Ring & Pinion question

[moparbeepbeep]

I am guessing that I have 3:92 gearing in my

70rr​

383 3spd auto as it's running around 3900 rpm at 55mph.

I've been reading a lot of the different forums but still a lot to take in.... I'm wandering about 3:23 vs 4:10 … I'd be able to drive to shows and cruises and down the highway without sounding like it's doing 100... Also can I get away with just a ring and pinion gear or will there need to be a lot more done ?

Looking forward to your input.

 

[funknut]

I went with a 3.55 gear in mine and it can cruise reasonably at 60 without feeling like it’s turning too many revs. To me it feels like a good compromise, but opinions vary.

There are calculators online that can tell you your RPM based on trans, tire size, gear, etc. I’d play with one of those and see what you prefer.

And yes, you’ll just need to change the ring and pinion, and whatever other worn parts may be in there like bearings, seals, etc.

 

[pro-streeter]

You have more of a race gear in the car. If you are not into Friday night grudge racing you should go with the 3.23.
This will make it much more enjoyable for street use. The ring and pinion can be changed. You will need someone to set the gear lash. Also it would be good to replace all the bearings and seal if you are unsure of condition.

 

[beanhead]

Hold on here....is your tach working right? You'd have to be running much bigger (like 4.88) gears and a tiny rear tire to turn that rpm at that speed. I have 275/60/15s (28") tires and 3.73 gears, 2700 at 55mph. To know your gear ratio, jack the rear end up, put it in neutral and turn the driveshaft by hand, count how many times it turns compared to one full turn of the tire. If you really do have 3.91 gears, your tach is malfunctioning or there's a serious slippage issue in the trans/converter...

 

[Demonic]

Do the twist and count and figure out what you have. It's not hard. I really like my 3.55's in my Charger.

 

[66Satellite47]

Agree with Bean, jack the rear up rotate the tire exactly 1 revolution while watching the U joint, count how many turns the U joint makes. Then you know what you have. A 3.91 with a small 26" tire should be under 3000 at 55 mph.

 

[Demonic]

Oh- if you don't have a Sure-Grip, you'll have to rotate the tire twice, and count the pinion rotation.

 

[Charged440]

Another vote for the 3.55s, I'm using that with a 275-60 tire. Seems a nice compromise between highway and street.

 

[66Satellite47]

Demonic, I believe you are mistaken.

 

[Runner71]

Im doing 55.9mph @ 2900 rpm with a spanking new set of 3.91, 275 60 15, 28" tire

 

[Daves69]

Im doing 55.9mph @ 2900 rpm with a spanking new set of 3.91, 275 60 15, 28" tire

hmmmmm.....
https://www.strangeengineering.net/gear-ratio-calculator/

 

[Runner71]

hmmmmm.....
https://www.strangeengineering.net/gear-ratio-calculator/

View attachment 947513View attachment 947512

Well thats theoretical numbers but give or take 100rpm and the speedometer is slightly of if i check with gps. Also im not 100% @ 55mph as im rolling in kmh. But no were near 3900rpm even at 68mph/110kmh highway speed but deff not enjoyable. Spin the tires and count

 

[33 IMP]

Something is wrong with the op's numbers. Either he's got a monstrous gear in the car, his tach is way wrong, or his trans/converter is absolute garbage.
Now it is just diagnosis time.
One...find out what the rear gear really is. Two....beg,borrow,steal a tach/dwell meter, to double check the tach. Three....check for burned/low fluid in the trans.

 

[moparbeepbeep]

Thanks for everyone's input... I will first check tire vs drive shaft rotation to verify gears... even though the tach is after market I feel it's close to accurate. (at least at idle).. I'll get back with everyone when completed.

 

[Demonic]

Oh- if you don't have a Sure-Grip, you'll have to rotate the tire twice, and count the pinion rotation.

Demonic, I believe you are mistaken.

Kinda. To be clear, it depends on weather one or both tires are off the ground. Try it if you have an open rear.

 

[33 IMP]

Determining rear gear is easy with a sure grip. If you have an open, usually with both tires off the ground , turn one wheel, the other wheel turns the opposite direction, and the driveshaft doesn't move. With one tire on the ground turn the other two turns to get the gear ratio.
Check your aftermarket tach for a cylinder number switch.(4,6 8,10,12). If it's wrong, it won't be off much at idle, way off going down the road.

 

[66Satellite47]

OOPs, Guess I've been too long without a one wheeler!

 

[moparedtn]

Speedo might be a tad off as well. Easy way to verify is with an app on your cell phone.
Oh, to toss in another set of data for an example, my GTX has 3.55's and 28" tires (275/60/15).
2800RPM is 65MPH, pretty much dead on.

 

[beanhead]

Something is wrong with the op's numbers. Either he's got a monstrous gear in the car, his tach is way wrong, or his trans/converter is absolute garbage.
Now it is just diagnosis time.
One...find out what the rear gear really is. Two....beg,borrow,steal a tach/dwell meter, to double check the tach. Three....check for burned/low fluid in the trans.

Absolutely!
Do a little detective work before tearing into it! Nothing's more deflating than spending time and money only to find out there's still an issue..

 

[33 IMP]

Just for giggles, I calculated op's gear ratio, if his numbers are right. With a 27" tall tire, and 6% converter slip, his rear gears would be....... 5.37. So either 5.17s or 5.57s. Is this a mud bogger? Sorry, just kidding.