To figure out what you have, follow this:
Chock your front tires
Jack up one rear tire
Set trans in neutral, disengage parking brake
Mark driveshaft and tire with tape, chalk, etc.
Rotate tire EXACTLY 2 times while counting how many times the driveshaft turns.

This will give you a visual indication of your gear ratio.

If the driveshaft turns 3 times, it's 3.08
3 1/4 turns = 3.23
3 1/2 turns = 3.42
etc

Or if you want to drain the rear diff and put fresh fluid in you can pull the rear cover and look for markings on the ring gear. The gear ratio will be indicated by 4 numbers with a colon or backslash in between them; XX:XX XX/XX

First # is the quantity of teeth on the pinion gear, second number is quantity of teeth on the ring gear. Divide ring gear by pinion and it'll give you your numerical gear ratio.

 

Rotate tire EXACTLY 2 times while counting how many times the driveshaft turns.

I thought it was one complete turn of the tire not two. While turning the tire one turn count how many times the driveshaft turns. Also the gear that is turning the speedometer cable could be the wrong one for the rear gear ratio.

 

It turned 3 times. so...3.08? If so, should it be at such a high RPM at 60mph?

 

No, something is not right.
With about 26in tires, and a 3.08 rear at 3000rpm SHOULD be in the area of 76mph.

I'm basically in a similar situation. I did have a 3.07 12bolt in the 70, and ran across a very nice 2.73 12bolt and added a posi to it. Hiway driving is really nice now.
Many people are replacing 2.56, 2.73, 3.08 rears with ratios such as 3.73 and installing over drive transmissions. So I would think if would be fairly easy to find a good, complete rear in the 2.73 range pretty reasonable.

 

Not sure if this matters ..... the tires are p225/60/r15

 

Look online foe a speed calculator and punch in your numbers. Like DZ said somethings not right. Are you tach and speedometer reading correctly? The numbers you posted are more like a 3:73 ratio.

 

Could it possibly be a loose stall converter?

 

3,000-3,200 rpm around 60 mph is what my 3.73 rear with a 27" tall tire does, I agree somethings not right if you have a 3.08 rear gear

 

I will pull the cover this weekend and get the exact gear. I will post as soon as possible. Thank you all!

 

How are you measuring your speed? Check with a GPS or another trailing vehicle you know has a correct speedometer.

Your speedometer may just be off.

 

Sir, I would bet you have 3.31 ( the most common ratio at least in SS 12 bolts) + those short 25" tires does get you over 3000 at 62. 25" V 27" tires is worth around 0.3 of ratio.

Tom maybe right, you may want to find a set of 2:43 or 2:73 gears for the 10 bolt or stuff a 200R4 in there , alternatively. For 3X the cost of a gearset installed, mind you.

 

This may help you determine your gear ratio that you have or want to have, but you will need to measure your tire diameter.

 

I'd remove the rear axle cover and count the teeth to make sure about the ratio you have. No need to swap the whole rear end if you're not into racing or any stuff like that. Keep your current housing and just change the gear ratio for something like 3:08, and maybe add a posi and you should be good.
Good luck,
Claude.

 

Gearing Calculator | RPM and Tire Diameter to km/h - mph with Shift Tracer (blocklayer.com)

225 60 15 = 25.6 inches
60mph
5% convertor slip
2823 rpm = 3.42 ,,, with no slip 2687
2670 rpm = 3.23 ,,, with no slip 2550

 

Transmission? Engine? powerglide (2 speed)? 283? If rear is 'higher' (lower numerically) than 3.31, will it be a 'decent' overall driving car? I think not.

Is not Tom's (DZAUTO) car a 400 sbc V8 and a 4 speed manual transmission? A very different combination.

Recommend consider the entire power train combination when contemplating changes. A 'mismatch'? Not good.

Pete

 

Transmission? Engine? powerglide (2 speed)? 283? If rear is 'higher' (lower numerically) than 3.31, will it be a 'decent' overall driving car? I think not.

Is not Tom's (DZAUTO) car a 400 sbc V8 and a 4 speed manual transmission? A very different combination.

