Best gear ratio for windy rural roads street car.

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Gtsjosh

Gtsjosh

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Been having a ton of fun with my car but looking for a little more from the 3.23 gears. Car is a mostly stock 340 , 4 speed. Has an avs2 650 carb , electronic ignition , and is tuned pretty well. At some point I’ll be doing a cam and Intake manifold but would like to get gears sorted first. Tires are 245/60/15. May possibly go up to 255 in the future because I think they will fit my current setup.
I live in Southwest Missouri. Lots of rolling hills and windy roads. I do a lot of cruising around 50 mph and don’t plan on much highway cruising. So a 65 mph cruise speed is good with me. I’m leaning towards 3.73 over 3.91 as they seem like a good compromise without getting too extreme. Would love to hear from some people more experienced than myself. My main goal is building a really fun responsive street car but I doubt the car will ever get built beyond 400hp or so.
 
Personally I think the 3.91s would be overkill- 3.55s should be fine, maybe the 3.73s with the bigger tire.
 
Since you indicate not much freeway travel over 50mph, I would lean toward the 3.91 gears.
Here is the formula for RPM

(MPH x gear ratio x 336) ÷ tire diameter in inches.

In this example with 3.91 gears at 50 mph with a 245 60R15 the result is

50 x 3.91 x 336 ÷ 26.6 = 2469 RPM.

Play around with the formula and variables to arrive at an RPM suitable to your driving style. In my experience 340's come alive with a little bit of gear.
 
Garrett Ellison said:
Any particular cam and intake in mind?
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probably the Edelbrock air gap intake manifold. Honestly not to sure on cam. I don’t have a lot of experience there. Figured I’d call Lunati and comp cams and give them my specs and how I plan to use the car then bump their recommendations off this groups. Nothing too extreme though, just a fun street cam.
 
Professor Fate said:
Personally I think the 3.91s would be overkill- 3.55s should be fine, maybe the 3.73s with the bigger tire.
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Having had a 340 3.55 combo, I find the 3.55 gears to be a "lost in the middle" gear. It isn't a great performance gear, nor is it a great highway gear. Now with a 408 stroker, different story all together. My experience is that 340's used for spirited driving like 3.73 or deeper gear.
 
cpearce said:
Since you indicate not much freeway travel over 50mph, I would lean toward the 3.91 gears.
Here is the formula for RPM

(MPH x gear ratio x 336) ÷ tire diameter in inches.

In this example with 3.91 gears at 50 mph with a 245 60R15 the result is

50 x 3.91 x 336 ÷ 26.6 = 2469 RPM.

Play around with the formula and variables to arrive at an RPM suitable to your driving style. In my experience 340's come alive with a little bit of gear.
Click to expand...

thank you. I’ve been playing around with some calculators online which is kind of what got me leaning to 3.73 and 3.91. Mostly looking for the best feeling 0-60 or maybe 0-80 times. Right now I’ve been downshifting a lot with the 3.23 and the 3.73 or 3.91 will do a better job keep Keeping me in the powerband. Right now I’ve been shifting around 5200 but after cam and springs will be more comfortable bringing that up some.
 
Gtsjosh said:
probably the Edelbrock air gap intake manifold. Honestly not to sure on cam. I don’t have a lot of experience there. Figured I’d call Lunati and comp cams and give them my specs and how I plan to use the car then bump their recommendations off this groups. Nothing too extreme though, just a fun street cam.
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For not a lot more money, I'd call a cam grinder and have a cam spec'd to your particular combo and driving style. Jim at Racer Brown cams would be a good call.
 
In my opinion a well-balanced 340 with a 391 rear end is a thing of beauty a match made in heaven so to speak people get a little touchy about it winding a little but actually 340s piston speeds with a 391 are probably about the same as a 408 stroker with a 355 gear So there's that. 354-355 is a better combo if the car seeing the highway often cruising speeds over 65. I run 391s with a 26-in rear tire four-speed on country roads it's a thing of beauty and still capable of a 130 mph no problem.
 
I’m also debating just ordering another thirdmember and keeping my 3.23. Either way what is better , the cone or clutch style sure grip or should I just do a truetrac ?
 
