What rear gear

bobfil · 10-25-07, 04:04 PM

I have a 72 vette 350 with 4 speed , and a one step above stock cam with intermediate heads. The car has 3.36 rear gears and runs well, but I would like a little more jump from the start.What gear should I be looking to put in ,3.55 or 3.70? Thanks for any help, Bob.

65Tuxedo · 10-26-07, 12:10 AM

Don't rule out a lower first gear, depending upon what trans you have now. I just went to a 3.09 from a 2.20. The difference is incredible.

Glensgages · 10-26-07, 06:44 AM

Quote:

Originally Posted by bobfil

I have a 72 vette 350 with 4 speed, and a one step above stock cam with intermediate heads. The car has 3.36 rear gears and runs well, but I would like a little more jump from the start. What gear should I be looking to put in, 3.55 or 3.70? Thanks for any help, Bob.

..... with a 4-speed and 3.36 gears, if you still want more grunt outta the hole, can I assume you have a close-ratio (2.20:1 First Gear ) transmission?

Generally-speaking, a 'one step above stock cam' will increase upper RPM power, at the expense of losing low-end punch, so it's possible you've decreased off-idle power, and the tall gearing won't let it get wound-up quick enough:
going to a deeper (higher-numerical/lower-mechanical ) gear will put/keep the motor in it's RPM power-band better.

I think you have to ask-yourself what are your intentions and useage for the car (i.e. - is cruise RPM important at highway speeds, or is the car used mostly in-town, for stop-and-go driving, where accelration is more-important ) before determining if the gear should be changed:
please keep-in-mind that 4-speed/stick-shift '77-'79 Z28s, with wimpy 175 HP LM-1s came with 3.73s from the factory to help off-the-line performance.

Switching from 3.36s to 3.55s is a increase of just 6% (not-much ), while a swap to 3.70s is over 10%, and the price for the swap is the same either-way, so you may want to consider 3.70s as a minimum:
the ratio for calculating MPH at RPM with your combination, in 4th Gear, would be:

RPM x .006 x 1/2 TH (Tire Height ) divided/by GR (Gear Ratio ) = MPH

or

MPH x GR div/by .006 div/by 1/2 TH = RPM

'Assuming' you are running a 255/60R-15 rear tire (approx 27.4" tall ), with 3.70:1 gears you'd be turning 3150 RPM @ 70 MPH, or, if limiting RPM is more critical to you, 2500 RPM would be 56 MPH.

Hope this helps!

fine69 · 10-26-07, 09:53 AM

Quote:

Originally Posted by bobfil

I have a 72 vette 350 with 4 speed , and a one step above stock cam with intermediate heads. The car has 3.36 rear gears and runs well, but I would like a little more jump from the start.What gear should I be looking to put in ,3.55 or 3.70? Thanks for any help, Bob.

I have 3.70:1 in the Vette and 3.73:1 in the Camaro. I like this ratio as I do not do much long distance highway traveling in either one. With respect to performance - both cars are great off the line. In the Vette, I often chirp the tires un-intentionally from 1st - 3rd gear.

A couple of weeks ago I was in city traffic driving the Vette, going about 20 mph maybe more. I gunned it in 2nd gear, and was surprised that I spun-out and squeeled wheels!

The drone of the RPMs when on the highway (usually I-70) does take some getting used to....

GTR1999 · 10-26-07, 07:00 PM

If your car is stock then the base eng with M-20 muncie 252 came with 336's. It was and is a good street combo. If you want to feel the difference then go to 373 aftermarket gears. 370 was the stock ratio but the 373 gear has more teeth in the R&P. With 225 70-15 tires you'll be about 3400 @ 70 mph. If you go as far to change the gear you should also polish and tune the posi to extend the life of the case and use solid steel clutches.

bobfil · 10-30-07, 10:32 PM

Quote:

Originally Posted by Glensgages

..... with a 4-speed and 3.36 gears, if you still want more grunt outta the hole, can I assume you have a close-ratio (2.20:1 First Gear ) transmission?

Generally-speaking, a 'one step above stock cam' will increase upper RPM power, at the expense of losing low-end punch, so it's possible you've decreased off-idle power, and the tall gearing won't let it get wound-up quick enough:
going to a deeper (higher-numerical/lower-mechanical ) gear will put/keep the motor in it's RPM power-band better.

I think you have to ask-yourself what are your intentions and useage for the car (i.e. - is cruise RPM important at highway speeds, or is the car used mostly in-town, for stop-and-go driving, where accelration is more-important ) before determining if the gear should be changed:
please keep-in-mind that 4-speed/stick-shift '77-'79 Z28s, with wimpy 175 HP LM-1s came with 3.73s from the factory to help off-the-line performance.