Recommend consider the entire power train combination when contemplating changes. A 'mismatch'? Not good.

Pete

That is correct, but it is a very compatible combination. The SB400 is a well built healthy engine with a moderate cam profile, thus, very agreeable on the street and still strong. The transmission is a built M22W(wide ratio M22) and the rear is a 2.73 12bolt posi (was a 3.07 12bolt). NO, it is not a rocket from a dead stop, but the 400 has plenty of torque to get it rolling with virtually no clutch slippage.
Because of the 2.73 rear, 60-70mph in 1st gear, getting into Interstate traffic is just too smooth. And 75mph on the interstate in 4th gear is like having a 3.73 rear with an OD tranny. It's been a good combination that I'm very happy with.

This is when the 70 had the 3.07 rear. Embarrassed a few 396 cars!

 

I have a 1968 Chevelle that I bought 4 months ago. It runs at 3000 to 3200 rpm at 60 to 65 mph. I do not need low end power due to my age. (No burn outs for me) I need a rear end that will cruise on the interstate. I have never changed a rear end out or changed gears. I need advice as to what is best? I feel with my mechanical skills I can accomplish either. (YouTube Vids / friends)
What is the best gears to buy if I go that route or what complete rear end do I search for to buy?
I don't know the ratio of this rear end. It is a 10 Bolt NON posi.
Any advice is GREATLY Appreciated.

View attachment 736821

 

Hey, on your axle is the ID number you can decode that to find out what gear ratio you have

 

That assumes you still have factory gears....which is NOT a given in 2022.

 

Personally, I think your speedometer isn't reading correctly.
I run 3.36 rear gears on 255/60/15 tires and cruise at a much lower RPM than you are reporting.
That said, here is some advice that hasn't been given yet:
It's my personal belief that gear swaps are best left to professionals who do it for a living.
If you're mechanically inclined and can research how to swap gears, it's very doable...on face value.
But you'll probably wind up with whiny gears b/c they aren't set up perfectly.
The local rear end guy here has my gears factory quiet...or better. Money well spent, IMHO.

 

The only thing you are going to save changing gear ratios is fuel economy. 3000-3200 isn't going to hurt anything.
You should verify the speedometer with a GPS app on your phone before doing anything. Sounds like it's an original rear which is probably a 2.73 or 3.08.
Also, from someone that has built a bunch of rears, if you don't have the tools, don't attempt it. It's not that simple.

 

Also, from someone that has built a bunch of rears, if you don't have the tools, don't attempt it. It's not that simple.

THIS....^^^^^^^^

 

hi
yoke holder
drag gauge - dial tension wrench
pinion height basic tool
carrier shim driver
housing spreader
test bearings
dial indicator and stand
work shop vise and bench no toys

install kit either shims/crushable spacer
or
solid spacer will require lathe
Lsd set up clutch or Torsen /trutac

Setup time will be half the time taken for the job
Shimming the diff is either right or wrong takes time

 

I was in the same situation with my 70 Model. Instead of changing gears or rear end. I changed transmissions.

 

I owned a Corvette with 3.08, great highway cruising gear.

 

Your car will fit a 28" tall tire in the rear. I would borrow a set from a friend and see how you like it. If yes buy new tires...... if no change gears or add overdrive transmission. My guess is going from 25" to 28" rear tires will lower rpm 3-400

 

I have seen with some converters my heavy 57 4 door will slip over 10%
Add more weight or more speed and closer to 15%.
Supposed to be a 3000 stall 12".
Terrible way to get 3000 stall.

 

If it was me, I'd change the rear to a 12 bolt with the gearing I would want.

 

Fantastic advice from you all. I plan on pulling to cover Saturday and will update. If I change the gears I will pay to have it done after the great advice from ABOVE.

 

You shouldn't have to count the teeth if you can find the ratio code thats stamped on the side of the ring gear or the end of the pinion gear.It will be like a two digit high 30's number along with another two digit lower number.Something like 41-11 or 39-11 or a set of numbers similar to that.
If you find that you can divide the larger number by the smaller number and you'll have your ratio.