Gear ratios and driveability

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Go Mango

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My 70 Dart came with 3.91 sure grip.
I feel that driving it 70 mph for extended miles is asking a lot of my 340. I have the stock 14" wheels with 245/60-14 tires. My buddy works at NAPA in southern Indiana, and hes got a customer with a 69 roadrunner 383 with 3.91 sure grip that he drives on long hauls. But it's got 15" wheels.
Am I thinking correctly that the 15" wheels are changing things for him enough to allow better highway speeds?
 
I'd go 15 inch tires & then go with a 4:10 gear.
Well...I ain't gonna do that. It looks too cool the way it is. But are the 15" wheels (larger) why hes got so much better highway driveability?
20180813_190414.jpg
 
Overall tire diameter affects the final drive ratio.

If a taller 14" is available you could try that?

For me & all around driving (no track) I prefer a 3.23 with 14" tires & a 3.55 with 15" tires.

3.91s at 70 MPH can be done, but it is not something I want to do myself for very long.
 
Overall tire diameter affects the final drive ratio.

If a taller 14" is available you could try that?

For me & all around driving (no track) I prefer a 3.23 with 14" tires & a 3.55 with 15" tires.

3.91s at 70 MPH can be done, but it is not something I want to do myself for very long.
Thanks! That's a good answer! My tires are really oversized for those damn skinny wheels, but they seem to work, so I'm ok with that. I was really just curious as to how a 383 could do it easy and I cant!
 
Google / search "gear tire speed calculator" and you will find HUNDREDS of sites that you plug in your gear ratio and tire size, and it will compute engine RPM at various speeds.

In the 70's when "I was a kid" we had to do it the old fashioned way........measure around the tires and USE MATH
 
throw some 2:94 gears in there and enjoy. :)
I grew up with a 72 chevelle that my dad bought new. It had the Buick/Olds/Pontiac 10 bolt rear end that had 2.56 gears. And I put 15" Corvette wheels on it. It was cool, but couldn't get out of it's own way!
I really like the 3.91s!
 
15inch steelies help withs the high rpms also helps with braking. I went from 14 to 15 big difference on the highway. I have a 15x8 inch rim with 255 60 nice and wide and has a good height. But that's in a 68 barracuda.
 
Rim diameter has nothing to do with it, the overall diameter of the tire sets the ratio. Now, there aren’t a lot of tire options for 14” rims so I guess that does come into play some.

A 245/60/14 is only 25.6” tall, which is fairly short for a rear tire. With those tires and 3.91’s you have to turn 3,600 to do 70 mph.

If your B-body buddy is running 255/60/15’s, which would be pretty easy, those are 27” tall. He only has to turn 3,400 rpm to do 70 with those. If he has 275/60/15’s, which wouldn’t be shocking, those are 28” tall and would mean he could cruise at 70 at just under 3,300.

Tire diameter makes a big difference. But freeway speeds for long hauls with 3.91’s is asking a lot. 3.55’s would make life a lot easier at freeway cruising speeds.
 
I'm really ok with what I've got. I've just been beat up a little over not wanting to drive long distances.
The wife wanted me to go to a car show on the opposite side of indy on Halloween, I told her she was crazy! Then it was "what's the point of having it if you can't drive it?" I told her its because it's a effing race car! And that was that.
 
Many, many moons ago, I used the stock 3.91's in my 383 Challenger R/T 4spd, around town, and swapped a 2.94 pumpkin in it when a road trip was in order.
I often wondered what kind of top end it had w/ the 2.94's. I redlined 3rd gear once, we was movin'! (Still had the 3.91 speedo gear.)
Of course, I was a lot younger and much more energetic then!
But, I digress...
 
Ordered my '70 Cuda 340 with 4-speed and 3.91 gears. Came with F 70 14 tires. Ran about 3,000 rpm at 60 mph. Cruised the roads in western Kansas for hours at 80 mph (4,000 rpm). Assuming you have the 340, it has an almost perfect bore to stroke ratio and can wind all day. It will not hurt the engine, if it is in good shape. People have become used to modern cars that turn 2,000 rpm at 75, which makes the older cars seem odd. If it is a hobby car that seldom goes on long trips, keep the 3.91s it will be a lot more quicker, but you will have to settle for a 120 mph top end.
 
