67 speedo reads 50mph, GPS reads 60mph, what gear? HELP

[William B.]

My 67 vette speedometer reads 50mph, checking with gps I am going 60 mph, does this mean my speedometer is reading slow? it should be reading 60mph.
Currently installed is a green large gear with 22 teeth. I have an original 4 speed transmission and rear end (with 3:70 ratio), tires installed are P205/75/r15.
In reading charts 1 tooth equals 5mph, I assume I now need a 20 tooth large gear and can not find any listed?
Am I missing something or do I have a problem with the transmission?

[Edward D.]

Re: 67 speedo reads 50mph, GPS reads 60mph, what gear? HELP

As you surmised the problem with a low reading speedo is usually the pencil gear in the side of the trans, generally not the trans itself although there are two different possible worm gear diameters on the tailshaft. Speedo reading depends upon the worm gear on the tailshaft and the number of teeth on the pencil gear, and the tire diameter. Your 205s are close to the original tire OD so it probably isn't that. Try a pencil gear with fewer teeth and see what you get.

[William B.]

Re: 67 speedo reads 50mph, GPS reads 60mph, what gear? HELP

I can not find the plastic driven gear in 20 teeth large gear, if I use a 20 tooth small gear it does not work at all,

[Edward D.]

Re: 67 speedo reads 50mph, GPS reads 60mph, what gear? HELP

Like I said there are two different worm gears possible on the tailshaft. The 1.84" OD gear uses 17 to 22 tooth pencil gears that are .810" OD. The1.76" OD gear uses 22 to 24 tooth pencil gears that are .870" OD. The 20 tooth blue, or 21 tooth red are both used with the 1.84 OD worm gear. The smallest that will work with the 1.76" gear is the 22 tooth green. If you have a 1.76 OD worm gear on your trans tailshaft and the 22 tooth green gear reads too slow then you have the wrong worm gear on the tailshaft. Only way to fix that is remove the trans, pull the tailshaft housing, the gear, and press on another gear. Or, i believe there are aftermarket gear conversion devices that allow you dial in a change although I've never used one of those.

[Joe L.]

Re: 67 speedo reads 50mph, GPS reads 60mph, what gear? HELP

I can not find the plastic driven gear in 20 teeth large gear, if I use a 20 tooth small gear it does not work at all,

William------


As Edward mentions, if you have the GREEN 22 tooth gear currently installed, then there is no available gear of lesser tooth count that will work with the transmission's internal speedometer drive gear. So, you could look for a lifetime and you'll never find one because, as far as I know, no such gear has ever been made. In order to go to a lower tooth count drive gear, you would need to partially disassemble the transmission and change the drive gear.

However, the GREEN 22 tooth gear is the correct gear for a 1967 Corvette with 7.75-15 tires, 4 speed transmission, and 3.70:1 rear gear ratio. So, you have the correct gear for your application assuming that you are using tires with the same or nearly the same revs per mile as the original 7.75-15's. The revs per mile of the original tires was 767. The revs per mile of most 205-75-15 is about 770 (the Goodyear Assurance is 771). so, we're talking less than about a .5% difference. That's a negligible difference and well within normal accuracy of these speedometers. So, I cannot understand why your speedometer would be 10 MPH off at 60 MPH. If this is actually the case, I would say there's a problem with the speedometer instrument rather than a problem with the speedometer gear.

By the way, if this actually was a situation in which you required a lower tooth count speedometer driven gear with your existing drive gear, the speedometer reading could be corrected with the use of a speedometer adapter. A speedometer adapter is a small "gearbox" which attaches to the speedometer fitting on the transmission and to which the speedometer cable is then attached. The "gearbox" overdrives or underdrives the speedometer cable by the precise amount required to obtain an accurate speedometer reading. GM actually used these in PRODUCTION for some applications, including Corvettes. They were also used to achieve "certified accurate" speedometers for certain applications where that was necessary (e.g. police cars). Speedometer shops can determine/make up an adapter to correct speedometers.