Switching from 3.36s to 3.55s is a increase of just 6% (not-much ), while a swap to 3.70s is over 10%, and the price for the swap is the same either-way, so you may want to consider 3.70s as a minimum:
the ratio for calculating MPH at RPM with your combination, in 4th Gear, would be:

RPM x .006 x 1/2 TH (Tire Height ) divided/by GR (Gear Ratio ) = MPH

or

MPH x GR div/by .006 div/by 1/2 TH = RPM

'Assuming' you are running a 255/60R-15 rear tire (approx 27.4" tall ), with 3.70:1 gears you'd be turning 3150 RPM @ 70 MPH, or, if limiting RPM is more critical to you, 2500 RPM would be 56 MPH.

Hope this helps!

Thanks ,but I don't have a close ratio (2.20:1 first gear trans) It's the wide ratio. Looking at getting either 225-70-15 or 235-70-15 tires, 27.4'' to 28'' tall . Looking for mostly in town driving , but also occasional highway. So you are saying 3.7 would help me off the line and not to bad on highway? How will wide ratio affect?

TWINRAY · 10-31-07, 10:32 AM

IMO, it wouldn't be worth the effort going up one notch to the 3fifty's. You're definately gunna get some pull off the line with a wide ratio box and 3seventies. But be aware, it's gunna get a little "busy" on the highway. In my '67, I have the 4.11 with the CR box - I certainly wouldn't go higher in my car than a 3.70 with it's 2.20 first gear. If most of your driving in off the highway, then it sounds like the 3.70 would be a good choice. My 4.11 is pretty useless on the highway - above 60mph things start getting kinda "busy." I reserve my '69 for trips - it has the 3.08 rear .

Glensgages · 10-31-07, 11:40 AM

Quote:

Originally Posted by bobfil

Thanks, but I don't have a close ratio (2.20:1 first gear trans) It's the wide ratio. Looking at getting either 225-70-15 or 235-70-15 tires, 27.4'' to 28'' tall . Looking for mostly in town driving , but also occasional highway. So you are saying 3.7 would help me off the line and not to bad on highway? How will wide ratio affect?

The motor won't turn anymore RPM in 4th Gear with either close-ratio or wide-ratio transmission, as 4th Gear is 1:1 in both:
'assuming' a typical 27.4" street-tire is out-back, 65 MPH would be about 2950 MPH in 4th Gear with 3.73s.

Let's think about changing rear gear-ratio as it'd relate to us pedaling a 10-speed bicycle, but only being allowed to select certain speeds on the bike:
with 3.36s being mid-range, let's 'assume' (there's that word again..... ) that the highest speed on the bike you are permitted to use is 7th speed, so with your 4-speed transmission, it'd be like pulling-out from a dead-stop (off-the-line acceleration ) being allowed to use only 4th, 5th, 6th, and 7th speed of the bike....

Without increasing your leg-strength (TQ ) or leg-speed (higher RPM ), you are suddenly permitted to use 3rd, 4th, 5th, and 6th speeds instead:
it'd be easier to move off the line, but you'd have a slightly-lower top-speed, because you could only pedal (engine RPM ) so-fast.....
this might simulate a switch from 3.36s to 3.55s.

Using 2nd, 3rd, 4th, and 5th speeds of the bike might simulate going to 3.73s, and being allowed to use 1st, 2nd, 3rd, and 4th would be like having 4.10s:
it'd be HELL off-the-line, but you'd run outta steam before you'd attain a comfortable cruising speed, unless you have a lotta leg-speed (RPM ), or tall rear-tires.

Hope this better illustrates how deeper (higher-numerically/lower-mecahnically) rear-geaing can help off-the-line acceleration with no-change in power being made from the motor:
you are simply putting that power to the pavement quicker and more effeciently.

TWINRAY · 10-31-07, 12:03 PM

A wide ratio helps launch a car better with a high rear gear (low numeric) like a 3.08 to a 3.55. Actually a 3.55 isn't what I'd call high. There is more multiplication
ie. 2.54 first gear x 3.36 rear = 8.53.

A close ratio box starts with a 2.20 low gear and finishes with a 1.1 top gear ( a 4 speed with no overdrive). There is less rpm drop between shifts which makes for (IMO) more comfortable driving but at the "expense" of needing a low rear (high numeric) to launch it- ie. 2.20 first times x 4.11 rear = 9.04 ( or 8.14 with a 3.70). If I did a lot of highway driving, my choice would be a wide ratio box with either a 3.08 or a 3.36 rear. I love cruising in my '69 with 3.08 gears. At 80 mph, it's only at 3000 rpm which makes for a very comfortable cruise at good mpg. I don't cruise over 60 in the '67 cause I'm all ready around 3 grand (or so) with 4.11's.

1/4 mile cars normally go with a wide ratio and road racers with the CR