Pretty much what 240Demon said...... I wouldn't be too nervous if readings were a little low as long as they were all about the same. It's also not going to tell you much (compression check) unless the valves are adjusted correctly. Blowby could be a good indicator unless the car has been sitting a long time, there will be a lot of blowby then.

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1985 300D Turbo ~225k
2000 F350 (Powerstroke) 4X4, SWB, CC, SRW, 6spd ~148k
1999 International 4900, DT466e (250hp/660 ft/lbs), Allison MD3060 ~73k