Currently I'm running 205/50-15 ES100 tires (slightly smaller rolling diameter than stock 195/55). On several occasions while driving on the interstate, I have timed myself between mile markers over a certain distance, and then calculated my actual speed vs. indicated speed.

I have found that my speedometer reads about 4-5 mph fast at highway speeds, so an indicated speed of 75mph is actually around 70-71 mph (~5% error).

However I have also noticed that my odometer is almost exactly accurate. For example, I record my odometer reading at a certain mile marker, then 100 miles later I record again, and I have found that my odometer is off by maybe 0.1-0.2 miles over a distance of 100 miles (~0.1% error).

How is this possible, don't the speedometer and odometer both measure from the speed sensor in the transmission?

I have done both of these tests several times and get the same result every time.

 

Most countries have laws about speedometer accuracy. Particularly Europe, where a manufacturer can get some serious fines if the speedo reads even a little bit low. They're lax about it reading high. What's even more silly (if it's true) is that even if the owner puts wrong-size tires on it... the manufacturer might still get a fine. So guess which way the car-maker wants the error to go?

There's no lop-sided penalty for inaccurate odometers.

 

patrick bud , when you get back to memphis go to the inspection station on washington and they will test it for you , they got this machine sorta like a dyno

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by JimBlake &raquo;</TD></TR><TR><TD CLASS="quote">Most countries have laws about speedometer accuracy. Particularly Europe, where a manufacturer can get some serious fines if the speedo reads even a little bit low. They're lax about it reading high. What's even more silly (if it's true) is that even if the owner puts wrong-size tires on it... the manufacturer might still get a fine. So guess which way the car-maker wants the error to go?

There's no lop-sided penalty for inaccurate odometers.
</TD></TR></TABLE>

Yeah I don't really care about the inaccuracy (at least I'm going slower than what it actually shows, less likely to get a ticket), but I was just puzzled as to why the speedo could be so far off but the odo is nearly dead-on accurate.

 

Maybe someone pulled the speedo needle off and it lost it's calibration. Is it off linearly ie 5 mph off at 25 35 45 etc or is off by a ratio. off 5mph at 70 buy only 2.5mph off at 35. If it's linear you can try to move the needle to compensate.

 

The car was bone stock when I bought it, owned by a woman who bought the car brand new in 94 before she met her husband, so I highly doubt the needle has ever been removed before. I'm pretty sure it's off by a ratio, due mostly to the smaller tires (although it was off a bit when I first bought it, when it had stock 195/55 Michelins on it).