foxmoth
I don't think checking the tachometer with a stobe would be practical every time I fly.
Mariner9
The approach was still flyable and of course the power setting depends on a number of factors, but without the tachometer one doesn't know what power setting one has selected and that doesn't make life any easier. It's quite possible to do an approach without an ASI too, but I'm not going to try!
Yes - I probably should have realised there was a problem when doing my power checks, but the mag checks were fine. It's even possible the problem developed during take off. The a/c had been flown previously that day by an instructor and he obviously hadn't noticed it.
It's possible too that the error was not linear over the whole range - being greater at the high end.
The propellor was of the fixed pitch type.
BackPacker
I found it out myself - some time after leveling off after take off. This would have meant I had done my power checks at 1200 rpm rather than 1700 rpm. Also if the engine had not been producing full power on take off I would have been misled into thinking it was. I probably would have realised this by sound alone, but one can't always be sure.
The flight was charged by time and not tacho hour so I wasn't over-charged.