Greg Laden's Blog

The following is from a report filed with Physics News Update in September, 1998:

ANOMALOUS ACCELERATION. Data from several spacecraft, including Pioneer 10 and 11, Galileo, and Ulysses, provide evidence for an unexplained, weak, long-range acceleration, a new report shows. Position and velocity information is derived from radio signals sent from the craft to the Deep Space Network back on Earth. Any change in velocity over time can be ascribed to a variety of known sources: the sun and planets, the solar wind, the Milky Way, the Kuiper belt, etc. But even after taking this all into account, as well as other possibilities such as the presence of dark matter in the solar system (only a millionth of a solar mass of dark matter could reside within the orbit of Uranus, it is estimated) or gas leakage from the vehicles themselves, a small acceleration in the direction of the Sun---8 x 10-8 cm/sec2 for Pioneer 10---remains unaccounted for. ... [source]

And this is from a more recent report, at the end of February, 2008 (Earth time):

A new look at the trajectories for various spacecraft as they fly past the Earth finds in each case a tiny amount of surplus velocity. For craft that pursue a path mostly symmetrical with respect to the equator, the effect is minimal. For craft that pursue a more unsymmetrical path, the effect is larger. In the case of the NEAR asteroid rendevous craft ... for instance, the velocity anomaly amounts to 13 mm/sec. Although this is only one-millionth of the total velocity, the precision of the velocity measurements, carried out by looking at the Doppler shift in radio waves bounced off the craft, is 0.1 mm/sec, and this suggests that the anomaly represents a real effect, one needing an explanation. [source]

I'm sure an Earth Car exhibits a greater anomaly, say, in the odometer reading due to wearing down of the tire. In other words, this is a very tiny amount.

Spooky.