Well, if it's revving really high, change up! They're electronically limited to a max of 4,800 rpm or so, as I remember, so in theory it can't get terribly high. They're very leisurely in their acceleration after around 50 mph - not exactly a good overtaking car - so be patient. You don't say how long you've driven it flat out, but unless you've got a very long straight you don't see the top speed. Any warning lights? Little yellow picture of an engine lighting up anywhere on the dash? Is it limiting itself because the ECU has detected a fault? Usually it might benefit from a service - get some new filters and fluids in there and check that there's enough air and fuel finding its way through. Then see if there are any fault codes showing - find someone with diagnostic kit or take to a garage if you haven't got one yourself. If it's able to rev right through the full range I'd suspect it's not the accelerator potentiometer (or 'driver demand' as it's sometimes called) itself that's faulty, but something to do with what the ECU does with the information.
