Hey, i have a 82 280zx and everytime i drive it the speedometer seems to be off by 10 MPH...i have no idea why...what should i do?

 

do you have different than stock tires... taller or short tires always affect your speedometer unless you get it recalibrated

 

^^^ Indeed... diff size tires or differential swap (mis-matched diff ratio to speedo calibration) can throw it off.

 

well i the stock tires are p195 and i just bought brand new p205....should i take it to a mechanic..? or what do i do?

 

If you put on bigger or smaller tires than stock, you change the circumference of the tire, which means that the tire actually covers a different distance per revolution than stock... check out this nifty lil tire calculator: http://www.miata.net/garage/tirecalc.html

Plug in your stock tire size & then the new tire size. Going from 195 to 205 shouldn't make a 10mph difference, but it will be off a lil. Are you sure it's 10mph off?

 

when i switched to my 130mph speedo because my son destroyed my 80mph'er I had the same issue. My speedometer was always off by quite a bit, even at lower speeds. 10-15mph difference in actual speed and what my speedometer was reading was the norm. The faster i went the further off it would be.

 

there is a gear in the tranny that needs to match the rear end to get the correct speed on the speedometer. If someone switched rear ends or the gear in the tranny you will have this problem. But it should be a linear change in speed ie if 10% off it will always be 10% off not 10 mph off. So are you saying when you are doing 30 your speedo reads either 40 or 20 depending on which way it is off. If that is the case sounds like the needle was put on the speedo shaft off by 10 mph.