Top end is probably about 50 or so, maybe a hair higher. Your readout will be effected by the 27" tires like Bear said. With stockers on my old analog AC speedometer, it would read 59-60 (way optimistic too), but never would go over 54 with the 27's on it (slightly optimistic). On a GPS, I could get 52-53 out of that 99 model AC 500. The 400 will probably top out similar to it.