Table:
text: 1971 United States Grand Prix
Classification
Classification
Driver Constructor_ Laps Time_Retired Grid
Chris Craft Brabham - Ford 30 Suspension 27
Sam Posey Surtees - Ford 15 Piston 17
Henri Pescarolo March - Ford 23 Engine 20
Nanni Galli March - Ford 11 Wheel 23
Reine Wisell Lotus - Ford 5 Brakes 9
Peter Revson Tyrrell - Ford 1 Clutch 19

Question: What is the average starting grid with a time/retirement piston and laps less than 15 for the 1971 United States Grand Prix?