To deploy effective communication systems in vehicle cavities, it is critical to understand the time variation of the in-vehicle channel. Initially rapid channel variation is addressed, which is characterised in the frequency domain as a Doppler spread. It is then shown that for typical Doppler spreads, the in-vehicle channel is underspread, and therefore the information capacity approaches the capacity achieved with perfect receiver channel state information in the infinite bandwidth limit. Measurements are performed for a number of channel variation scenarios (absorptive motion, reflective motion, one antenna moving, both antennas moving), at a number of carrier frequencies and for a number of cavity loading scenarios. It is found that the Doppler spread increases with carrier frequency, however the type of channel variation and loading appear to have little effect. Channel variation over a longer time period is also measured, to characterise the slower channel variation. Channel variation is a function of the cavity occupant motion, which is difficult to model theoretically, therefore an empirical model for the slow channel variation is proposed, which leads to an improved estimate of the channel state.