68hemi

Which gear do I need?
Tires are 225/70 x 15"

Steve59

my 59 is a posi 3.70 BW T-10, stock gears and tires took a 22 tooth gray gear. There is a gear calculator on the net, just plug in your stats and go. ps, I think it was the smaller 22 tooth gear.

sorry originally said gray gear, it's actually green.

Chuck Gongloff

Best, and most accurate way to do it is to take your car in its' current configuration on a road test with a Garmin GPS device. The GPS device will give you your accurate speed.

Compare that with what is shown on the speedometer.

You can then get a "factor" using fractions. Pull your original gear and count the teeth. Use your calculated factor to see how many more...or less teeth will make your speedo more accurate.

GPS shows 60 MPH
Speedo shows 50 MPH

You need to "speed up" the speedometer

50/60 = 0.833.

Example: Your current gear has 24 teeth. 24 x 0.833 = ~20 teeth.

Fewer teeth will speed up the speedometer, more teeth will slow down the speedometer.

Yellow6t7

The speedo driven gear for 3.70 gears is a different size diameter than most others. The diameter of the gear itself is larger. Not sure where you can find one.
When I was looking a few years ago I could not find one so I used a speedo gear ratio adapter.
If you find a place to get a gear please post here.

68hemi

I just changed the trans in the car to a 1961 BW and it has no speedo gear in it at all right now.

I have looked at the online calculators but not much help. M

y 225/70 x 15" tires are 27.4" tall. I can not find the height of the original 6.70 x 15" tires but the charts say the closest size is a 205/75 x 15" which is 27.1" tall.

If the speedometer reads too high, a drive gear with one more tooth will slow it down approximately 5 MPH 55-60. I don't know which direction to go with regards to my tire size.

The original speedo gear for the cars with 3.70 is a green 22 tooth.

There is a 21 tooth red gear for original 3.55 and a Power Glide.

There is a blue 20 tooth gears that I find no factory application for.

There is a yellow 24 tooth gear for original 4.11 rear gear.

68hemi

Is this what you are referring to? https://www.corvettecentral.com/c3-6...7%26count%3d11

Yellow6t7

Thanks , but I was referring to the driven gear which attaches to the speedometer cable at the transmission. The drive gear for the 3.70-4.56 gears is a different size than say the drive gear for a 3.36. It is a smaller 8 tooth gear, 1.77 Dia.

DZAUTO

One more time.
There is a big diameter and a small diameter INTERNAL drive gear which is pressed onto the main shaft (some later Muncies used a plastic drive gear which was retained by a spring clip-------------I always replace the plastic gear with a pressed-on steel gear). MOST, MOST, MOST, MOST of the time, the drive gear has 8 teeth.
The larger diameter drive gear is 1.84in.
The smaller drive gear is 1.76in.
The larger drive gear mates with the SMALL plastic gears that have 17, 18, 19, 20, 21, 22 teeth.
The small drive gear mates with the LARGE plastic gears that have 22, 23, 24, 25 teeth.

YES, YES, YES, YES, there are TWO, repeat, TWO 22 teeth gears. The small one (which mates with the LARGE 1.86in. drive gear) is gray.
The large 22 teeth gear is green and mates with the small drive gear (1.76in).

GENERALLY, but not always (BUT MOST OF THE TIME), a car with a 3.70 rear will have an 8 teeth DRIVE gear and a 22 teeth plastic DRIVEN gear.

A SMALL (17-22 teeth) plastic gear cannot be used with a small drive gear (the teeth won't mesh).
A LARGE (22-25 teeth) plastic gear cannot be used with a large drive gear (the large drive gear will chew up the teeth on the large plastic gear).

Here are the large and small 8 teeth drive gears.



Here is a comparison of large and small 22 teeth driven gears.

68hemi

So, with my 3.70 rear and a 61 BW trans will these gears work with it?

https://www.corvettecentral.com/sear...=1957&count=11

CaptainEgg

Have you taken out the gear to see what you have now?

SWCDuke

Both the 225/70R-15 and OE 6.70-15 tires are approximately 27.4" inflated OD and have a nominal revs/mile spec of 760 per the various tire manufactures specs and Corvette AMA specs. The 205/75R-15 is nominally 766 revs/mile. Current reproduction 6.70-15s are about an inch greater OD with correspondingly fewer revs/mile due to the modern DOT requirement that they be inflated hot out of the mold, which expands them.

Duke

Chuck Gongloff

I'm puzzled.

To know where you want to end up, you have to know where you started.

Anything else is pure guesswork.

Yellow6t7

ditto

68hemi

I don’t know if it’s that easy to determine if you have the large or small steel drive gear when it is installed in the car? I am going to assume that the trans came with the standard 3.70 gears and install the green 22 tooth gear. The worst that can happen is that it is wrong and it will chew up the plastic gear if it is wrong.The worst that can happen is that it is wrong and it will chew up the plastic gear if it is wrong.

Yellow6t7

If the 22 tooth gear is the smaller diameter and you have the smaller drive gear the driven green gear will not spin at all.
If the 22 tooth gear is the larger gear and you have the larger drive gear you will chew up the green driven gear.

65GGvert

Take this for whatever you like. I have a 60 with a 61 BW T10 4 speed. I have a 3.70 stock rear. Mine uses a green 22 tooth that I ordered for a 61 BW with no issues. I didn't concern myself at the time with large or small gear, I replace the stock with what said it was stock. My tires are 205-75/15 and are 27.1 inch diameter and the speedometer is off from a GPS about 2 mph at 50. Close enough for me.

68hemi

This sounds like the answer to me for my car. Who did you order your speedo gear from? Is there a difference in the gears between BW and Muncie transmissions?

65GGvert

Zip Corvette Parts

59BlueSilver

BTW, there is a speedometer app that uses GPS you can get for your iPhone. I use Speed Box.

Steve59

I had my 1959 BW rebuilt for my 3.70 posi rear by Ed Harnett here in Lansdowne Pa. Told him what I was running and he put in a 22 tooth green gear. As stated above about 2-3 mph difference from the GPS. Pretty damn accurate for a speedo with 1959 technology.