100 now with the original gears and at about 3100 when I hit the overdrive. Also if I drop in my gears w/o having a tune how will affect the cars performance?

[SpectorV]

you wont need a tune for rear end gears it wont effect it. The only thing you will need is a way to calibrate the speedo. You should be able to run a chip or a speedcal unit (depending on year/tranny)

For an auto (since the gearing is worse than a manual) 4.10s are generally a good choice but you really need to do a tlok since its the same amount of work.

I cant remember the exact percentage that the rpms will go up but if memory serves me right from stock to 3.73 it will be about 250-300 rpms more than what it currently is at~

[JROC]

Look here.
Tech Equations - ThirdGen.org

http://www.mustangevolution.com/foru...ula-22451.html

[KyleUSAF]

you might want to get a tune too while you're at it..instead of getting a speedcal, then a tune if you plan on taking it that far

[megax29]

Quote: Originally Posted by JROC Look here. Tech Equations - ThirdGen.org http://www.mustangevolution.com/foru...ula-22451.html

JROC: This info seems very helpful if I were able to determine all the numbers to plug into the formula. My car is an 04 3.9L automatic and my tires are stock 225/55/16. I need to know rear end ratio, transmission gear ratio and tires' radius in inches. I don't know what type of transmission I got. I've taken it to a few shops to ask about gears and some say its got an AOD and other a 4rw70 or something like that, what transmission do I have?

[megax29]

Quote: Originally Posted by SpectorV you wont need a tune for rear end gears it wont effect it. The only thing you will need is a way to calibrate the speedo. You should be able to run a chip or a speedcal unit (depending on year/tranny) For an auto (since the gearing is worse than a manual) 4.10s are generally a good choice but you really need to do a tlok since its the same amount of work. I cant remember the exact percentage that the rpms will go up but if memory serves me right from stock to 3.73 it will be about 250-300 rpms more than what it currently is at~

If 3.73s give you rpms of 2400 when cruising at 70mph that's perfect for my liking. How about 4.10s?

[megax29]

Any input anyone? Doesn't n e one have 4.10s?

[I_luv_HP&trq]

Quote: Originally Posted by megax29 JROC: This info seems very helpful if I were able to determine all the numbers to plug into the formula. My car is an 04 3.9L automatic and my tires are stock 225/55/16. I need to know rear end ratio, transmission gear ratio and tires' radius in inches. I don't know what type of transmission I got. I've taken it to a few shops to ask about gears and some say its got an AOD and other a 4rw70 or something like that, what transmission do I have?

you have a 4r70w transmission with an overdrive hence the AOD.
with 4:10 you'll be around 2750 or so at 70 MPH.
try this RPM calculator....Gear Comparison
It's a bit easier to input your tire size info.

[megax29]

Hey man thanks that calculator was really helpful. I think 4.10s maybe to much for me since I do a lot of hwy and 3.73s may not be enough so I'm thinking of going with 3.90s. Do you know of any shops that can do this job at a reasonable price in the bay area? I've asked at 3 shops and all 3 of them have quoted over $600 out the door.

[Zim]

I can tell you I roll at 2800RPMs at 80mph with 4.10s. My old are with an auto and the same gears cruised at the same RPM as well. 70 youll probably be around 2500-2600 RPMs with 4.10s, I dunno about 3.73s

[bbunt302]

With 3.73s, I do 80 at about 2600 RPMs.

[megax29]

I do about 2450 rpms at 80mph with my stock gears. I think they're 3.27