Saxo problems

[Saxocar]

Hi,

I decided to buy a Saxo last week - 2001 1.1 Basic runaround car.

within 1 week i have already encountered a problem - My rev counter now starts at about 8000 rpm and my speed o meter now begins at about 10mph.

So basically when i am going 10mph it will say im doing 20 mph..Not much fun in central london where its speed camara central.

I've seen everybosy say just disconnect the battery? - Is this the best solution and is it really that simple to fix this issue?

anyhelp would be great

Thanks