Advice needed with rear gear decision

[John M.]

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.

So you don't feel like your winding out running at 70 MPH with the 3:70?

I had an LS6 Chevelle with the M-22 and 3:73 and cruising at 3200 RPM seemed a bit much. But then again, that's a big Block so the 350/350 may wind out a little better after I freshen it.

[John H.]

Re: Advice needed with rear gear decision

My '69 Z/28 was built with an M-21 and the optional 4.10 axle. I do a fair amount of highway driving with it, and 3850 rpm at 70 mph finally got to me - swapped it out for a 3.55 (with a new Series 3 Posi unit). Mistake. It's great on the highway, but around town the 2.20 low gear and 3.55 axle don't work very well, especially with the low torque of the 302. I think it'd work fine with an M-20, but it in retrospect, it wasn't a great idea with the M-21.

[John M.]

Re: Advice needed with rear gear decision

It's amazing the varying opinions on this one.

Just enough negativity to scare me away from it

It makes sense they didn't offer lower than 3:70 with the close ratio for a reason.

I'm sure it would work, but my guess is you'd be leaning heavy on the clutch in town, and getting spanked by 4 cylinder Hyunda's off the line.

[Stuart F.]

Re: Advice needed with rear gear decision

John;

I'm not certain what yoiu mean that they didn't offer anything higher than a 3.70 with a close ratio. Are you speaking of a particular year or engine option? My 63 has a 3.36 with a close ratio and I'm lovin it. I wouldn't waste my gas on any unworthy 4-cylinder car at a stop light anyway. And from 30 mph on up, I'll eat any stock small block with any lesser gear, incl. any fuelie! But the choice is yours; jump me now and I say goodbye to you later! The wide ratio box is certainly a nice combo for all around street driving, but I like to use my tranny a lot for braking and to keep my rev band narrow under serious acceleration. Works for me.

Stu Fox

p.s. do ya think that will keep this going?

[Bob S.]

Re: Advice needed with rear gear decision

John, you pay your money and make your choice....

Having owned 2 M-21/3.70, and 1 M-20/3.36 L-79's (in addition to a '68 M21/4.10 Z/28 similar to John Hinkley's), I feel the M20/3.36 is a little more compatible with stop-and-go driving than the M-21/3.70. I find the need to feather the clutch a little more with the M-21/3.70 to get rolling smoothly.....something I don't remember ever having to do with the M-20/3.36.

The close ratio with the 3.70 has about 4% less torque multiplication than the M20/3.36 combination. About half of the difference could be compensated for by using 205/70-15 tires rather than 215/70-15's.

Regardless, my experience with that Z/28 made me fall in love with the M-21 gear spacing, so the M-21/3.70 combo is really my favorite.

P.S. I couldn't imagine anything less than 4.10's with the M-21 on a 302 Z/28.....just no bottom end!

[Dale S.]

Re: Advice needed with rear gear decision

I have not heard anyone give the alttitude of where they are using these gear ratios. I guess the posters are at sea level. Dale at 5290 feet with a wide ratio tranny.