Tracklogs' figure for the BCM route (scenic, I think?) was about 12,000m when I did this.
I think the GPS (Etrex Vista HCx with barometric altimeter) came up with a figure of around 7,000m when I rode the 2004 route last year (it's claimed to be 8300m by contour counting). The GPS being much closer to reality. Tracklogs seems to get it wildly wrong in that sort of terrain, for the reasons already described in much detail. I don't think you can apply a fixed scaling factor since the amount of error seems vary widely.
I have frequently observed very accurate summit heights from my older Extrex Vista and the HCx - e.g. 609m summit of Black Hill above Nenthead in the Pennines being shown as 610m - when you account for the fact that the GPS unit is on the handlebars, that's pretty accurate.
Maybe when I start some hill repeats later this month I'll take the gps round with me and log it all and see how consistent it comes out.
