I forget the reasoning behind it, but I read someplace that a speedometer will read 1-2 mph too fast up to about 80 mph, 2-3 from 80 to 110, then they tend to vary in inaccuracy above that. This was back when I had my 97 M3 and we used to always wonder why the speedo would show one speed and the OBD would show another speed typically just a few off and more so at higher speeds.

 

you know what you can is go at max speed, reset your average MPH meter and see what your actual speed is. i suspect the speedo doesnt lie that much and that at 140 youll be going 136 or 137.

 

The towns around here love to put those SMART (Speed Monitoring Awareness Radar Trailers) on the side of the roads; I've observed my IS consistantly reads a few MPH high, even at slow (35-40) speeds.

 

even radar is off a little bit sometimes. the only reliable way to test your speed it with known distances (measurements you find on the freeway that are there specifically for this) and a stopwatch. even your handheld gps is inaccurate to some degree when it comes to velocity (speed and direction).

 

If you are doing this make sure you have at least the right tires. Non-summer tires could be a bad thing waiting to happen.

 

I'll give a lay person's explanation since I recently had to address this problem for a 1966 mustang I was restoring. The speedometer gear has a certain number of teeth, e.g. 10, 12, 14, 16, or 18. The gear type will vary according to a formula that takes into account the tire size, tire profile, rear axle ratio. In most cases, no gear will be a perfect match, so you try to pick the one that is closest to true speed at a particular speed--say 50 mph. My car was reading 15-20 mph fast so I had the gear changed out twice and was able to get within 1-2 mph accuracy or (2-4%).

Assume the percentage accuracy stays the same, the faster you go, the bigger the error will be in mph. So if I'm going 100 mph, the speedometer error doubles in absolute terms to 2-4 mph, but is still the same on a percentage basis.

 

That makes sense. I wonder if the CVT trannies have better accuracy?

 

Not really, this is 2006, not 1966. The speedometer in the IS is electric, positioned by the computer based on speed pulses coming from a magnet and coil at the output shaft of the transmission. Unlike the '66 Mustang where lifeisgood had to choose a specific gear, Lexus engineers are free to devise an accurate mathematical formula, including a correction factor for different rear ends and tire diameters, to convert pulses to vehicle speed.

 

Finally, the information I knew was out there. Why some people stay stuck in the past is beyond me. There (may) be a slight difference between tires over or under inflated but otherwise the speedo is 'right on' period.

End of story........lets move on

 

It's 2006 not 1996. Ten years ago they did use the driveshaft speed, but now they decided since they have wheel speed sensors on all four wheels it makes more sense to use the direct wheel data and not the driveshaft. The sensor on the transmission produces a signal SP2, it is used in conjunction with NT, the input shaft speed. The skid control ECU produces SP1 and sends it to the combination meter. The skid control ECU does not have a direct input from the transmission.

 

any way to remove the limiter ?

 

 

My RD tracks speed and its spot on with my speedo. :shrugs:

 

..........

 

..........