# Tire Size

I have a 2000 Hyundai Elantra with tires 195/70 and the label inside the driver’s door calls for 195/60. I find that my speedometer reads about 3mph slower than the actual speed and I wonder how off my odometer must be. I have roughly figured that moving from mile marker to mile marker on the interstate at one mile intervals my odometer doesn’t quite turn over. My guess is that for every mile I actually register 0.95 mile. Is there a calculation or chart for the diference in the odometer with these incorrect tires? Thank you for your help.

Looks like you calculated just about the right error. Go here to see what it is:
http://www.miata.net/garage/tirecalc.html

How do you know the speedometer reads slow? Normally a speedometer in intentionally calibrated to read fast, to help you prevent speeding tickets and then blaming them on the car. We need to know how fast you were going before we know that 3 mph difference is meaningful. For example if you were driving 3 mph it would be a big difference if you were driving 60 mph it would be a lot less meaningful.

There is about a 6% difference in the speed displayed by your speedometer with those two tyre sizes.

I don’t know that a speedometer is calculated to read too fast or slow. Just having tire pressure monitor systems calculate pressure based upon tire rotational differences tell you that with many driving around with too low tire pressure will have a faster than actual reading w/o any intent by the manufacturer.

So, aspect ratio does make a difference in speedo reading as does wear, as does tire pressure which is affected by temperature and atmospheric pressure. That’s why the margin of error and why your most reliable reading is your GPS speed monitor device and road side timing of mile indicators. When GPS (or Radar) and the like are made to react quickly (or cheaply) enough, present systems will be a thing of the past.

Until then, accept the margin of error, and adjust to the things you do have control over (tire size and the like) cause it’s all normal, plus OR minus. Being accurate is the present speedometer exception, not the rule.