Gear ratio options for a 69 350/350 M21

[John M.]

I currently have a stock 4:11 rear in my 69 350/350 M21.

I read that the 3:70 was standard on the 350HP with M21 and the 4:11 was optional.

That dosen't make sense to me as both of these gears seem a little steep for highway cruising. It dosen't seem like I read the correct information if these are in fact the only 2 options with this configuration.

I prefer to have a 3:36 gear or similar for some good highway cruising but really don't want to put in an incorrect set-up or ruin low end performance with the gearing in the M21.

Can somebody tell me what was optional in 69 and perhaps share some personal experience with the most desirable set up with the L46/M21 for performance and drivability?

I'll need to order a new rear soon as I'm attacking suspension and rear end first.

Thanks

John McNeely

[Warren F.]

Re: Gear ratio options for a 69 350/350 M21

John,

The L46 engine option came with two different drivetrain combinations. If one ordered the 350 cid/ 350hp engine with an M20 wide ratio transmission, std rear axle ratio was 3:36 with 3:55 ratio as an optional performance ratio.

If one ordered the L46 engine with an M21 close ratio transmission, std ratio was 3:70 with 4:11 as the optional performance ratio.

[Arland D.]

Re: Gear ratio options for a 69 350/350 M21

John,

My 69 350/350 had the 3.70 rear and it seemed like the perfect setup to me during the fifteen years I owned it.

Arland

[John M.]

Re: Gear ratio options for a 69 350/350 M21

John,

My 69 350/350 had the 3.70 rear and it seemed like the perfect setup to me during the fifteen years I owned it.

Arland

Thanks Arland,

It wasn't winding up too high at highway speeds?

That 4:11 rear leaves me doggish after 75 MPH. I can't imagine the 3:70 offering much relief.

Of course freshing up the motor will likely make a world of difference.

So the question is, is it worth the cost of regearing?

[Arland D.]

Re: Gear ratio options for a 69 350/350 M21

As I recall (large loophole) it tached somewhere around 3500 at 70 mph. It was used for everything around the Bay Area and San Francisco. A trip to Texas had me wanting better highway gears but it all depends on how you are going to be using your car. Regards,

Arland

[Terry M.]

Re: Gear ratio options for a 69 350/350 M21

My LT1 with 3:70 (standard gear for that motor) does about 3200-3300 at 70, which is nicely into the power curve of that motor The 350 hp motor is not that different. I think the 3500 at 70 is more like 4:11 range.

The 3:55 with M20 was the hot set-up back in the day. Good gears for acceleration and reasonable cruise speed as well.

We have gotten too used to modern cars with their low RPM cruising. Modern engines are built with their torque band much lower in the rpm range to allow for this. Of course 6-speeds help too.

[Alan T.]

Re: Gear ratio options for a 69 350/350 M21

Terry, glad to hear your comment about the 3:55/M20 setup. That is what my '70 LT-1 has, and in the early years of my ownership of the car I thought it was "inferior" to the 3:70/M21 and several owners of LT-1's poked fun saying their car would be quite a bit quicker than mine in a race. Later I learned that in the day, those-in-the-know ordered the 3:55/M20 as the combination was actually the "hot" setup, confirmed by studies of GM engineers who often used that combination in Bob Wingate's specially-built Corvettes. Anyway, only one way to know for sure - you and me drag 'em for pink slips

Alan

[John H.]

Re: Gear ratio options for a 69 350/350 M21

I prefer to have a 3:36 gear or similar for some good highway cruising but really don't want to put in an incorrect set-up or ruin low end performance with the gearing in the M21.

You would NOT like a 3.36 (or a 3.55) with the M21 transmission for in-town driving, although it would be fine with an M20.

[John M.]

Re: Gear ratio options for a 69 350/350 M21

And that was my next question, whether to go with the 3:55, 3:70, or stick with the 4:11.

I can't imagine the 3:70 offering up much relief from the 4:11, but I intend on doing some highway driving so I guess it's worth the cost.