Inaccurate odometer

[CragGriff500]

For some time I have been suspicious of my odometer being inaccurate. I seem to have covered loads of miles without actually driving that much. Last night I traveled from my home to Nottingham, a journey which according to the sat nav was 30 miles. Now, my odometer increased 40miles! Not very impressed at my odometer recording an extra third per mile covered. I have original wheels with toyo proxy 225/50r15 and 245/45r16 on the front and rear. The speedometer seems pretty accurate. Has anyone noticed any similar problems? Did you try and re-calibrate the odometer somehow or just leave it alone? I wouldn't mind if it was under reading. Thanks in advance guys.