Speedometer problems in vintage cars

Question: I have a problem with my 1976 Triumph TR6. My speedometer says one speed while those police radar check points say another, and there can be quite a difference. When I am doing 55 mph my speedometer reflects about 15 mph over that, and the faster I go the larger the discrepancy becomes. I must drive using my tachometer to gauge my speed. Nothing that I know of has been changed on the car. The speedometer cable is driven off of the distributor, not the transmission like American cars. How can I go about getting this problem fixed?
Answer: I’ve owned a few of these cars myself, and I want to tell you that the problem you’re experiencing is not uncommon. I hope that makes you feel better. Any other questions?
Actually, there are a few things that can cause the problem you’re having. I was confused by your statement that your speedometer was driven by your distributor, because it should not be. I checked with Mike at British Car Service in Holbrook to confirm this for me. He confirmed that I was correct, so I suspect that what you believe to be your speedometer cable is actually your tachometer cable, which is in fact driven off of the distributor. Your speedometer is driven off of your transmission in a very conventional manner, although it does go through a “counter box.”
The first, and most common cause of this speedometer error is a change in tire size. If you or a previous owner changed to a smaller, or lower profile tire, it will spin faster at any given speed. This faster tire rotation speed will cause the transmission to spin faster, and since a gear in the transmission drives the speedometer, the speedometer will indicate a higher speed. The original tire size on your car was 185/15 which had a “total height” (measured from the top of the tire to the bottom when inflated) of about 26.3 inches. You can easily measure yours, and if it is smaller, you’ve found your problem. The original style “redline tires” are being reproduced in the proper size but my experience is that they have a slightly larger “total height” (about 27 inches) which will cause your speedometer to indicate a speed that is slightly slower that your actual speed.
A modern 205/70/15 tire has a “total height” that is virtually identical to the original tire and will work well on your car.
The other possibility is that the wrong “speedometer drive gear” is being used in the transmission. There are two gears that are available for TR6s. The application is determined by the year of the car. Since they will both fit in your transmission, it’s not unusual to find the wrong one being used. The “tooth count” only varies by one, but this is enough to account for a significant error.