You need to set a datum... I.E. Find a straight bit of road, pref on a hill, then do some test runs at various speeds and establish some landmarks that the car reaches at certain speeds..
E.G. On my local Mway, if I am doing 40 MPH at the top of the sliproad, then booting it, I should be doing 70 mph by the time I pass under the 2nd bridge.
Note:- My local Mway slopes uphill toward the East and that's the bit I test my cars on.. If I do the above test in my ZX then the car will struggle to reach 70 mph before I crest the hill. But if I use the gears manually and keep the revs well up, I can get it up to 85 mph over the same distance..