Having checked my speedo against a portable sat nav reading, I am horrified to see it is way out.
At true 30mph it is showing 39mph and gets progressively worse.
Is this common with Morgans, mine is a 1995 plus 4, and any suggestions as to the possibility of getting it recalibrated or is it a replacement job.

Curiously, my speedo was regularly 5mph-9mph out (under reading) against sat nav. When I replaced my tyres, all 4, I found it back to parity at low to midrange speeds, and 2-3mph under at motorway speed.

I bought it with full set of new tyres fitted so nothing to compare previously.

It's not legal Colin.

At least you are very unlikely to get caught speeding, if you go by the speedo reading!
But you might be pulled for causing an obstruction.

I suspect someone has fitted tyres with a different profile, so effectively changing the wheel size and altering the speedo drive gearing.
You don't mention the model and the wheel/tyre size fitted.

Construction and use regulations allow a speedo tolerance of -0% to +10% so all speedos should overread. To accommodate manufacturing tolerances most are towards the 10% error but 39mph at 30mph is outside the limit.

True but since its never checked by officialdom, that doesnt matter.

I would be a bit cautious comparing with a satnav because they arent htat accurate on speeds paricularly if unudlations and hills are involved. Their speed calculation is the distance between two calculated points on the geoid and the time difference. That said the satnav wont be 30% out or anything like it. Check wheel and tyre sizes. Check if the speedo has been replaced - does wonders for resale value sometimes!

If the speedo is digitally calibrated, another possibility is that someone has inadvertently changed the calibration number.

I cannot remember the numbers or the button sequence required to display the number - hopefully someone will be along soon who knows what they are talking about.

The law allows more than that Chris, it's up to 110% + 6.25 mph over read in the range 25 to 70 mph. A cars speedo could legallly be indicating 50.25 mph when actually travelling at 40 mph

Sadly mine under reads by c 10%. MMC say it is legal, hence my GPS speedo. I know the GPS has faults but that combined with rpm & gearing is helpful