Advice needed with rear gear decision

[John M.]

I have a 69 350/350 M-21 close ratio trans geared with a 4:11 rear.

I really want to run a 3:55 gear but was advised against it due to the gearing of the M-21.

With 3:70 being the standard gear, I really don't want to second guess GM engineering just because I want a decent highway gear.

Would you advise against this swap due to poor drivability off the line?

[Bill M.]

Re: Advice needed with rear gear decision

I have a 69 350/350 M-21 close ratio trans geared with a 4:11 rear.

I really want to run a 3:55 gear but was advised against it due to the gearing of the M-21.

With 3:70 being the standard gear, I really don't want to second guess GM engineering just because I want a decent highway gear.

Would you advise against this swap due to poor drivability off the line?

If the 3.55 is unacceptable, swap the close-ratio for a wide-ratio. You'll have the same overall ratio in 1st, 2nd, 3rd as a 4.11 with a close ratio.

[Stuart F.]

Re: Advice needed with rear gear decision

I've lived with a 3.36 and close ratio tranny now for 45 years, and that's with an L-76 solid lifter engine. I still have the original clutch too even after 44.5k miles. Actually, once you get rolling which is a technique you learn quickly, you shift far fewer times. Often, in city traffic, I start in 1st and may/may not get into second by the next light. Highway driving is a lot of fun too as you can use 3rd as an extra umph! passing gear on two lane roads. However, you usually have enough torque to manage most situations in 4th.

Stu Fox

[John M.]

Re: Advice needed with rear gear decision

If that's the case, then you would think a 3:55 should be reasonably drivable with the M-21.

I just wouldn't want to burn up the clutch and shudder everytime I start out in first or run doggishly until it hits 50MPH just for the sake of reasonable RPM highway cruising.

On the other hand, I really don't want to be winding out at 75MPH.

I was hoping the 3:55 would be a happy medium but that's quite a bit of coin just to experiment.

[Dennis C.]

Advice needed with rear gear decision

If you do a 3.55, that will be very, very driveable. You may even ask yourself why you didn't do a 3.23 or a 3.08. Probably depends on the "traffic area" you live in. I feel 4.11, 4.56, 4.88, etc are great for Saturday night or the drag strip, but nothing else.
My current '62 is a 300 hp, wide ratio 4 speed with a 3.08 screw. Most "driveable" (old) Corvette I've ever owned.
Of course, my '67 Chevelle 396 SS was pretty fun, too

[John M.]

Re: Advice needed with rear gear decision

If you do a 3.55, that will be very, very driveable. You may even ask yourself why you didn't do a 3.23 or a 3.08.

Your referring to use with the M-21 close ratio?

[Dennis C.]

Advice needed with rear gear decision

Certainly 3.55, 3.23 with close ratio box will be great. 3.08 screw will be happier with wide ratio box or "open highways." City stop and go are a different animal - I hope I never have to deal with again in my lifetime.

[Stewart A.]

Re: Advice needed with rear gear decision

Every time I change to lower rear gears I'm happy on the highway and really pissed off at the lights. ''It's up to you''. Unless you put 6 forward gears in the car there is no solution.
Which the judges would love to see

[Dennis C.]

Advice needed with rear gear decision

Stewart - You make a good point for Saturday night - however, when I drive my car on a "road tour" from Lyle, WA to Williamsburg, VA, seems like I shift through the gears about six times over the 3,000 mile 1 way trip... Guess ya just gotta make a choice and do what'cha do... Best, Dennis

[Timothy B.]

Re: Advice needed with rear gear decision

John,

My choice is the wide ratio and 355 gears. Install new 355 gears in your original rear housing and keep the 4.11's if you ever decide to go back or sell the car. Do the rear overhaul now and change the transmission gears at a later date if you desire.

[John M.]

Re: Advice needed with rear gear decision

Thanks to all for the advice.

I'll try the 3:55 and go through the tranny later if desired.

[Jack H.]

Re: Advice needed with rear gear decision

An RMC member with a '65 M21/4.11 on his 365 HP convertible changed to an M20/3.36 configuration and was VERY happy with the results...

The wider ratio 1st gear of the M20 vs. M21 compensated for the rear end change without affecting the 1:1 cruising profile of 4th gear for highway use.

[Jay G.]

Re: Advice needed with rear gear decision

John, The 3:55 gear is very streeetable and Hwy friendly. My 67 Chevelle SS 396 has that set up with factory M-20. The Best overall (Not radical) off the line and Hwy gear. With taller tires I'm at 2200 RPM at 70MPH. Ranch Country were we live. Need a Hwy gear. Wish my 67 Vette had em. It's M-21 with 3:70 gear. Factory stuff.

[Jim T.]

Re: Advice needed with rear gear decision

The 3:70 IMO is a decent highway gear, my 68 L79 with M21 and 3:70 does just fine on the interstate with gas mileage at 70 about 17-18 mpg. High, not the highest rpm shifting out of first gear at 60 and second gear over 80. Uphill start from a stop is a delevoped technique.
Really liked the gearing in my 64 convertible 327/300, it had a M20 and 3:08.

[Tim S.]

Re: Advice needed with rear gear decision

I have come to really like the 3.55. I have it in a 65 375hp and had is in a 63 360hp. The torque of the 350/350 should pull it along real nice. I have another 65 365hp with 3.36's and IMO its kind of a dog. Once it's rollin and on the highway, its a nice gear. 3.70's are a nice ratio as well but will provide some RPM in the 65-70mph range.

Tim