[ho428]

Ok guys, pull off your shoes, you made need fingers and toes to figure this one.
Ok, so it's really not that bad.

68 Firebird on the road course, I run various tracks and can't afford to swap gears every time I change tracks.
A track like CMP and RRR are tight and technical, while Road Atlanta and VIR are high speed tracks.
Been running an M-20 and 3.08 since I already had it but I'm changing the tranny and getting a 9" over the winter. Running a 365, solid flat tappet with HRE heads, great engine up to 6500, then runs out of breath by 6800, which is my rev limit anyway.
The sweet spot is about 4000-6300 it seems.

Looking at switching to an M-22X tranny, 2.199-1.506-1.174-1:1, out back I'm having a tough call between the 3.50 and 3.70, using 26.7 OD tires. With the 3.70 I'll loose top end at RA, but face it, over 120 and areo on that car sucks anyway, I'd rather get out in front off the corners, if they have the kahunas to chase me down at 130-140 and out brake me in a 40 year old car, more power to them.
So far I'm leaning toward the 3.70 but the tight 2nd-3rd of the X might work better with the 3.50 where I could pull them a little longer, and 3rd-4th is nothing.

Another option is the std M-22, 2.199-1.64-1.274-1:1, little wider spread and I am dealing with a Pontiac that has a decently wide power band.

[boater bill]

Using your tire diameter, I am thinking the 3.36 or 3.5 would give you enough gear to outpull out of the corners and have enough top end to keep them in your rear window.
If you can pull to 6500 RPM in 4th, you are doing great. My Sprint fell flat at about 5K in 4th.
I like the gear spread of the M-20 and using the engine torque band to negotiate the track. The 2-3 shift is negligible for the M-21,M-22 and that doesn't favor the wide torque band you are making.

Hope this helps.

[ho428]

Thanks Bill,

The 3.50 seems to be the best compromise.

The M-20 3-4 rpm drop is killing me though. 4th is unusable at CMP, I can run the entire track in 2nd-3rd if I want, which fits the power band pretty good, but I'll hit 4th a few times to keep the rpms down since I'd essentially be winding it high for nothing.

At Road Atlanta I hit 4th once real good down the long back stretch, the other two times just briefly because I run out of third.
Not sure how the M-20 would do with a 3.50 gear, I think 2-3 would be great, but the 3-4 drop ???
I tried to find a set of 8.2 3.55 gears to try but never did.

[gtoroadracer]

If I may help with this choice, I am running the Auto gear m-22 road race gears for years now I have changed the 2:75 gear ratio over to a 3:50 gear and what a world of difference but I would not run a higher gear like a 3:70. With a 6,800 rpm redline an a 25.5 tall tire I am almost topping out 4th gear at the tracks I go to which is about 160 mph.