Gearing on a BB

[vipergts]

My 66 BB seems to revs its socks off.

I recon the gearing must be too low or high (?) I can't use 1st and 3rd is pointles. In any gear if I give it some welly it's all over in a second and I'm having to find the next gear.

What should the gearing be. In not too technical terms. If I'm doing 60mph in 4th what revs should it be.

Also maybe how many prop turns to a full wheel turn.

As you can tell, I'm not technical.

Thanks Guys....hope you can help.

 

[67HEAVEN]

Since there were so many different rear end ratios available from the factory, it's hard to say what the revs "should be" at 60mph.

You can jack the car up and count the number of revolutions of the driveshaft vs. revolutions of the wheel to determine the ratio on your car. Remember safety if you plan on getting under. Detailed instructions (including dealing with poistraction) are included on this link...... http://www.csgnetwork.com/rnggrpinioncalc.html

Report back to us on the numbers and we can make some suggestions. Also, measure your rear tire "height" as any change from original sizing will make a difference in rpms. Mine are 26.5" tall.

In my case, I have a 3.08:1 rear gear and although I've changed out the M-21 Muncie for a Richmond 5-speed, 5th gear is still 1:1, just like your 4th. My revs at 60mph are approximately 2,300 rpm.

 

[67l36driver]

With a 3.36, 3000 rpm gets 70 mph in 4th gear. Even then 2nd gear starts are do-able with an M21 close ratio.

 

[67HEAVEN]

I used the calculations when choosing a transmission for my project. They may help you understand the relationship between various transmission gears, rear-end ratios and tire size.

 

[JohnZ]

Also maybe how many prop turns to a full wheel turn.

Mark both wheels and the driveshaft, and turn both wheels one full turn, and count the revolutions of the driveshaft. The number of driveshaft turns will tell you the rear axle ratio. 3-1/3 = 3.36, 3-1/2 = 3.55, 3-3/4 = 3.70, 4-1/8 = 4.11, 4-1/2 = 4.56.

Assuming the gears inside the diff housing haven't been changed, there's a stamping on the bottom flange of the diff housing between the rear spring plate and the camber strut bracket that shows the ratio code and the date the diff was assembled; will require heavy cleaning to see it.

 

[vipergts]

John.....Might you be suggesting my car is dirty?:L

Thanks guys for the great response. If the weather gets better next week I shall take the car out and check the details and report back.

Thanks again.

 

[vipergts]

Right......I have the information.

One turn of the wheel represents 3.75 turns of the prop.

Probably more ureliable because of speedo and rev counter callibration at 70mph the car tells me it's doing 3750 rpm.

My Brother in Law has a 67 small block which tells him he's doing 2750 rpm at 70.

Your help as ever is much appreciated.

Also the date on the diff is 11 19 65

 

[vipergts]

Also.

Is this a relatively easy job for a qualified mechanic and probably more importantly, what parts should I replace while I'm in there?

 

[JohnZ]

Right......I have the information.

One turn of the wheel represents 3.75 turns of the prop.

Probably more ureliable because of speedo and rev counter callibration at 70mph the car tells me it's doing 3750 rpm.

My Brother in Law has a 67 small block which tells him he's doing 2750 rpm at 70.

Your help as ever is much appreciated.

Also the date on the diff is 11 19 65

If it was a 3.70 axle (assuming stock tire size - 27" diameter), 70mph would be 3224 rpm; 3750 rpm at 70 mph would be between a 4.11 and 4.56 axle.

The small-block example (2750 at 70) would be between a 3.08 and 3.36.

Looks like some speedos and tachs need to be calibrated.

 

[vipergts]

Thanks again for your help.

I've ordered the 3.08 gear and recon kit for the diff whilst I'm there.

Thanks