The tach is usually off by a percentage, the speedo is usually off by a set amount plus percentage. If you limit yourself to 10mph over the posted limit (posted = 65, speedo says 75, actual = ~71), you'll stay under the proverbial radar of 99% of traffic cops. I've verified my odometers are spot-on with the correct tire, so they probably built it into the speedo to err on the side of caution. Similar to the stupid idiot anti-lockout on the driver's lock.
The coolant gauge is accurate for what it's designed to do. It basically tells you if the motor is NOT w/in optimal temp range instead of telling you relative temp. It will rise linearly up to ~160F, then barely move from that spot until 220F or so then rise linearly again. I know those numbers are off, but I think the actual values are in the helms manual.