Compression ratio and operating speed for early engines are rarely mentioned - but both are quite low, due to the metallurgy and designs. Car engines are (usually) a step up from hit & miss engines - which can run slower than 1 rotation per second (60 RPM). I've heard a single cylinder Olds running perhaps as slow as about 100-200 RPM anyhow - with a later style carb. (which has better adjust-ability) The brush could be similar. I expect the C/R might be around 4:1 - maybe even less, probably not much more. I've thought more than once about digging up a tachometer that can measure these cars - remember, they may be running on a buzz box or magneto ignition, so an external voltage source for an electrical tach is needed, or a mechanical / optical tach.
I searched through my 1918 Dykes Automotive Encyclopedia, and did not find good listings for either. Did find a reference about SAE horsepower ratings - such as a car listed as a 25 / 30 HP car - the 25 would be at 1000 RPM, and the 30 would be as tested on a dynomometer (no RPM listed)
I've seen the 1908 Ford model T listed as 4.5:1, and engine speed of at least 1600 RPM.