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Should I Change Gears in Trouble?

Laurie S.

Laurie S.

Well-Known Member
Staff member
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Trouble's rear end is out right now for me to strip and repaint to repair the damage my former mechanic did when he put a floor jack under it without padding. I've had a couple of people suggest that my 3:50 gears should be changed to 3:25. This would be the time to do it, so I would appreciate some opinions.

Basic info: car has a built-up 390 (410 hp on engine dyno) and a C-6. Once it's back on the road, I would mainly be driving it to local shows and trailering to shows out of state. I'm too nervous to park the car at a motel with no security, so it would go in the trailer. However, I would like to drive it to Phoenix, Knott's, Albuquerque, and maybe Las Vegas--basically anywhere I could get to a safe location in a day's drive.

Will the 3:50 gears be too screaming for the highway, and should I swap out? What's the downside of swapping out? I don't care about gas mileage, after all, it is a 390 and I can always get a bank loan. I want a fun muscle car to drive and I could be talked into a few burnouts.
 
Really just depends on your driving style. If you're one to want to cruise along well above the posted speed limit on the highway you may want to change it out but just going to a 3:25 wouldn't be worth the effort and cost, IMHO. Plus, short line burst fun will be diminished if you swap out the current gear for something more highway pleasing.

Since Trouble isn't a frequent driver I say leave it as is.
 
Even 3:25 will tach some on the freeway. I've got 3.27 (8.8) in mine and 4th gear is ok at 60 mph, but I also have the T5 and don't usually run it in 4th much. On my other car (chebby) I just switch from 3.31 to 3.55 knowing I'm not going to like it at freeway speeds, but I also know I won't be taking it more than an hour away so it wasn't a huge concern.

What transmission, and rear tire size do you have?
 
my 68 has 3.25 in it and a t5.

I wish it had 3.55 in it because I would actually like the highway cruise RPM better in OD ;)
 
"Horseplay" said:
Really just depends on your driving style. If you're one to want to cruise along well above the posted speed limit on the highway you may want to change it out but just going to a 3:25 wouldn't be worth the effort and cost, IMHO. Plus, short line burst fun will be diminished if you swap out the current gear for something more highway pleasing.

Since Trouble isn't a frequent driver I say leave it as is.
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Same here Laurie
 
"stangg" said:
Even 3:25 will tach some on the freeway. I've got 3.27 (8.8) in mine and 4th gear is ok at 60 mph, but I also have the T5 and don't usually run it in 4th much. On my other car (chebby) I just switch from 3.31 to 3.55 knowing I'm not going to like it at freeway speeds, but I also know I won't be taking it more than an hour away so it wasn't a huge concern.

What transmission, and rear tire size do you have?
Click to expand...

It's a C-6 and the rears are 15x7 wheels with 245 x 60; the fronts are 225 x 60.
 
In Shag, I'm running 3.55 with a T5z which has a .63 5th gear and sometimes wish I had put in 3.73, but it is running so well right now I'm going to leave it.

Laurie,
I agree that for all the work and expense, going from a 3.55 to a 3.25, I really don't see you changing RPM's all that much. Maybe 1,500 rpm's difference at say 65 mph?
 
It sounds like I should stick with what I have, thanks! When I originally was putting the rear end together, my friend let me drive a couple of his cars and I really liked the feel of the one with the 3.50 gears. No speeding in Trouble, I think it will be too much of a cop magnet as it is. LOL
 
With 245/60 -15,

3.25 : 2500 RPM @ 60
3.55 : 2700 RPM @ 60

3.25 : 2900 RPM @ 70
3.55 : 3150 RPM @ 70

As others have stated it comes down to your driving style... If you like driving 70 on the freeway, and don't like reving the motor to 3K, then numerically lower gears might be more comfortable. If you like drive 60, then 200 RPM isn't worth it in my opinion. For me personally when it gets into the 2800+ range, it just feels like the engine is begging for another gear...
 
"stangg" said:
With 245/60 -15,

For me personally when it gets into the 2800+ range, it just feels like the engine is begging for another gear...
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Try driving highway speeds with a 4.11 rear gear and 23" tall tires!


and some wonder why I rarely take it on the highway anymore :hs [nb]that is, when it's running[/nb]
 
A 410hp 390 should not have any problems in the torque department so I doubt that you will notice much difference between the 3.25 and the current 3.50 performance wise. A 3.25 is a pretty good all around gear. It seems to me that with any driving style you will be taching 3000 rpms for long stretches which is alot of work on a big block. Lots of heat and lots of fuel. For lots of highway driving a 3.00 is a better choice. Heck I had a 2.79 in my Mach when I had the 400hp motor in the car and with that much torque I could not tell much of a difference between the 2.79's and the 3.50's I installed later on. The difference is much more noticeable in a bone stock, small cube engine then it will be with a modified big block on street tires. I tend to agree with the others that the gear swap is probably not worth the effort unless you would be willing to go to a 3.00 gear.
 
"Laurie S." said:
What would I lose by going to a 3.00 gear?
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Sounds to me (to use the words my wife throws at me all the time) like you got a bug up your ass to change something.......you don't want to make the car slower do you? :nut
 
I have a mild 429, maybe 400 - 450 HP? I have had a 4.11, 3.70, 3.00 & 2.50. I also have a C6. I still have a stock converter. I weighed my 67 at the garbage dump and it was 3,400 lbs. with full tank and me in it. Even with gobs of torque, the 3.00 & 2.50 seat of the pants feel is a bit sluggish. I debated on a 3.50 but I do drive mine long distance and highway speeds. The 3.70 was the most fun to drive, 4.11's just too low. The 3.70's are too low for long hauls at $4+ a gallon. I averaged 16 mpg on the Power Tour both 2010 and 2011. I had the 3.00 in 2010 and the 2.50 in 2011. I ordered a 3.25 and if I ever get it I'm hoping it will be a good compromise. I keep breaking stuff though. I broke the 3.00 housing before Power Tour last year and the 2.50 (both junk yard stock diff's), is clunking and acting funny, I think I broke some spider gears.
 
"dodgestang" said:
Sounds to me (to use the words my wife throws at me all the time) like you got a bug up your ass to change something.......you don't want to make the car slower do you? :nut
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No, I don't want to change anything, but I want to have some good arguments for when the pressure comes to change, and it is coming. Also, wanted to make sure that I do have the right combo for the car.
 
I'm not sure about the C6, but in SWMBO's 66 (stock 289) the C4 shift points were better when I put a stock 8" rear back in with 2:79's. It had 3:25's in a 9" that was acting up so I swapped rears just so we could drive it, then she liked it so we left it
 
a three speed trans with a 3.25 rear gears don't match up gear wise.
If it was a 4 speed auto or manual , it would suit better.IMHO.
 
"Laurie S." said:
What would I lose by going to a 3.00 gear?
Click to expand...

I'm sure if you took it to the track you would probably be a few tenths slower, but it really depends on how your engine is setup. Since you know what hp the engine makes I assume you had it on an engine dyno. Where did it make its peak torque? Also do you have a stall converter in your C6 and if so what rpm is it suppose to stall at? All these things will determine how much the gear swap would effect your performance.
 
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