speedometer and odometer not reading correctly
 

My speedometer I think is reading faster than I am actually going, for instance if it says I am going 25 I clocked through one of those mobile speed reader things at 17-21 somewhere in there. I think this is also affecting my odometer as I believe (or hope, otherwise I am getting awful gas mileage) I think that the odometer is reading less miles than I have actually traveled. I was wondering how I could check this and correct or calibrate the speedometer some how?

Thanks for any info