[Duke W.]

Re: 67 speedo reads 50mph, GPS reads 60mph, what gear? HELP

More information is needed. You need to take readings at several speeds and compute the percent error ... like the speedo and GPS readings in 10 MPH increments from 30 to 80 MPH. Is it fairly consistent at all speeds? Post the data. As Joe said, a 22T gear is correct for a 3.70 axle and tire revs per mile near the same as OE.

Also state the tach reading at 60 MPH indicated on the speedo.

One cause could be that it actually doesn't have a 3.70 axle. Even if the axle code indicates a 3.70 you don't know if a different ratio gear set was installed sometime in the last 50 years.

Duke

[William B.]

Re: 67 speedo reads 50mph, GPS reads 60mph, what gear? HELP

Thanks everyone, I will jack up the car and try and figure what ratio I have, and then make more readings at highway speeds, give me a couple of days.

[Donald O.]

Re: 67 speedo reads 50mph, GPS reads 60mph, what gear? HELP

I had the same problem in my 67's speedo. I had been getting my more accurate speed from my tach, or so I though. Correct gear in transmission, correct new blue driven gear and bullet housing and gaskets. I contacted our own Joe Ray (1011) and sent speedo and tach to him for calibration. Very fast turn around and VERY fair pricing. I put a new speedo cable on as well, Joe's suggestion.

Speed and tach are now accurate.

Don

[Dick W.]

Re: 67 speedo reads 50mph, GPS reads 60mph, what gear? HELP

Good possibility that the magnets in the speedo head have lost strength, they need to be degaussed and remagnetized. If that is the case, Joe Ray is your man

[Tom B.]

Re: 67 speedo reads 50mph, GPS reads 60mph, what gear? HELP

With your tire diameter, drive gear and driven gear right it can only be the ring and pinion ratio or the speedometer calibration. If your drive shaft turns 3.70 times for one revolution of the tire it can only be speedometer calibration. Of course if you find that the rear end ratio is not a 3.70 then your speedometer head may be just fine. If it's not a 3.70 then you will either have to change the ring and pinion to a 3.70 or the speedometer drive gear on the trans tail shaft so you can go to the small diameter driven gears to get the correct reading on the speedometer.

Tom

[William B.]

Re: 67 speedo reads 50mph, GPS reads 60mph, what gear? HELP

Thanks for everyone's help, I have confirmed I do have a 3:70 rear end, transmission is original to the car, green 22 tooth gear is good, speedo reads consistent not jumpy, so I am assuming everything is good and I must pull out the speedometer.
I am going to drive it some then pull the head out and send it to Joe Ray

[Richard G.]

Re: 67 speedo reads 50mph, GPS reads 60mph, what gear? HELP

Below are the calculations for your speedometer.
Speedometer are calculated to read 60mph at 1000 rpm cable speed.
You could confirm your tachometer calibration as it should be 2850 @ 60 mph with your gears and tires.
1036 rpm vers 1000 rpm comes out to a 3.6% error at the speedometer.

				Tire revs per mile
Tire height inches				770
Differential ratio		3.70		2849	drive-line/engine rpm per mile in 4th
trans shaft gear		8		2.75	spedo gear ratio
Trans output gear		22		1036	RPM of spedo cable @ 60 mph

[William B.]

Re: 67 speedo reads 50mph, GPS reads 60mph, what gear? HELP

Thank you for the info, I will check with my tach

[Donald O.]

Re: 67 speedo reads 50mph, GPS reads 60mph, what gear? HELP

Thank you for the info, I will check with my tach

William,
I had been using my tach as my speedo, 23mph per 1000 revs in 4th gear (3.36:1 posi rear). Then I checked tat against my DPS speedo. Turns out BOTH the speedo and tach were off.
Having BOTH calibrated by Joe Ray solved the two issues.
Don