The tires are exactly the size called for in the MB manual. I know the speedometer is correct, I've seen the Digital speedometers beside me and they register within a mph or two of just what I am running. Plus have had a fiend of mine follow me in his '98 Chevy Astro-Van. No the speed is correct. I was already warned by a county sheriff - I used the excuse tires were off size (liar), no it is correct speed registering. I have already done the 60mph test for 5 miles straight, it is right on the money.
When I got the car, I was lucky to get 82mph, but after running 70 to wide open most of the time, it has gradually gotten to where it will run up to 90 with ease. Just got back from eating, ran 80-90+ most of the 18 miles back on the freeway. I can throw out of gear at 60 and coast a bit over one mile. (flat asphalt road)