Inaccurate Tachometer?

[infernosg]

I originally posted this in the FAQ sticky, but that isn't the right place to ask questions (mods please delete that post) so I'm starting a new thread. For those who want to skip to the point of this thread, ignore the next part.

*Example calculations*

Based on transmission gear and final drive ratios I made a spreadsheet to calculate engine speed for a given road speed and tire diameter because I wanted to see the impact of switching to 5-speed Miata gears and a 4.30:1 rear end. To verify my spreadsheet I went out driving and recorded various gear/speed/RPM numbers. What I found was that the numbers I was calculating were significantly (to me) lower than what the car's tachometer was telling me. Below is an example.

Speed: 30 mi/hr
Tire diameter: 24.7 in.
Gear Ratio: 1.366:1 (3rd gear)
Final Drive Ratio: 4.10:1

So based on that information the engine speed should be calculated as follows:

Tire circumference = 3.1415926 * 24.7 = 77.597 in.
Vehicle speed (in./min) = 30 mi/hr * 5280 ft/mi * 12 in./ft * 1 hr/60 min. = 31680 in./min
Tire rotational speed = 31680 in./min * 1 rev/77.597 in. = 408.632 rev/min
Driveshaft speed = 408.632 rev/min * 4.10 rev/rev = 1673.879 rev/min
Engine speed = 1673.879 rev/min * 1.366 rev/rev = 2286.519 rev/min

So theoretically, at 30 mi/hr in 3rd gear my engine should be spinning at about 2300 RPM. However, when driving I notice the tachometer is actually reporting about 2600 RPM. This trend is repeated for just about every gear/speed I recorded.

*Question Below*

My question is, how "off" are our tachometers usually? Is there a way to recalibrate them or am I S.O.L. and forced to compensate in my mind (will make tuning later on a bit of a pain)? The other possibility is I'm completely screwing up the math, but the fact I'm off by ~300 RPM doesn't make me believe this is so.