All 2st Speedometer re calibration?

Hi all,
Today I fitted my WRE 125 with a pair of 17" SMS4 125 wheels, the wheels were a straight swap but now the speedo reads out did to a smaller diameter. Obviously I can use a GPS speedo but this isn't ideal as I don't want the bike indicating a higher mileage than it has especially with 2 strokes becoming rarer and rarer. Therefore is there anyway to re calibrate the dash to display the actual speed I'm travelling with the 17"s on?
Thanks,
Ryan

Found something on how to do this on a KTM and the trick is to disconnect speedo from the permanent feed on bike´s battery ,so as to let it run just on its internal battery!Then you should get a reset WS (wheel size) message!
http://www.thumpertalk.com/topic/597179-how-to-recalibrate-ktm-speedo-for-17s-wpics/

I haven't been able to get the WS message to show, so I'm going to order a trail tech vapor