What can be the problem? rings, head gasket, valves. Anything else???

 

if they were the same i would be happy could just be your guage thats why its a low reading, try someone else's i know my reads in the middle compared to my friends his one reads low like your and my other friends guage reads high. If you had numbers that were like 175 140 120 160 then i would be worried.

 

That was my first thought. Thanks.

 

Also car does not smoke at all. But it is running very lean. I check the plugs, or it can be very old.