Transmission Speedo Gear

[Murray S.]

I just finished changing the rear differential in my 65 Corvette to a 3:55 ratio (correctly date-coded, etc). I suspect the previous differential was either a 3:73 or 4:11 ratio from a 70 or newer Corvette, based on the RPM the tach was showing (3500-3600 RPM at 60 MPH).
I installed a new Speedo Driven gear (red) which is supposed to be correct for this ratio. My RPM at 60 MPH now shows 3100; I thought it should read 2900 RPM.

I noticed that Long Island also lists a Speedo Steel Drive gear for 3:08 to 3:55 gear ratios.

Would this be the cause of my problem?
Also, how difficult is it to change this gear?

[Steven B.]

Re: Transmission Speedo Gear

Murray, a couple of other things to verify; Is the speedo accurate? Is the height of the rear tires equal to the originals? These can also throw off the speedo reading.

[Murray S.]

Re: Transmission Speedo Gear

I cant vouch for the accuracy of the speedometer; my friend was following me and we compared speed readings after a short drive.
As for the tires, they are Coker bias-ply original type 7.75-15.

[Bob R.]

Re: Transmission Speedo Gear

I believe the drive gear from Long Island is the aluminum gear that is installed on the rear transmission shaft itself. There is one size used on transmissions from 308 to 355 and a smaller gear from 373 to 411. For example if you take a transmission set up for a 411 and try to use a 308 on it you will have to change the internal aluminum gear and the external plastic gear. To go from 308 to 336 you only have to change the plastic gear. To change the internal gear you have to remove the tail shaft and then remove the old pressed on aluminum gear.

[John H.]

Re: Transmission Speedo Gear

I just finished changing the rear differential in my 65 Corvette to a 3:55 ratio (correctly date-coded, etc). I suspect the previous differential was either a 3:73 or 4:11 ratio from a 70 or newer Corvette, based on the RPM the tach was showing (3500-3600 RPM at 60 MPH).
I installed a new Speedo Driven gear (red) which is supposed to be correct for this ratio. My RPM at 60 MPH now shows 3100; I thought it should read 2900 RPM.

Murray -

I'd check your mechanical tach calibration/accuracy; based on its readings,

3500 rpm at 60 mph with 27.2" tires (your original setup) says it had a 4.56 axle.

With a 3.55 axle and 27.2" tires (your current setup), it should read 2640 rpm at 60 mph, not 3100; 3100 would be for a 4.11 axle.

[Joe L.]

Re: Transmission Speedo Gear

I believe the drive gear from Long Island is the aluminum gear that is installed on the rear transmission shaft itself. There is one size used on transmissions from 308 to 355 and a smaller gear from 373 to 411. For example if you take a transmission set up for a 411 and try to use a 308 on it you will have to change the internal aluminum gear and the external plastic gear. To go from 308 to 336 you only have to change the plastic gear. To change the internal gear you have to remove the tail shaft and then remove the old pressed on aluminum gear.

Bob and Murray-----


That's pretty much the way it is. However, a few comments:

1) the speedometer DRIVE gear mounted on the mainshaft was originally steel. If the one that LI sells is aluminum, that's not the original material but it should work ok;

2) For 1965, the "large" DRIVE gear should be the correct one for a 3.55:1 rear gear ratio and the 21 tooth red DRIVEN gear should also be correct;

3) IF the car previously had a 3.73:1 or a 4.11:1 rear gear AND IF the speedometer worked and was accurate, then the transmission must have a "small" DRIVE gear installed. A 21 tooth, red DRIVEN will not work properly with such a DRIVE gear. Further confirmation that the car has a "small" DRIVE gear installed would be if a green, black, yellow or orange DRIVEN gear was removed from the transmission.

4) If you have a "small" DRIVE gear installed, the smallest tooth count DRIVEN gear you can install would be a green, 22 tooth gear. If that gets you close on calibration, you're done;

5) Even if you have an incorrect DRIVE gear installed, there is a way around having to remove and partially disassemble the transmission to change it. You can use a speedometer adapter to achieve proper calibration. These are installed between the transmission and the cable. If you go this way, the best route would be to go to a speedometer shop to have it done;

6) The best way to check speedometer calibration is with a GPS unit. If you are within 3-5% of the GPS reading, that's about as close as you are going to get.

[Wayne M.]

Re: Transmission Speedo Gear

Murray -- At this stage, the best info you could provide us is what driven gear you removed from your trans (as Joe L. mentions). That would give us some idea what DRIVE gear is installed on the mainshaft. You need the "144" gear [the big gear shown in the thumbnail, because it has a slightly larger dia. and thickness than the [small] "145", used for 3.70 and 4.11 final drives.

But if you had a 4.56 (as John H. suggests the readings might indicate), then you may have removed a steel driven gear, as shown in the big pic, immediately below. This drive gear is larger still in diameter but thinner than either of the 144 or 145.