C2 Speedometer Error

[George C.]

I have a 65 roadster 365 hp, 336 rear gear and have a speedometer that reads low. I have read several archive mesages but still have an unanswered question. My speedometer error grows with speed. For example according to the handheld GPS it reads low by about eight miles per hour at 45 and about 12 miles an hour low at 65. I am guessing this can't be corrected by the driven gear in the transmission. Can anyone help me understand the problem?

Thank you in advance.

George
#36809

[Michael W.]

Re: C2 Speedometer Error

The % of error in both your examples is roughly the same (~18%). You want a driven gear with 18% fewer teeth on it, or a driving gear with 18% more teeth if that is not available.

[Charlie P.]

Re: C2 Speedometer Error

I'm not an expert, but it seems to me that your condition IS characteristic of incorrect gearing. Your percent error appears quite consistent- 17.7% at 45 mph, and 18.4% at 65. I think that if the gearing is wrong, the percent error will remain constant at all speeds, and the gap in absolute measure will widen as speed increases. Therefore, it would seem that a gear change should correct the problem.

[Joe L.]

Re: C2 Speedometer Error

George-----

Assuming that your tires are close to stock diameter, you require a speedo driven gear which is a 20 tooth BLUE gear. If you pull out any other gear, that's your problem. If, per chance, you pull out a 22 tooth (GREEN) gear to a 25 tooth gear, then that means that your DRIVE gear will not be compatible with the 20 tooth BLUE gear. That creates much more of a problem in getting your speedo corrected. But, we'll cross that bridge if we come to it.

[Wayne K.]

Re: C2 Speedometer Error

George,

You indicate that the speedometer has the error. Does the actual mileage (odometer) also present the same error. If so then the gear change should correct the problem but if only the speedometer reading is off and the odometer is correct then I think your problem is with the calibration of the speedometer itself.

Wayne

[George C.]

Re: C2 Speedometer Error

Joe,

The tires are Coker Gold Line radials, as I recall they are roughly 28 inch diameter so pretty close to the original 775 tire. I like talking colors. The driven gear is a tan or beige color, I haven't seen it in a while and I didn't count the teeth when I had it out. I will check that out on Saturday. I'll check the catologs for a new Blue one.

Thank you,

George
#36809

[George C.]

Re: C2 Speedometer Error

Wayne,

The odometer is also off, I guess that helps keep the mileage down.

Thank you,

George,
#36809

[Joe L.]

Re: C2 Speedometer Error

George-----

The tan or beige gear is called "NATURAL" color and it's a 19 tooth gear. If the gear is, in fact, 19 teeth, that would not account for anywhere near the error that you, apparently, are experiencing. That gear should produce only about a 5% error.

[George C.]

Re: C2 Speedometer Error

Joe,

That doesn't sound like good news. I was calculating the 336 gear based on the numbers in the sales brochure, the tachometer reading and the GPS. The rear-end cover is dated 66, so that is not original, and it calcs to a 336 gear, I think it has an AM code, not positive about that without checking it again. I think the transmission is original, but that doesn't mean the drive gear wasn't changed somewhere along the line. Any suggestion on where to start?

Thanks again,

George

[Joe L.]

Re: C2 Speedometer Error

George-----

First, you better confirm what driven gear is actually installed and what the rear end ratio actually is. "AM" would indicate a 3.36:1 IF the gears currently installed in the case are the original ones.

[George C.]

Re: C2 Speedometer Error

Joe,

Thanks again, I will start with making sure what the rear gear is.

George