I want to lower my gears. With the current 410, doing 55 down the road is rediculous. I didnt open it up yet, but i am assuming i have a 4 series carrier. I know that i can go to a 373 without changing the carrier. My question is, is it worth the trouble to go down to a 373, or is it gonna be real close to what i already have?

If you want to get this car out on the highway a lot then changing to a 3 series and a set of 3.42 gears is probably going to be the best way to go. You could also go to a 5 speed with overdrive. Taller tires also helps.

x2

A nice used set of 3.31s would also be a nice alternative with a new Eaton 3 Series Carrier.

Do you have a M20 or M21?

The first gear ratio of a M21 with anything less than a 3.55 Ring and Pinion is too high.
From the Factory, M20 Transmissions were used with gears up to 3.31. M21 Transmissions were outfitted with 3.55s and up.

I went from an M20 with 4.10s to 3.42s. Great gear for the highway. Still pretty steep for the freeway though. Go down as far as you can if your need is freeway driving.

If you have MS Excel, pm me with your email and I will send you my calculation sheet that let's you change gears/trans/tires so you can see your RPM in each gear at given speeds.

3.31's is the best for both worlds, and even then, 65MPH would be the comfort interstate cruise. I went a little deeper to a 3.07 and cruise 70 mph pretty nice.

x2

A nice used set of 3.31s would also be a nice alternative with a new Eaton 3 Series Carrier.

Do you have a M20 or M21?

The first gear ratio of a M21 with anything less than a 3.55 Ring and Pinion is too high.
From the Factory, M20 Transmissions were used with gears up to 3.31. M21 Transmissions were outfitted with 3.55s and up.

I'll have to disagree with that. The '70 has 3.31's and an M22. I haven't had any problem with it being sluggish from a stop. Sure it doesn't pull like a set of 4.10's with a wide ratio trans. But it is still plenty good for a street car.

I'll have to disagree with that. The '70 has 3.31's and an M22. I haven't had any problem with it being sluggish from a stop. Sure it doesn't pull like a set of 4.10's with a wide ratio trans. But it is still plenty good for a street car.


I think he meant to say or imply that typically this was the case, or that the majority of cars sold off of the lot were like this. You could order nearly anything I think (My car was original M20 w/4.10s for example).

Let us know your rear tire size as well, this will play a factor.

I am not sure if its a 20 or 21, I have to change into 2nd gear by 5 to 10 mph the tires are 235-60-15s

I'm running a 26.5" tire with 3:73's and a M20, it is no fun at freeway speeds but in the city a blast!

With 26" tires, m20 ,& opt 307 gear imy 69 with mild 396 turns approx 2,400-2,450rpm @ 60mph.

Then when i installed 3.31's it turned approx 2,600-2,650 rpm @ 60mph with all else staying the same.

3.31s are the best all arround comprimise between reasonable highway cruise speeds while keeping some perf on the table too without an Od trans of some sort and running 26"-26.5" tires.

And your right 4.10's stink in every possible way for a street setup " without OD trans " & are useless for any crusing much above 50mph for any length of time unless somone likes to hear the motor at 3,500 rpms all the time and don't mind getting 8mpg/LOL!!!!!.

My mild 396(mild hyd f/t perf cam,tweaked q-jet,stock intake & exhaust manifolds) with 3.31's got 13.7mpg when i checked it last summer when driving in mixed city & a little open road driving,on open road cruise it got approx 15-15.5+ mpg @ 65mph @ 2,800-2,850 rpm.

I would not bother going to 373 from 4.10,there wont be enough cruise rpm reduction for all the $ & time involved in a gearswap. Buy good used or NOS 3.31's off ebay or from here in t/chevelle or buy new 3.42's and inc rear tire to 27" tall. I bought my 3.31's used from ebay and they turned out fine post install,been 4 yrs now,no issues. Approx 1 yr after i installed the used 3.31's i saw new NOS 3.31's on ebay which i bought to back up the used set of 3.31's in case they ever went by by because i like the 3.31's so much.

scott

Thanks guys.

My question is, is it worth the trouble to go down to a 373, or is it gonna be real close to what i already have?

Not 3.73 if you want to drive the freeway.
My 66 has 3.73 and stock tire size (7.70-14) I hate being on the freeway.
I would like 3.31s


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