Ordered my '70 Cuda 340 with 4-speed and 3.91 gears. Came with F 70 14 tires. Ran about 3,000 rpm at 60 mph. Cruised the roads in western Kansas for hours at 80 mph (4,000 rpm). Assuming you have the 340, it has an almost perfect bore to stroke ratio and can wind all day. It will not hurt the engine, if it is in good shape. People have become used to modern cars that turn 2,000 rpm at 75, which makes the older cars seem odd. If it is a hobby car that seldom goes on long trips, keep the 3.91s it will be a lot more quicker, but you will have to settle for a 120 mph top end.
I agree. I have the original 340. But it was rebuilt years ago by a guy named Sam Jones. He won the C/GS NHRA title in 1962 and 1963. He also won Gas Street Eliminator at indy in 1962. His shop is 10 minutes from where the original owner lives. Unfortunately, no records exist regarding the build.
 
I can tell you this. I've driven fast cars, but nothing like this. It's a monster. The 245/60s have no chance at all hooking up. That's okay! I LOVE it!
 
I grew up with a 72 chevelle that my dad bought new. It had the Buick/Olds/Pontiac 10 bolt rear end that had 2.56 gears. And I put 15" Corvette wheels on it. It was cool, but couldn't get out of it's own way!
I really like the 3.91s!

our dart with 2:94 gears and 26" tires pulls real strong down low and can cruise the highway with ease at 80mph..:)
 
If you were closer I'd trade you my 3.23 sure grip, i have a 3.91 gear set ready to go it would be easier to just drop the center section.
 
A little off track here, but what I don’t understand are folks that won’t lay into it out of a red light but see no problem winding the motor up to 3k or 4k on the freeway. Talk about efficiency...
 
Heck I’ve got 3:23’s and 225/60/15 tires (around 25.6” tall) and I don’t even like doing 70 for more than a few minutes. I have notes at home that I took down the other day, noting the rpm’s at 40, 50, 60, and 70. I believe 70 mph was like 3450 rpm.

I’m considering two options, because I’m on the highway a lot. Install a set of 2.94’s or even 2.76 gears, although that’s going to kill my around town performance.
Or option 2 is to install an A500 tranny for the overdrive. And maybe 3.55 gears, which will give me good performance around town, and make it cruise very comfortably on the highway.

Of course option 2 is a lot more expensive and a lot more work. I just feel like there really is no good compromise on gearing unless you throw in overdrive.
 
A little off track here, but what I don’t understand are folks that won’t lay into it out of a red light but see no problem winding the motor up to 3k or 4k on the freeway. Talk about efficiency...
Short term high rpm, maybe ok.
Long term high rpm, maybe recipe for disaster?
Call me a coward, but I'm really not sure what my engine is good for, and I don't want to find out the bad way!
 
Heck I’ve got 3:23’s and 225/60/15 tires (around 25.6” tall) and I don’t even like doing 70 for more than a few minutes. I have notes at home that I took down the other day, noting the rpm’s at 40, 50, 60, and 70. I believe 70 mph was like 3450 rpm.

I’m considering two options, because I’m on the highway a lot. Install a set of 2.94’s or even 2.76 gears, although that’s going to kill my around town performance.
Or option 2 is to install an A500 tranny for the overdrive. And maybe 3.55 gears, which will give me good performance around town, and make it cruise very comfortably on the highway.

Of course option 2 is a lot more expensive and a lot more work. I just feel like there really is no good compromise on gearing unless you throw in overdrive.
I've got a 4 speed, and like I said, I love the car the way it is. I just wanted an explanation for my questions.
 
Install a set of 2.94’s or even 2.76 gears, although that’s going to kill my around town performance.

i think you would be surprised at how well the 2:94 performs..

i had them in my yellow dart with a stock 74 360 and a 28" tire and was real happy with them... now in jamies dart with a mildly built 360 and 9.5" converter its a rocketship...
 
I had 4.56 gears with 245/60/14 tires in my 68 dart with 340 and 727. I believe it was 3000 rpm at 45 mph. Way over 4000 rpm at 60 mph. I drove to the track once that was 130 miles away. It was a very long drive. I got pulled over by the popo for going too slow. Kim
 
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