I've got a Series V with overdrive. I've just done some work on the engine and it's running really well now, very smooth, starting nicely, idling smoothly and so on, the gears change nicely and the clutch seems ok. I just can't shake the suspicion that the car is lot slower than it should be. I'm reasonably confident that an indicated 4000rpm on the tacho is accurate, but when I go out and use my satnav as a speedo (so much more accurate than the one in the dash!) it tells me I'm doing about 50mph, in overdrive top, at an indicated 4000rpm. This seems way too slow. Can anyone with a standard Series V tell me what kind of speed I should be reaching at 4000rpm, and has anyone got any ideas why I'm so slow?

With 5.60 x13 or 165/80 x 13 tyres the OD 4th gearing is about 20.3 mph/1000rpm.
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So 50 mph is almost 2500rpm. 4000 should be 81mph. If you have the higher non-OD axle, with OD, you get 22.5/1000 - near enough 3000 at 70.
Best way to check speedo is on a Mway or dual carriageway where they have 100m or 200m posts - they are marked with the distance. 16 or 8 to a mile.

It turns out that although the car was running well, the coil was doing really funny things to the tacho which meant it was reading all over the place. I've just replaced the ignition and coil with Pertronix kit, and the rev counter is now rock solid over the whole rev range.
Having done a couple of runs it seems the relationship between speed and revs, according to the instruments, is exactly what it should be.
May I suggest my own preferred method for checking the speed? Buy or borrow a satnav, and set it to show your speed. Much easier than trying to count mile markers and drive at the same time!