Is there a way to check my speedometer's accuracy. It seems when I'm going along with the speed of the traffic <70> I'mn going 80-85. Is there an adjustment?

Thanks, first post

 

Did you put smaller tires on your car? That would make your speedometer read high, but it would have to be a significantly smaller tire for it to be off by 15% like you state. There is not an adjustment other than replacing the sensor that is connected to your transmission with the correct model.

To check if it's actually reading correctly, find a highway that has mile markers on it. Find a time when you can drive 20+ miles with the cruise control set and not have to turn it off due to traffic interruptions. It also needs to be flat so that you don't drop from your cruise speed when you go up a hill or coast above your cruise speed going down a hill. Set the cruise control at 60 mph, and time how long it takes to go 20 miles using the mile markers on the highway. It should take exactly 20 minutes (but it won't in reality).

To find your true speed, divide the number of miles by the number of minutes it took, then multiply by 60. Example: it takes 21:15 to drive 20 miles. (20/21.25)*60=56 mph. This would mean you're actually going 56 mph when your speedometer reads 60 (approximately 5% low).

 

No on the tires, all stock. Do you know if the sensor is electrical or mechanical (like a speedo gear)? Not mentioned was the fact that it all seems fine as far as accuracy until I'm over 55-60. After that is when the noticeable inaccuracy happens.

I understand the mile a minute method, I did not know the formula other than that, so thanks for that. I've been able to account for the bigger tires on my Jeep without changing out the speedo gear but this is my wife's car so, well, you know.

Thanks

 

I think the sensors are typically electro-mechanical... as in the transmission drives a gear in the sensor, which then outputs a voltage proportional to the speed. But I'm not sure. Maybe there is something funny where the speed sensing system doesn't understand that you're in overdrive? That's just a wild guess though.

Or, maybe it doesn't seem to be off at lower speeds because 15% difference at 40 mph is only 6 mph, while 15% difference at 80 mph is 12 mph which would be much more noticeable.