Here's a good engine RPM calculator you can plug your numbers into. I send this to my customers when they are trying to pic a gear for our rear ends.  Click HERE...

If I recall the stock Turbo 350/400 torque converter runs at about 85-90% efficiency which relates to something like a 200 RPM higher rate with the same rear end ratio (compared to a 4spd) due to slippage. The above calculator compensates for this. You can jack the rear end of the car up, have someone turn your rear wheel exactly 1 turn by hand while you count the times your drive shaft turns over. You want to mark it with a fixed ref point off to the side of the drive shaft so when your down to that .36 .55 .70 .10 of your last turn you can figure your gear. Its doable....for example the dif between .36 and .55 is 68 degrees of drive shaft turn. You can visually see this with masking tape and sharpie marks. Hope that makes sense!
Both tires will need to be off the ground with a posi....turn the wheel forward.




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My Christmas present to ME! Bought Sat 11-30-2013. 72, 350 4 spd, Silver with blue interior...Can't you just hear Pablo Cruz playing in the background?

I can't speak for the autos, but there is a reason that for many years the 3.36 ratio came standard with the L-48 4 speed cars.  It is a good all around ratio that does not have you screaming along just going 60, but still decent enough off the line.  I wouldn't dream of changing it considering I've rebuilt for a pretty good bump in power (alleviating the need for lower gears), I plan to keep my original borg-warner (vs putting a 5 speed OD in for a lot of money) and I use this car as a cruiser vs a drag racer.   

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What's also helpful, is knowing what 4-speed trans you have.  Hopefully, you have the Muncie wide ratio trans with the lower 1st. gear (2.52).  If you have the close ratio (2.20 first) then you might want to consider nothing numerically less than 3.55 ratio rear gears.  

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1973 BB 4-Speed Coupe Project Car

Definitely 3:55.i run them with my T 400 auto. Rpm matches.

You have a 4.56 rear end.

http://www.prosystemsracing.com/calculate.html


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I think we should place some bets  . We be all over the board....

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My Christmas present to ME! Bought Sat 11-30-2013. 72, 350 4 spd, Silver with blue interior...Can't you just hear Pablo Cruz playing in the background?

Ok Feet, here's how to check the rear end gear ratio:

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1973 BB 4-Speed Coupe Project Car

I think that should tell the tale....
Without knowing the tire diameter, there's no way to calculate the ratio from just RPM/MPH. Jacking it up and spinning the tires, and counting the driveshaft revolutions will get you just about as close as you can without actually looking at the gear set.
There are only a certain number of ratios that will fit into the stock(original) diff housing. In this case, I think the 3.55 is all that would work, without changing the diff carrier. The 3.70 and 4.11 ratios that were available for the year took a specific carrier that would only be used for those two ratios. However, we also do not know if any of the stuff has been changed, so really, it's a crap shoot, and we're all just guessing....well...everyone except Bob, who happens to have the 3.55 gears, and turns the same RPMS at the same MPH....




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Joel Adams
C3VR Lifetime Member #56    
My Link


(click for Texas-sized view!)             NCRS

"Money can't buy happiness -- but somehow it's more comforting to cry in a CORVETTE than in a Kia"

My stock Silver Ann  and the Pace car both with the L-82, 4 speed and 3.70 tachs 3500 somewhere between 60 -65. I'll pay closer attention next time I'm out.
Maybe you got the 4.10 if doing 3500 right at 60mph. Converting to 4 spd they may have wanted that extra get up and go. 


|UPDATED|4/15/2014 2:35:25 AM (AZT)|/UPDATED|

Adding a little more precision Make a chalk mark on the driver shaft where it stops Using a sewing tape to measure the distance between the two marks and divide by the circumference to get the fractional portion. Example. If the the circumference is 5". And your final mark is 1.9" past your starting mark you would have a 3.36, 2.6 =3.55, 3.5"= 3.70. Actually even better in centimeters.

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http://www.vetteprojects.com/bmv