I've always used a measuring tape. I make sure all 4 wheels are off the ground with a stand. I measure from rear axle nut to front axle nut, both sides. Then I will measure from the top, front, bottom and rear of the front wheels, wheel to wheel. It's always worked for me.
Dad of #523 Noah Free
2014 150-300 TMXA Champion
2014 Mid-America Arenacross 0-90 open Champion
2014 Mid-America Arenacross 71-90 Champion
2012 Ohio Valley Indoor 0-90 Auto Champion
2011 TMXA 71-90cc Mod Champion
2012 Mid-America Arenacross 0-90 superquad champion
2012 G-Force National Team
969 motorsports Team rider
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