Talk Morgan Forums Morgan Sports Cars Classic Morgans (pre-CX) Speedometer Calibration

Having checked the car speedometer reading against my sat nav gps reading I believe that the speedometer is under reading across the range. At an indicated 30 the gps shows 34 and at 60 it shows 63. Even allowing for a bit of leeway as the speedometer is way off to the left of my normal line of vision its still not right, as evidenced by the roadside 30 limit warning signs illuminating when I drive in to a 30 limit under an indicated 30mph which doesn't happen in my other car. Looking at the VDO website it looks relatively straightforward to recalibrate however I wonder if anyone knows the answer to the following two questions not answered on the website.
1.In order to change the pulse per mile calibration is it simply a case of taking the percentage difference and applying that to manually calibrate the speedometer? I have a strange feeling that as the calibration range is between 500 to 399,999 it won't be as simple as that.
2. If I can't manually calibrate then the auto calibration option will be the one to go for but where are there reliable mile marker posts?
Any help gratefully received as I have a clean license and would like to keep it that way but still remember getting two speeding fines in a week, one in a car the other on a motorbike, and don't want to repeat that if possible.

Can't answer all your questions but you could always look for the white painted lines set 90 degrees to the kerb that the police use for checking the calibration of their speedos. Usually they are 440 yds check at indicated 60 mph against a stopwatch.

Did you just change your tyres?

Are they the single short dashes by the side?

Charles
Everything is standard at the moment with 195/60 tyres as it arrived from the factory. I will probably change to 195/65 at some stage to improve ground clearance and that reduces wheel revolutions per mile from 833 to 808 because of the extra 0.8 inch diameter of the tyre and I would then need to re calibrate again but if I can crack the speedo adjustment now I can make another adjustment later if I need to.
Keith
The VDO instructions say you need to drive for exactly a mile to auto calibrate, doesn't matter what speed it's the distance that matters. If there are a number of these markers over a mile I am in luck but I don't know where any are.

They are usually only a quarter mile. What you could do is measure the mile with your GPS that will be accurate. I suggest finding a quiet straightish piece of road and make your own markers, two white pieces or 2x1 should do it and measure using GPS.

Charles they can be at the side or in the middle of the carriageway. Usually on a fairly straight road. Measure about 4 inches by 18 inches. There are plenty about if you look for them.

Keith
Cheers, good idea.

Never known what there for thanks.

Easiest check is to get a GPS based speedo on your phone ...lots of free apps available.

I changed everything in the driveline and totally confused the speedo on my '82 plus 8 - 225/45/15 tyres instead of 205/60; R380 box instead of LT77 (mine might have expected a different final drive); 3.08 diff from a Holden V8.

It felt a little quick for the speedo readings so I got an app and found I was doing 57mph at an indicated 50, with the same proportional error at other speeds.

I think it will be too hard to recalibrate so I'll just remember the adjustments to be made on the run. (I am open to advice about how to recalibrate though)

I know people who have used bicycle speedos too on devices from cars to race karts, although they can take a bit of setting up and might not have enough range of values of wheel diameter

Terry
(Canberra)