What would cause a tach to be off? By my estimate, I read 1500 when the engine is at 1000RPM. And if my calculations are correct, I get a reading of 3000 at 2000RPM. Have I been lugging me engine all this time? If my calculations are correct, that means I have been operating at 1800 - 2500 RPM

 

May I ask what calculation you are using to think the tach is incorrect? A mechanical drive tach either works or not (broken inner cable)
A normal idle is about 900 to 1000 rpms. Less than 900 the engine will stumble and die, unless it is in exceptionally good tune.
I just wonder what makes you think it is not reading correctly?

 

Am I correct in assuming that for each pop of the exhaust on 1 cylinder, the engine revolves twice? If so, I calculated the RPM by counting the number of pops while running on one cylinder at a specified indicated RPM. At an indicated RPM of 1500, I counted 40 pops in a 5 second period. I multiplied the number of pops times the number of revolutions per pop (2) to get the total revolutions in a 5 second period. I then multiplied the number by 12 to get the total number of revolutions per minute. (960). If my method is accurate, I get 960 at an indicated RPM of 1500. I then set the RPM at 3000 indicated and counted the revolutions using the method I described above and got around 2000.