I agree with Paul, but suggest a longer inter-ping time.
Teckel, who wrote the NewPing library after a lot of experiments with these sensors, found that less than 30ms between pings increases the chance of false readings due to picking up late echoes from the previous ping.
A robot travelling at e.g. 30 kph (is that fast for a robot?) will travel only 25cm in 30ms.
There more information here about NewPing: NewPing Library: HC-SR04, SRF05, SRF06, DYP-ME007, Parallax PING))) - v1.7 - Sensors - Arduino Forum