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Rear end ratio change

Longknife

Longknife

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Hello. I have 69 rr with a mildly built 383 4 speed. It has a 3.23 open rear end. I am wanting to put in a sure grip with a lower ratio. I am running a 28 inch tall tire. I punched in the numbers to a gear ratio calculator and it said I need 3.46 gears so a 3.55 would be about perfect! Now I am thinking maybe going a little lower maybe 3.73 or 3.91, This car is mostly for cruising, maybe a couple of 100 mile runs a season (oh and a random burn out here and there)!! Any opinions? Thanks, Ed
 
3.55 and 28 inch tire is a pretty good combo. If you do 100 mile trips and cruise with the car, you don't want to go any lower.
 
Hey Ed, I put 3:55s in my '69 SuperBee, 383 & 4spd. I loved 'em ! Car would light the tires up, and cruised great. I wish I'd have kept that one...
 
Longknife said:
Hello. I have 69 rr with a mildly built 383 4 speed. It has a 3.23 open rear end. I am wanting to put in a sure grip with a lower ratio. I am running a 28 inch tall tire. I punched in the numbers to a gear ratio calculator and it said I need 3.46 gears so a 3.55 would be about perfect! Now I am thinking maybe going a little lower maybe 3.73 or 3.91, This car is mostly for cruising, maybe a couple of 100 mile runs a season (oh and a random burn out here and there)!! Any opinions? Thanks, Ed
Click to expand...

a 3.73 and 3.91 are higher than 3:55 and 68 is right 3:55 perfect.
 
Go with 3:55s. I have 3:91s in my 70 / 383 / 4-speed and it's okay for around town but too steep for highway cruising. I'm looking at 3:55s too.
 
Based on what you said, "This car is mostly for cruising, maybe a couple of 100 mile runs a season". I think you already have the perfect combo. That's what I am running and in the same type of driving you describe and I wouldn't think of changing it. Just my $.02.
 
Ma Mopar never put a 3:73 in any cars to my knowledge. You need to get the correct speedo gear for the trans
with your tire height.
 
You won't like the 3.91's if you do much 60 mph or faster driving. I have run 3.91, 3.55, and 3.23's in my Road Runner with 28" rear tires. 3.55's are not bad, but if you have a stroked motor with the torque curve down low, I would seriouly consider the 3.23's.
 
Longknife said:
Hello. I have 69 rr with a mildly built 383 4 speed. It has a 3.23 open rear end. I am wanting to put in a sure grip with a lower ratio. I am running a 28 inch tall tire. I punched in the numbers to a gear ratio calculator and it said I need 3.46 gears so a 3.55 would be about perfect! Now I am thinking maybe going a little lower maybe 3.73 or 3.91, This car is mostly for cruising, maybe a couple of 100 mile runs a season (oh and a random burn out here and there)!! Any opinions? Thanks, Ed
Click to expand...

Ed, can you get more specific on how much highway driving in a year? Are you saying 2 - 100 mile highway trips per year and that is about it? Also, what is your meaning of "cruising".
 
I’m running a 3.73:1 with a 28” tire.
I’m happy with it, and I drive it in the country (55MPH) and on the freeway a lot.
Keep in mind, when new, these cars had short tires (25.5 or so)
A 28” tire makes a taller gear shorter.
 
vintage chromoly said:
I’m running a 3.73:1 with a 28” tire.
I’m happy with it, and I drive it in the country (55MPH) and on the freeway a lot.
Keep in mind, when new, these cars had short tires (25.5 or so)
A 28” tire makes a taller gear shorter.
Click to expand...

I was a bit surprised by most everyone's responses. I'm the guy always recommending 3.23s for 440 auto cars. A 383 - 4 speed and a 28" tire is a different story. With limited highway driving, 3.91 is where I would land. The 383 needs so much more help than a 440 on the torque side.
 
Last edited:
Longknife said:
Hello. I have 69 rr with a mildly built 383 4 speed. It has a 3.23 open rear end. I am wanting to put in a sure grip with a lower ratio. I am running a 28 inch tall tire. I punched in the numbers to a gear ratio calculator and it said I need 3.46 gears so a 3.55 would be about perfect! Now I am thinking maybe going a little lower maybe 3.73 or 3.91, This car is mostly for cruising, maybe a couple of 100 mile runs a season (oh and a random burn out here and there)!! Any opinions? Thanks, Ed
Click to expand...

I would do both. As in you have the 3:23 set up leave it as is just replace the center chunk with a lower gear one like you asked about.
Prob the 3:73 or 3:91 with sure grip. No longer than it takes to swap them you can always pop the 3:23 chunk in if your doing a cruise.
I would not for 100 mile ride , but power tour leg or something like that.
 
I have the 3.91 gears with 28" tall tire and it's great around town but not highway friendly at all
At 70 mph I am doing about 3300 RPM. To much for 100 miles
I stay off the highway when possible but I have my own trailer
In your case I would use the 3.55s
 
Your going to shorting engine life,use lots more gas
at highways speeds & the noise level will
drive you nuts with 3.90.
I'd go no lower than 3.54or 3.55 unless
you stay in city always.
 
Thank for all the replies. BSB67, Most of my "cruising" would be going to local car shows and cruises within 50 miles of my home. mostly at lower speeds and on 55 mph highways. I do want to be able make a couple longer trips, not over 100 miles on the open highway.....Thanks again ,,,Ed
 
Longknife said:
Thank for all the replies. BSB67, Most of my "cruising" would be going to local car shows and cruises within 50 miles of my home. mostly at lower speeds and on 55 mph highways. I do want to be able make a couple longer trips, not over 100 miles on the open highway.....Thanks again ,,,Ed
Click to expand...

That's what I thought. So far, I'm still leaning toward the 3.91, if it were mine. If your motor is nearly stock, maybe the 3.73.

What has been done to your 383? Cam, headers, intake, heads, and compression ratio?
 
'69 RR, modified 383 with a rebuilt 727. 8.75 sure grip with 3.23, 255X60X15 on the rear. I'll take it out for a 1-2 hour drive putting anywhere from 50-100 miles on the freeway and canyons without issue. Still able to pass mostly everything without excessive RPM's. I would not recommend anything more than 3.55 for cruising with occasional longer drives.
 
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