I would not be concerned…
Carl
I bought timing light with rpm ability. I always use that to check now. The XJS gauge reads 50RPM low.
Or the timing light is off by 50 RPM ?
As my former comments. No concern. real test is no stall, no creep.
Drive and enjoy…
If you still want to hunt. Access to another car that has compatible ignition and a tach. Try there.,
There are non automotive RPM test device. cheap, I doubt it. Compare with one of those…
Way out :: Look up an RPM vs MPH formula. Go for a drive and record, Speed read, and rpm read. Might not work. Small error here and there. 50 RPM worth…
Carl
You don’t need anything expensive. Most, medium cost, multi-meters (which any home mechanic should have) will show a pulse measurement up to at least 2000Hz.