I personally have always run factory third members with no issues and I have put them through hell including drag radials and 5,000 RPM hole shots. Just because I'm an idiot don't try this at home LOL
 
I am going to take the 3.23 out of my Duster 340 4-speed, 275/60/15 I have a 4.30 gear which should wake it up.
The only reason I used the 3.23 was because I had them when I built the rear.
I drive mine the same as you, short hops, and 2 lane highway, in hilly WV.

I used to change the gears out on Friday from 3.23 to 3.91 and change them Sunday back to 3.23 for the fun factor, but the 3.91 were just not deep enough.
 
340s 833's an 3:91s go together like baseball hotdawgs apple pie and mopars! thats way mine was set up and i loved it specially on the curvy back roads! now it would do hwy speeds but i didnt like the rpms! 10ish mpg no matter how ya drove it, was a bit over cammed lol!
 
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I wouldn't bother with a ratio change of less than about 1/2 point. In other words, 3.23 to 3.73. Or 3.55 to 4.10. I wouldn't bother going from 3.23 to 3.55.
That said, the way you describe your use, I would lean toward the 3.91s
I have 3.91s in my big block B, and I commuted 20 miles one way at 70mph with a 440 and 4.10s for ten years. That rpm shouldn't bother a 340.
 
IMO, You already have the perfect gear;
But I'm biased; since 1970, they've been my go-to gear.
>The next time you are out horsing around, and you are wishing for more power, every time that happens, look at the tach. If you do this often enough, you may get a pattern that will point you to a more favorable gear ratio. My guess is that you may need more engine. Not a cam, not gear, not a bigger carb; just more cylinder pressure or more cubes.
>Gearing can only do so much. Once you start bucking the wind your engine has to actually make power. Every time you double the speed, it take double double (4times) the power, or more. Altitude changes also cause trouble. IIRC every 2000ft is one jet change. If you're going uphill, your engine will go rich and lose power. If yur coming down, jetted for the top, she will go lean and maybe melt stuff.
>Alternatively, if you are not using first gear, why are you lugging it around?
What I mean is, once you realize that all you ever use it for is to take off with, then you might think to DOWNGEAR the back, to move the ratios especially second and third, to a better roadspeed..

Here's how it works; With the standard 2.66-box and 3.55s your roadgears are;
9.44-6.82-4.97-3.55 let's say you wanted 3.73 type performance which is
9.92-7.16-5.22-3.73 Plus 5% everywhere. How about 2.76s
........7.34-5.30-3.86-2.76 Plus 6.6% in 5.30ratio..... how about 2.94s
........7.82-5.64-4.12-2.94 plus 13.4% in 5.64ratio.You see what's happening? You Gain a hiway gear but lose a starter gear

If you run out of power in third gear with 3.55s/4.97 roadgear, your 340 is in trouble.
> If this happens at low rpm, you need a bigger engine.
>If it happens at high rpm, you need more airflow up there, which usually points to a cam and heads, or a bigger engine.
>If it happens in the midrange, more cylinder pressure may help.
 
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My basic recipe has treated me well in flat country to mountain roads;

3.23 if with 14" tires.

3.55 with taller 15" tires.

(the one exception for me there is with a C-body, 3.23 with 15" tires is perfect)
 
@Gtsjosh did you wver decide on gears? I'm near the Springfield area and going through the same delima now. Currently have 3.23s but wanting more gear for more spirited driving.
 
DentalDart said:
@Gtsjosh did you wver decide on gears? I'm near the Springfield area and going through the same delima now. Currently have 3.23s but wanting more gear for more spirited driving.
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Not yet. I’m actually leaving the country for work for about three months today. I spoke with doctor diff and I think I’m going to just order an entire third member while I’m gone with 3.91 from him and keep my 3.23 third member in the shop to swap in should ever take a long road trip with the car. I’m down in cape fair btw.
 
391's 489 case sure grip . This rear is out of this Duster it had 38 k on the car . I am keeping the rest of the rear for my Demon I have a 355 sure grip for it and I need the housing mine was braced
DoctorDiff is out of stock. Just looked and they get $1295 for aftermarket parts. This rear is factory built low miles and in great condition. Car was rotted hard sat on the Jersey shore.

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