78.2 MPH;
The motor car's versatility is legendary. It overwhelmingly won every reliability trial and distance record, dominated the great Alpine Trial of 1913 and won the Spanish Grand Prix of that year.
In May 1907, Claude Johnson drove the car to Scotland and back. This run was a precursor to the Scottish Reliability Trial for which the motor car was later awarded a gold model by the RAC.
The original idea was to drive 10,000 miles without stopping the engine, but the Silver Ghost proved so reliable that the target was raised to 15,000 miles. Despite a stall at 629 miles, when rough roads shook the petrol switch to the off position, the Silver Ghost ran faultlessly for 40 days and nights.
A further challenge was designed by Napier for Rolls-Royce to compete against them in a run from London to Edinburgh followed by high-speed runs at Brooklands. But the challenge was to complete the distance without changing gear, as opposed to how far you could travel. The car, driven by Ernest Hives, averaged 24.3 mpg between London and Edinburgh and attained a speed of 78.2 mph at Brooklands.

http://www.conceptcarz.com/vehicle/z11360/default.aspx

I believe they only had 6 cylinders, top speed that I could find was about 80MPH.