View Full Version : Changing gears in diff.
Im thinking of changing the gears from 308 to 355, has anybody out there started with 308s and changed to 355s , did it make a big enough improvment for the cost and work?
Walt
Gordon Levy
11-08-2018, 11:37 PM
Depends on your engine package. A 302 or 347 will notice a big difference. A 408/427 not as much but it is noticeable.
CraigS
11-09-2018, 07:18 AM
Generally 1 step isn't worth the expense but 2 steps is. 355 has been overall the most popular gear for FFRs. You list yiur car as donor build from 89 GT. If it is a 302 I'd go to 355 in a heartbeat.
Railroad
11-09-2018, 08:01 AM
If you are going to buy new gears, do the math on cruise rpm with your T-5. As said, engine size dependent, I would consider 3.73s.
Yes, my car is a stock 302,
Thanks for the responses!
Walt
GoDadGo
11-09-2018, 08:51 AM
If you are going to buy new gears, do the math on cruise rpm with your T-5. As said, engine size dependent, I would consider 3.73s.
Here Is Some Math Based On A 25.7" Tall Tire:
3.55 Ring.......63 O/D......70.09 MPH......2,050 RPM
3.73 Ring.......63 O/D......69.95 MPH......2,150 RPM
3.90 Ring.......63 O/D......70.02 MPH......2,250 RPM
4.10 Ring.......63 O/D......69.56 MPH......2,350 RPM
1. The question is what RPM is your engine happiest at or at least will tolerate without stumbling while in Overdrive?
2. Also, where do you shift the car and do you plan on making any passes at your local 1/4 mile drag strip?
3. The other question is at what RPM can you tolerate from a side pipe decibel level perspective?
4. We all know that when our engines hit certain RPM's they can be a bit tough on the ears.
5. How fast do you typically drive on the interstate may also be a mitigating factor.
5,500 RPM with 3.73's in 4th gear gets you to 112.74 MPH on the big end of the quarter mile.
5,750 RPM with 3.90's in 4th gear gets you to 112.73 MPH on the big end of the quarter mile.
6,000 RPM with 4.10's in 4th gear gets you to 111.89 MPH on the big end of the quarter mile.
Good Luck From The Dark Dart Side!
Thanks for the info. Go dad go, i wont be doing any drag strip, but i have thought about trying autocross, and i know 308s are not good for that, but my car is normally a street car.
So what would be the best setup for street and a little autocross?
Walt
GoDadGo
11-09-2018, 10:10 AM
So what would be the best setup for street and a little autocross?
Walt
Being a drag racer I still need to learn how to turn left and right.
Sorry, but I know very little about Autocrossing and/or Road Racing.
Regarding the street, where and what kind of driving you do I think matters.
Your gear choice may depend if you are you a back roads, highway or interstate cruiser dude.
Heck, you may want to go for a gear that works well when you are on the interstate going 70 MPH.
mburger
11-09-2018, 10:30 AM
I also am running a stock 302 with a T5 and 3:55 gears. The 3:55 gears will really wake your car up. I sometimes drive 70+ on the interstate and also cruise around town. No issues.
Ok, thanks guys i think 3:55 is where i'll go.
Walt
GoDadGo
11-09-2018, 11:53 AM
If your motor is basically stock, with only mild modifications, then I think your choice of the 3.55's will be darn near perfect!
3.55 Ring.......63 O/D......70.09 MPH.....2,050 RPM
Jeff Kleiner
11-09-2018, 01:07 PM
3.55 vs. 3.08 will feel like 50 horsepower seat of the pants and second gear will put a huge smile on your face ;)
Jeff
CraigS
11-09-2018, 01:52 PM
While 373s would give a little more grunt they have 2 short comings. 1st gets real short. In normal driving it's nice to be able to get across the street before shifting into 2nd. Also, for autocross 2nd could get too short depending on the local courses. No fun at all needing to hit 3rd for 150 ft and then back into 2nd. Other option is bounce off the rev limiter for those 150 feet. BTW, swapping the gears is actually pretty simple once you understand how it works. If you buy Ford gears chances are very good that the pinion shim will be re-used w/ the new gear. While you are at it, replace the clutches in the tracLock w/ new carbon fiber ones.
Thanks for the input, Jeff , just what i wanted to hear, Craig, i have the carbon clutchs ordered, Steve, looks like it would still make for good hiway driving!
Thanks again guys!
Walt