Two completely unrelated questions about my 1991 volvo 245

1) changed my main combination instrument from one with clock, speedometer & fuel gauge to one with a tach. everything went smoothly and everything works fine. Only I get a feeling that my speedometer is a bit optimistic. Of course it is just a feeling cause i have no other acurate way to know my speed but when i glance down it always shows ca. 10 km/h more than i expect (how fast i feel im going). In fact i remember having this feeling before i changed the combination instrument so it shouldnt be related to that and I havent been messing about under the car at all (no new wheels or tires). Actually could somebody please just tell me that nothing could possible have knocked the speedometer 10 km/h off and it is all in my head.

2)The lambda sond warning light came on again. In the old combination instrument it was a lambda symbol (inverted V) but in this new one it is a double curl (a pigstail with two curls) im pretty sure it is still the lambda sond light though, but could somebody confirm that. Anyway, the lambda light comes on about ones a month. The easy solution is to disconnect the battery (resets the computer i guess) then the light goes out for a month. More permently i could of course just remove the light buld in the warning light. But is there in fact anything to worry about? Does this indicate a faulty lambda sond or on the contrary that the sond works fine but that something else is wrong (since the warning light lights up). Is there something I can do myself to solve the problem that this warning light indicates? Does anybody have similar experiences with lambda sonds in old volvos?

Sorry for the long post

-Chris