Speedometer ok but miles pilling up faster :(

[manafas]

Hi
I wonder if someone has hd this problem or knows the possible cause/cure for it:
I purchased a 2001 XT600E about a month ago. It showed 13,000 miles and I had no reason to believe it had been tampered with, although its quite possible.
I am using the bike to commute to work everyday since, a daily return journey of around 100 miles.
Although the speedometer seems to show the correct speed, the mileage seems to be running at about 50% over the real distance made!
Is there any adjustment on the speedometer itself?
It even occurred to me that it could be registering kilometres instead of miles but then the odometer wouldn't say MPH but KMH instead.
Any ideas?
Many thanks for reading this.