Ok, I know I'm new here but here is my question.  I'm planning on having the rear end rebuilt this spring and I'm thinking about doing away with the original 3:36 gears.  Here are the particulars:

1969 350/350 M21 4spd. Convertible (Convertible=Heavy)

Rebuilt Original Engine:
Still has 11:1 compression
LT-1 intake
Holley 650 carb
Cam is slightly bigger than '70 LT-1 with solid lifters
MSD ignition and distributor
Headers
2.5" Dual exhaust with H pipe
Trans is original

Needless to say its built for the higher end and not for low end torque.  It's a dog out of the hole but really screams over 3500rpm.  It's all high octane fuel but  then we don't drive these cars for economic reasons.

Any thoughts on gear ratios?  I'm thinking about 3:55's or 3:73's.  I'm not concerned about fuel economy so much but I'd like to be able to drive 70 mph without feeling like the engine is screaming.  We live in the city and we don't take it on long trips (rarely over 60mph for more than 30 minutes) so we don't worry about overheating or the effects of living with high performance cars on long road trips. 

Any input from those that have experimented with gear ratios would be appreciated.  Especially those that have gone from 3:36 to 3:55 gears.  Will I even notice a difference?  Or will I have to go to 3.73's (and their inherent disadvantages) to notice anything? 

You will be able to notice the difference with the 3.55 gear set, compared to the 3.36, mostly at take-off. Your cruising rpm will be higher of course, but still acceptable.
Going from 3.36 to 3.73 will be a pretty good jump, and it will definitely be noticeable off the line. If you don't take longer trips in the car, then the 3.73 might be just the ticket.
You'll also need to change speedo gears with either ratio change, to keep the speedo correct.

The 3:55 is a good selection for what you describe as your usage. Anything much over 75 MPH and it seems like the engine is screaming it's guts out. Mine has the 3:55 with the ZZ-4 355HP, but with the 700R4 OD slushbox. No complaints at all. The OD trans cured my high speed screaming now turning 2300 RPM @ around 75 MPH where I usually cruise on long trips.

In your case the 3:73 with a 1:1 top gear ratio will run you nuts on the road.

Dave

Somewhere here there is an RPM calculator to determine engine rpm at speed if you know transmission, tire size and rear end ratio.

To All,       There is a site that will have all transmissions,gear ratios,tire size calculator along with a wealth of information..I think there is a few dollar charge for a 3 month access and well worth the charge relative to the accurate info. I have used it to install tires on the rear of vehicles to correct changes in the speedometer and to check out TACH's which were suspect. I then check this with the speed monitors the local authorities put on the side of the road to let you know how fast you are going.    The site is SMOKEmUP.com ,you have to type it in as it is shown here..Hope this helps..........Sal



(created "hot" link)Adams' Apple2008-01-25 20:15:13

Nuts...... I've been multiplying the tire diameter x 3.1416 for distance / rev and doing the math.....

That will work too. But measure from the pavement to the center of the wheel and double that for the effective diameter.

 I have 4;11 from the factory, its a bit low for any type of long trip driving but she comes outa the hole like a scalded dawg,nail it at 60mph in 4th and she leaps, but she loves gas,,all round Id go with 3;73,rd