(a bit "syncro 
)When I checked using my sat nav, the speedo is now reading pretty much exactly correct, whereas with std size tyres it was about (the usual) ~10% fast.
It has now occurred to me that the odometer will also be reading 10% less. So, when it now says I have done 200 miles on a tank of LPG, before I fitted bigger tyres it would have read 220 miles. This is not an insignificant difference, especially when comparing MPG with other folks vans
And for comparing MPG pre / post bigger tyres, I suppose I will have to add 10% (which in my case seems to work out right as it used to do 220 miles on a tank of LPG on the standard circumference tyres) ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Aside = this is before we get into the ins and outs of better MPG with the larger rolling radius v the higher gearing, requiring more frequent down-shifts, (though I have not noticed that on our relatively lightly loaded Tin-Top, though it does feel more "long legged" and more relaxed at 70 on the motorway).
