I have a pretty much stock 302 and c4 3spd. I am building an 8" rear for my mustang. I was looking at 3.80 gears. Will this be a good changeover? or should I be looking at a different gear ratio like 3.55's.
3.80's for a 8"
- Thread starter Hg80Design
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SadbutTrue
Founding Member
3.80s are a bit numerically high for a c4. You'd be pushing 3000 rpms on the freeway... will be really loud, eat gas and wear the engine pretty fast (though for some people thats either how they like it or thats not an issue). I'd stick with 3.25s to 3.55s (probably 3.40 or 3.55... my 3.25s didn't make a huge difference from the 2.8s or 3.0s that it came with, that was when I had a c4 before my t5 too).
I think 3.40 is a pretty solid all-around gear, if you can find it. 3.55 wouldn't be bad either.
I think 3.40 is a pretty solid all-around gear, if you can find it. 3.55 wouldn't be bad either.
SadbutTrue
Founding Member
In addition to the last post, with a stock 302 you're not really gonna be able to use the 3.80s to your advantage... the engine won't rev enough.
JC6715
Member
I just got done breaking in new 3.89's in my 9" using a C4 and my 1st and 2nd gear more or less disappeared (except when you really get on it). Extremely glad i will be running a 6-speed with my final combo.
If you're just street cruising and want to get on the freeways, I would definitely look into something less that 3.80's.
If you're just street cruising and want to get on the freeways, I would definitely look into something less that 3.80's.
So perhaps something like a 3.55's would be a better choice. The 2.80 that are in there presently just don't seem to have any go to them But I do want to retain highway drive ability if that is possible.
SadbutTrue
Founding Member
3.55s are fairly versatile and highway-able but might still be much if this is a daily driver that sees most of its time on the highway. If its just an occasional driver/weekend fun car then go with 3.55s.
If its something you need to live with day in and day out, get 3.25s or 3.40s.
My 3.25s were good with my C4. With the T5 it'd be kinda nice to have 3.55s. Thats about as far as I'd go personally. I'm not sure how people live with 3.89s and 4.11s... my first gear already goes by pretty quick with the T5 and 3.25s, with 4.11s it'd be completely and totally useless.
65fastbackresto
Active Member
I have 355`s and a C-4
If I could do it over again, I`d go down to about a 325, 2700 rpm at 55mph is kinda loud.
If I could do it over again, I`d go down to about a 325, 2700 rpm at 55mph is kinda loud.
wild70stang
New Member
- Nov 11, 2005
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Im just curious why most people want higher gears? I forgot the name of the guy, but he had 2.7 gears with a five speed, and to me that would seem like fun since your not stabbing at the throttle to feel alot of power and improved fuel efficiency is a bonus 
Decurion
Member
If you have a 3.80 gear, 25.15" tire (215/60/15) and are going 70 mph, you will be turning 3554 rpm. Thats kinda high to be going any kind of distance. I would go with say, a 3.25. Just for comparison I ran some numbers, all are assuming the same tire size and speed.
4.11- 3844 rpm
3.80- 3554
3.55- 3320
3.25- 3039 <~~ my choice
3.08- 2880
2.79- 2609
4.11- 3844 rpm
3.80- 3554
3.55- 3320
3.25- 3039 <~~ my choice
3.08- 2880
2.79- 2609
SadbutTrue
Founding Member
Im just curious why most people want higher gears? I forgot the name of the guy, but he had 2.7 gears with a five speed, and to me that would seem like fun since your not stabbing at the throttle to feel alot of power and improved fuel efficiency is a bonus
If you have a close-ratio transmission, the numerically higher rear gears are great. They provide a noticable bump in acceleration (via torque multiplication/faster revving - all part of the same thing), and with close-ratio transmissions/transmissions with relatively low (numerically) 1st gears can do it without becoming useless in 1st gear.
With my T5, anything beyond 3.55, imo would be a complete waste because my first gear would be useless. My trans first gear is 3.27 or 3.31, which with my 3.25 gears gives a 10.75 final gear ratio. I have been told that high 9s or low 10s for first gear is what you want unless you have slicks (and a drivetrain that can handle said slicks), and from what limited experience I have that seems to be accurate. First gear already goes by pretty fast and tops out early. With 3.73s it'd go by even faster, if I could even get traction (unlikely).
But if I had the closer-ratio T5 (with 2.95 first gear), I could go with 3.73s and have roughly the same 1st gear ratio as I have now with 3.25s. I would accelerate faster in every gear and my gas mileage wouldn't dip too much.
So basically you need to take into account your transmissions gearing, particularly first gear, when you choose rear gears.
I would say even more so than your trans' tallest (numerically highest, lowest ratio, whatever you wanna call it) gear, unless you don't care about performance and just drive on the freeway and want mileage.
Im just curious why most people want higher gears? I forgot the name of the guy, but he had 2.7 gears with a five speed, and to me that would seem like fun since your not stabbing at the throttle to feel alot of power and improved fuel efficiency is a bonus
2.7 gears with a 5-speed doesn't help anybody's fuel economy in any situation.
It certainly does not help with going fast either.I would go with the 3.25's. If you had a bigger cam I'd say go with the 3.40's
Decurion
Member
2.7 gears with a 5-speed doesn't help anybody's fuel economy in any situation.
Fox body Mustangs had either a 2.78 or the optional 3.08 rear gears, just for reference. I think a STOCK fox engine and 5 spd with those gears was optimal for mileage. Not to say its the best for performance tho...
I have 3.80s with a C4 with a mildly built 289 on my coupe.. its a lot of fun through the twisties but if I really wish i had gone with 3.55 or even 3.25.. I had stock cruising gears but edelbrock recomended a gear change with my Performer RPM package. last year i had the trans redone and built up. I went with a custom built 2200 stall speed torque converter with needle bearings. if i had the whole thing to do over id have put a 5 speed on it its crying for an OD.
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