In reply to Climbing Pieman:
> According to the neighbour it was expensive, partly as only specialised garages do it. Also, any puncture repairs or change of tyres and it of course has to be re done at specialist garages.
Garage near me does it - always seemed pointless for the average driver, as normal air is (as Gravy mentioned) 78% nitrogen anyway.
Incidentally, the last time I hired a road bike on holiday, the guy supplied it with a CO2 pump, but said if I used it, I needed to tell him, so he could deflate and reinflate with normal air. Seemed to think it was bad for the tubes, but I've never heard that from anywhere else. I've not had a problem with the couple of times I've used it.