Value movements have risen by 41% since the start of 2019.
A study of the used car market by cap hpi showed that March and August were the most volatile months and January the most stable.
Cap has reported market realignment through the first half of the year with values heading downward following an unusually strong year in 2018.
The declines slowed in August as the market stabilised with a fall of 1.0%, the smallest fall since March.
Average diesel car values are consistently dropping by more than their petrol counterparts, with August seeing a 1.1% fall compared to 0.8%.
Derren Martin, head of UK valuations at cap hpi, said: “The data shows the used car market is becoming more complex and volatile. Used vehicles reach the wholesale market through a multitude of routes that all impact on values.
“We also see the pace of the market accelerating as vehicles from the fleet and retail sectors are remarketed faster than ever. We believe that the most accurate data is achieved by combining technology and an experienced eye, especially as the whole system becomes more volatile.
“With the increasing number of models, powertrains, options, and acceleration in generations, it’s clear that the used market will only become more complex as time progresses.
“And while we don’t expect any major shifts in the overall market, we are seeing an increasing disparity and volatility between makes, models and powertrains as consumer tastes shift. We are strongly advising customers to use the real-time data available in our Live product.”