The above graph shows their oil consumption from 2000 to the most recent data. I have two sources - the annual data in the BP spreadsheet, and the recent quarterly data from the EIA. The two should not be assumed to be measuring the exact same thing. Overall, the CAGR from 2000 to 2008 is 6.7%, much lower than the growth in the car fleet. What gives?
According to researchers at Lawrence Berkeley Labs (with hat tip to commenter Sparaxis at TOD), in 2007, the breakdown of Chinese oil usage was as follows:
Given that, in 2007, the official size of the private car fleet was 23 million vehicles, the average Chinese car was using 5.5 barrels of oil per year. The ballpark number for the US would be 250 million cars using 10 million barrels of oil per day, giving about 15 barrels/year/car.
So a Chinese car uses about one third of the oil of an American car if we believe these various stats. This doesn't sound crazy. I haven't yet found a basis for estimating the average fuel economy of the deployed Chinese fleet, but it could easily be 50-80% higher than the US fleet. Multiply by a similar factor for the average Chinese car being driven less due to denser cities and lower incomes, and you could get to a factor of three.
If that figure stayed flat, then around 2017 when the Chinese car fleet reached US proportions, it would still consume much less oil - around 3.5mbd. However, as Chinese incomes continue to increase, and as cars increasingly move into the rural areas, the barrels/year is likely to get worse, unless restrained by high oil prices.
I will try to put this together into an uncertainty interval in future Chinese demand in a post coming shortly.