Picking rear gears

olds307 and 403

olds307 and 403

Comic Book Super Hero
Oct 14, 2008
4,788
113
Melville,Saskatchewan
At 75mph with my 26.1 tires and 3.42s I'm at 3300rpm, and it's not bad, sound wise, but it's annoying and puts unnecessary stress on an otherwise low strung engine. .67 O/D of a 200r4 would put it at 2200. Much More livable.
29.2" tire, .67 O/D, 4.30 at 75mph calculates out to 2486rpm for this calculator. Always been accurate +/-100 rpm for me.
https://www.crawlpedia.com/rpm_gear_calculator.htm
I get 2488 rpm at 75 mph and I find it within 50 rpm using GPS. That will put you at lower rpm than my planned 3spd TH350C and 2.78 gears with 26.3" tall tires and the converter locked up in my 70 Cutlass.
 
64nailhead

64nailhead

G-Body Guru
Dec 1, 2014
916
93
Upstate NY
You need to keep the rpm's in the operating range of the cam. A 500hp 383 is not going to want to cruise at much less than 25-2700 if it's NA. You're in a tough spot trying to cruise with a big cam. If you pick to high of a gear (lower numerically), then it will suck to drive in the 60-70mph range. We're running 3.73's with a 26.5" tire with a 200 and it's not bad. Cruising at 75 is fine. I would guess that you should be at a 3.73 or 4.11. The 4.30's will be too much unless you rarely cruise on the highway over 70.


\/ \/ if this is occurring, then you need a better converter.

Correction 75 at 2700-2800. What I am more worried about is if I want to take a long drive down the highway and over heating the transmission by not keeping it over the stall. And getting the car to hook.
The converter will not slip until power is applied if it's a decent converter. At cruise speed the converter will be 'locked' implying no slip. It will slip at a certain amount of torque, supplied by the motor, and resistance, supplied by the rolling resistance, but will 'lock' as the power is taken away. You can test this out by driving at a cruising seed and feeling the converter being locked and then snap a the throttle and see at what rpm it grabs. If you have an aggressive cam, then the 'snap' might be quite a bit. But when you ease back out of the throttle, then it should (if a good converter) lock up and drive normally. There will be a point where it will feel like it's slipping as the rpm's drop too low for the cam. It's somewhat of PITA to determine, but you'll get the idea.
 
Tc77

Tc77

Not-quite-so-new-guy
Sep 19, 2018
33
18
I live in Nj not many hills around here. Nothing is written in stone yet car not even close to being back on the road yet. Just trying to get a game plane on what’s next. Cam is a Howard’s cam with 642 lift. If I take it on a long trip down the highway it would only be once in awhile.
 
pontiacgp

pontiacgp

Canadian Prime Minister
Mar 31, 2006
25,151
113
Kitchener, Ontario
What about selling the 12 bolt and buying a 9" rear end so you can have 2 third members, one for street and one for dragging. Third members are easy enough to change when you get to the track
 
motorheadmike

motorheadmike

Comic Book Super Hero
Nov 18, 2009
4,041
113
Ottawa, Ontario Canada
Our wagon has 3.73s, 27.6" tall tires, and a 4L80E with a 3800rpm 9.5" lock-up converter. N/A this is a sub-optimal combo for the cam's rpm band and only somewhat satisfies the overlap (6.2°) - but, on the bottle it should be perfect. For highway driving and putting around it is nice.

The Monte is going to be running 4.56s, 29.5" tall tires, and a 200-4R with a 3000rpm 12" lock-up converter. The cam is a stock 2001 LS6 unit and has a tonne of negative overlap, so it will drive just fine in all conditions. Will it buzz on the highway? Sure. But, it will trap at peak engine speed at the track. I am willing to drive slower on the highway as a small sacrifice.

I don't suggest you compromise on gearing unless you like being disappointed or have one hell of a mitigation strategy.
 
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Tc77

Tc77

Not-quite-so-new-guy
Sep 19, 2018
33
18
Our wagon has 3.73s, 27.6" tall tires, and a 4L80E with a 3800rpm 9.5" lock-up converter. N/A this is a sub-optimal combo for the cam's rpm band and only somewhat satisfies the overlap (6.2°) - but, on the bottle it should be perfect. For highway driving and putting around it is nice.

The Monte is going to be running 4.56s, 29.5" tall tires, and a 200-4R with a 3000rpm 12" lock-up converter. The cam is a stock 2001 LS6 unit and has a tonne of negative overlap, so it will drive just fine in all conditions. Will it buzz on the highway? Sure. But, it will trap at peak engine speed at the track. I am willing to drive slower on the highway as a small sacrifice.

I don't suggest you compromise on gearing unless you like being disappointed or have one hell of a mitigation strategy.
What would you suggest for gear size?
I was trying to get a feel from people’s experience. I hate doing things twice
 
motorheadmike

motorheadmike

Comic Book Super Hero
Nov 18, 2009
4,041
113
Ottawa, Ontario Canada
What would you suggest for gear size?
I was trying to get a feel from people’s experience. I hate doing things twice
I totally forgot to mention my wife's old 2002 Z28 had 4.11s in it with 4L60E with a stock converter. That mod took it from a low-13 second slug and turned it into a mid-12 second machine. At the time I chose to do this the internet was still convinced it wouldn't help much - obviously they were wrong (LS1tech often is). Highway driving and my marriage suffered from this choice... and leaving the exhaust cutout open all the time didn't help either. LOL.


But, then there is my old Turbo Buick that had 3.08s with a TH400 and a 3300 rpm converter on a short sidewall 26" tall drag radial. Funny, the internet also told me the car would be slower with those gears vs. the stock 3.42s (or the 3.73s I took out of this car). The gearing change forced the car to remain lower in the rev-range where it made the best power (torque for days).


My point is it depends on the car and what you want to do with it. Have you taken it to the track? What RPM does it go through the traps at? If it is a dog through the traps (low in the powerband) you obviously need more gear. Our TBSS is like this; despite having 4.10s a built 4L70E with a loose converter on 28" tall drag radials and 650hp it still chugs through the traps (11.9 @ 116mph) well below peak power because it weighs 5300lbs.

But, if you want me to ballpark it? No less than 4.11s and no more than 4.56s.
 

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