Huh???

Any decent mechanic worth his salt can tell you that the spark plug has nothing to do with the speedometer/odometer malfunctioning. AFAIK, not only Mitsu engines require resistorized plugs, even Honda & Nissan engines have OEM "R" spec plugs. And those plugs are properly shielded from RF interference. That mechanic's diagnosis is highly doubtful.

Check the VSS (vehicle speed sensor). That's the common culprit for errant speedometers.