why does the service light come on at 40mph?

Dave,
Firstly be sure that it is the service light, usually a spanner symbol too. Despite the rather blunt response from Anon, the service could be due which would show the light. There are other reasons for this light to come on, some associated with "limp mode" where the engine loses power and some not. If this is a persistent problem I would suggest getting the fault code read by a mechanic to narrow down the reason.

Cheers now

Oldgrubber
April 2017

Yeah, it is probably telling you it needs a service,doh!

anon
April 2017