Speedo under reads?

[Tornado]

Hi, my new-to-me 2003 K12 1.0 speedometer seem to under read, compared to the revs and GPS. (I read the car is geared 18.6 mph per 1k rpm.) At 2k rpm the clock says 33mph, should say 37 or so. At 2.5k rpm, speedo 40, should be 46. My GPS can be hard to lock on, but when it does it seems to say speedo is under reading. That all make sense? Any way to prove real speed short of asking at the next police speed trap? Hi officer, mind if I drove by you again?

 

[vexorg]

Is everything standard?
What sized wheels and tyres?

 

[Tornado]

Thanks, wheels and tyres seem standard 165/70/r14. Tyres are an unusual brand, and a little perished but even so. Speedo seems out 10% or thereabouts . Can they be recallibrated on th K12? I think it's mechanical, maybe a different cog at the gearbox end? Garage I bought from says GPS is not accurate enough to recalibrate. Hmm. Maybe his obd2 reader might show the problem. Anyway, have a Cool Yule.

 

[Mika The Finn]

Tyre size will cause easily error that you described!
Look at this: Tyre Size Calculator

 

[Tornado]

Went back to the garage, showed him pix of revs and speedo being out of sync, he went for a spin with his OBD2 plugged in and managed to get the speedo pretty close to where it should be. Somehow. Cool, that's a relief.