Gear Ratio and RPM deciding on gears

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340inabbody

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Guys looking to decide over a 3.55 or 4.11 still. Have been reviewing my old threads as well as others. Ran some numbers and wanted bounce off conclusions from this and if I am missing anything. I used the Spicer calculator. Red dotted lines are approximated power band.

1. My 2.71 never makes it into the power band!
2. The 4.56 could hit my rev limiter just over 100MPH say if I was doing the 1/4 mile
3. Not a huge difference between the 3.55 and 4.11 at cruise 70 MPH

What is a reasonable RPM at cruise 65ish for a 340?
Am I missing anything? Does load or type of transmission alter the plots?

Thanks

IMG_4618.jpg
 
I run 4.56's and a 27 inch tire.

Need to know more about the combo.

It's mostly about what you can live with, and the rest of the car.
 
I have 26" tall rears and 3.55 gears on my Dart. It is plenty loud at 65-70 mph. I would not want to drive very far on the highway. I have thought about 3.73s to improve strip performance but would be reluctant to go far past that.

Good luck.
 
I run 4.56's and a 27 inch tire.

Need to know more about the combo.

It's mostly about what you can live with, and the rest of the car.

Road use?

Track only?

Details matter. ;-)
well, all we have to go on so far is "70 mph". IF his car needs to travel 70 mph for distance, I'd go 3.23's max, my opinion
 
I have 26" tall rears and 3.55 gears on my Dart. It is plenty loud at 65-70 mph. I would not want to drive very far on the highway. I have thought about 3.73s to improve strip performance but would be reluctant to go far past that.

Good luck.
100% agree. The back of my neck gets tense when it sounds like I'm in 2nd gear at 60 mph, and passengers ask "are you in drive?".
 
I went with 3.91's from 3.55's. Have not yet driven my car yet but my rear tires are 28". May not feel alot of rpm difference maybe but take offs will be better I hope!!:steering:
 
Road use?

Track only?

Details matter. ;-)

I want a hot street car that I can take on the track a few times a year at most since I don't have a lot of money to replace parts etc. But I would like to be able to ride a decent torque curve around town and the torque of the 340 starts above 3000 probably more like what I show in the graph at 3800. Wouldn't 4.11's have more torque around town than a 3.55? I don't plan on leaving the Phoenix metro area and tour with. I DO want performance and the 2.71 I have is a dog with the torque converter as is. I plan on working with Lenny AFTER I get the rear ratio in. There is always a split on what rear gear ration is best. All I know is I have a 904A 3 speed trans that idles at 35MPH lol which is not good.
 
well, if you going to be 55 mph or under, then go 4.11's. I don't know what you call a dog because some think a 1.8 60 ft time is a dog and others think a 1.8 is a race machine, but if it's a lazy pig that won't cross the street with 2.71's, then you probably have another issue.
 
I have a 340 runs pump gas and my power band is 700 rpm higher than your chart. My car is stock 3.53 and I can cruse 70 in it but not 80. I wouldn’t recommend 4.11 on the freeway for any distance.
 
I have a 340 runs pump gas and my power band is 700 rpm higher than your chart. My car is stock 3.53 and I can cruse 70 in it but not 80. I wouldn’t recommend 4.11 on the freeway for any distance.
Yeah the power band is a guess and every set up different. What kind of rpm are you at 70mph?
 

Yeah I was gonna say either this ^^^^^ or 3.91s.

HOWEVER, you STILL have not answered the question asked. WHAT'S YOUR ENGINE COMBO? Are we just "supposed" to know somehow? The cats knocked off my crystal ball and broke it a long time ago. Are we supposed to remember YOUR combo from another thread? Fat chance for my "old butt". PLEASE enlighten us if you want an answer remotely "accurate".
 
Yeah I was gonna say either this ^^^^^ or 3.91s.

HOWEVER, you STILL have not answered the question asked. WHAT'S YOUR ENGINE COMBO? Are we just "supposed" to know somehow? The cats knocked off my crystal ball and broke it a long time ago. Are we supposed to remember YOUR combo from another thread? Fat chance for my "old butt". PLEASE enlighten us if you want an answer remotely "accurate".

I agree. OP!
But ill say in general, with a 3.31 stroke that 3000-to 3800 is typical. Though in the next lane... the guy thinks 4200rpm down the freeway is where its at.
 
Yeah I was gonna say either this ^^^^^ or 3.91s.

HOWEVER, you STILL have not answered the question asked. WHAT'S YOUR ENGINE COMBO? Are we just "supposed" to know somehow? The cats knocked off my crystal ball and broke it a long time ago. Are we supposed to remember YOUR combo from another thread? Fat chance for my "old butt". PLEASE enlighten us if you want an answer remotely "accurate".
Sorry. My question was regarding the plots I made and conclusions rendered. The combination is a 340 dont have the CAM specs or compression specs as I got the engine without any docs. Its an aggressive cam run by an MSD 6AL steal stock heads Holley 650 double pumper with mechanical advance (I know) Mopar high rise intake and apparently a stock 904A trans. The current rear is a peg leg 2.71 in which I have a Spicer Sure grip 27 spline. I have it timed at 19 deg at idle 31 all in with a slow curve due to detonation. This detonation is partly due to the luging combo of a shitty rear gear 2.71 and and a converter (unknown stall). This is everything I know about the combo.
 
What mph are you doing at the track, and what’s your combo ??

but as a guess if you don’t mind the crusin rpm of 4:10 go with that.
 
3.91 or 4.10 gears are no fun at all if you use the stock non overdrive trans and do any freeway driving. If you just run around town or race they are great. 3.23 is the best compromise in my opinion. If the car is a “dog” with 3.23 gears, a set of deeper gears won’t make that much difference.
 
3.91 or 4.10 gears are no fun at all if you use the stock non overdrive trans and do any freeway driving. If you just run around town or race they are great. 3.23 is the best compromise in my opinion. If the car is a “dog” with 3.23 gears, a set of deeper gears won’t make that much difference.


How are deeper gears not gonna help if your car is a dog ??

what would it take if not gears to dial in your combo ?
 
Not talkin about the OP here, BUT.........

I know I've preached it till I am blue in the face. People come on here wantin to know "what gear" or "what converter" is best. They have (for example) a 340, 240-260 @ .050 cam, all the goodies and then curl up in the fetal position like a little sissie when I say "3600-3800 stall and 4.30 gears".

If you're gonna build one with "all that" up front than "what's out back" needs to match.
 
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