Speedo over-reading

[Matt7c]

What would cause my speedo to over-read?

Since getting my S2 up and running a few months ago, I have been a little disappointed that speed didn't seem so fast anymore. So I put it to the test with my hand-held GPS and discovered that indicated speed is about 12-15mph faster than is actually the case.

I have 155/13 tyres - can't recall the aspect ratio, but I'll check that tomorrow. If anything, I would expect my current tyres to have a fraction larger rolling diameter than the original 5.20/13s which would cause the speed to under-read wouldn't it?...?

Matt

[gjz30075]

Matt, are you saying that with no changes from a few months ago, your speedo is now over reading? Sounds like you're far enough off to warrant a change in the driven gear that is connected to the speedo cable in the tailshaft.


Greg Z

[Matt7c]

No, I can't say that its changed recently, it seems to have been over-reading ever since its return to the road 2-3 months ago after a 14 year rest.

[iain.hamlton]

Hi,

I also suspect the speedo on my +2 is over reading. I adjusted the tacho against a handheld meter which includes a tach. Now the speedo reads 20 mph per 1000 rpm instead of the 17.5 I was expecting. So either the gearing is different (possible because the car is bitsa-this-bitsa-that); the tacho is still out, or the speedo is out of calibration. Clearly, doing what you have done with GPS is the next step, which I'll do as soon as I can borrow a GPS.

In the mean time I found the following article which might explain what makes these speedos over read. I.e. the return spring becomes weak with age.

It also tells you how to calibrate (well adjust) your speedo.

http://www.tigersunited.com/techtips/Rh ... peedo1.asp

- Iain

[type26owner]

Matt,
A little investigating reveals that the 5.20-13 and P155/80-13 are the same diameter at about 23".

[Matt7c]

Iain, thanks for the link. Speedo rebuild now on the list of things to do when the sun stops shining!

[iain.hamlton]

Matt,
Just to let you know, I have now used the procedure at Tigersunited.com:

"Calibration of your speedometer is easy. If you have not serviced it, it ought to be close to the correct speed unless you have installed tires of a significantly different rolling diameter. To fine-tune your calibration, you need to drive a measured mile at exactly 60 mph indicated on the dial (you may use any other speed as well, but 60 is a nice mid-range number to use). Use a stopwatch to time your mile trip. Your actual speed is 3600 divided by the number of seconds it took to drive the mile. Now remove the speedometer from the dash, and remove the works from the case. Do not remove the pointer or face. Move the pointer to the indicated speed you used in the mile trip (60mph). This is done by gently holding the drag cup in the works. Double check that the pointer is still at the indicated speed. Now gently push the pointer to the calculated speed while being sure that the disk does not move. Now pull up some slack speedometer cable into the dash and reconnect the works of the speedometer without reinstalling the case. Carefully perch the speedometer in the dash opening without allowing the dash to contact any moving parts. Time another measured mile at exactly 60. If the time is not very close to 60 seconds readjust the speedometer. You should be getting very close to perfectly calibrated with one or two resettings of the pointer."

I used a GPS to find the actual speed rather than a stopwatch. The procedure works fine, except the speedo now underreads now at 30 and 40. (30 Mph actual reads ~26), but is close enough between 60 and 80; spot on at 60.

It only took one interation.

Most gratifying of all is that it validates my adjustment of the revcounter.


good luck and best regards, iain

[steveww]

All speedo even on modern cars over read. They are usually pretty good a low speeds then get worse as the speed increases. As long as it is close - why worry

[chrishewett]

I agree with the last comment. I prefer it to overead a bit when going past scameras at the limit. Why do peple brake to 50mph when seeing a scamera on national speed limit single carriagways?
Chris