mustangsavage1967

Im looking at putting 3.55 gears in but i wanted to know how much my top speed and rpms will change when i do. I currently have 2.79 (stock) for a 67 stang. I just want to know if anyone who has done the change can tell me what the rpms are at lets say 65 mph. thanks

derwin66

What size wheels do you have?

In my 65 I changed to the same gears and picked up about 600 rpm with 14 inch wheels. It was a good excuse to put an overdrive in though.

mustangsavage1967

i think they are 15 or 16

Gun Jam

regardless of your tire size if you are running x mph with your 2.79 it is going to take 700 more RPM to reach x mph with 3.55s

if your final transmission gear is 1:1 like with a c4 3 speed automatic the 3.55 gears will pretty much eliminate any practical long duration freeway driving. around town and short hops on the freeway will be okay!

SO if you drive a lot on the freeway you will want an overdrive transmission before you do the gear swap.

-Gun

kalli

what's teb size of tyres (not wheels) 225/60R15 ... soemthing like that. and what gearbox? 3/4 speed or a 5 speed with overdrive?

we can calculate your speed and rpm with this data ...

that 700 rpm is a pretty good ballpark value.
If you change the gears and leave everything the same the rpms will go linear with speed. 3.55/2.79=1.272 which means 27.2 percent increase.
So if he currently does 2500 rpm at 60mph (with 2.79 gears) he will see 2500*1.272=3180rpm @60mph with the 3.55 gears

TexasAxMan

Take the simple approach. Whatever your RPM at a given speed times the % difference in gear ratios. In you case (going from 2.79:1 to 3.55:1) multiply your RPM by 1.2725. So if you're currently running 2500 RPM at 60 MPH, you will run 3181 RPM with the new gears at 60 MPH, assuming the same tires and transmission.

musnicki

Posted a few years back for me, I'll repost for you.

http://www.5speedtransmissions.com/gear_calculator.html



Play around with that. You'll need the circumference of your tires.

Use this one: http://www.csgnetwork.com/tiresizescalc.html

James