Series 3 Speedo speeds

Hi, I have just bought my first Series 3 landy, a 1982 petrol 88"
It's a bit of a project, having been sat for at least 6 months in a Norfolk field, but it's fun doing bits to get it something like
So far I've put a new petrol tank and sender unit, but I'm not sure about the speedo
I know when you're doing 30 in a series it will feel like 90 but I do think it's reading incorrectly
I checked the transmission end and found it was only held in by one screw. I replaced the missing ones which seemed to help but I still think it's about 5mph out
Questions
Is this normal/acceptable for an elderly s3? (tyres are 205/r16)
Could the wrong speedo cable have been installed and would this make a difference anyway (I see there are a couple of alternatives)
Could the speedo worm be faulty?
Thanks in anticipation

quite normal , theres only one speedo drive for what you have and cable as long as it fits and turns wouldnt make any difference

You might have a speedo like mine where the needle is on wrong and reads everything out by a few miles an hour...but unlikely

You may have a speedo for smaller tyres there should be a TPM figure in small writing at the bottom that can make a smallish difference.

Alternatively you may have a set of range rover differentials fitted these make the car go 33% quicker (or rev 25% slower) but also make the speed read about 25% low so:

Speedo 20 = 27 mph
Speedo 30 = 40
Speedo 40 = 53
Speedo 50 = 67

Best thing is to measure it with a GPS at different speeds and find out if it's a fixed offset (i.e. 5mph out at all speeds) or a relative error (i.e. 25% too low) and we should be able to deduce from that what's up.

D

Many thanks for the replies, I won't bother changing the cable then. I'll check it with the sat-nav when I've got it a bit further on

Mine is about +2/-5 mph in accuracy, just get used to it I think is the easy answer (usually new cars are about -3mph I think) so not too bad really

If anybody has altered gearing diffs ect it will read incorrectly...or the wrong speedo head fitted....mine under reads by around 30%....a dot of white paint on the face of my speedo to match 10-20-30 mph ect does the trick for me.

Nick.

Again many thanks, I like the idea of checking with a Sat Nav and marking the correct speeds accordingly

I've just found my spare head unit and I'm going to send it off to Speedy Cables this week to get it recalibrated...they quoted me £65 when I asked though that was a year ago!

could be affected by diferent tyre/wheel size too

my other modern car overreads by about 5mph compared with sat nav, at least on the safe side for speeding tickets!