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Speedometer Calibration


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On a recent trip, I noticed that my speedometer was reading a bit higher than my Garmin GPS was indicating.

Since I have 17" aftermarket rims, I am thinking its normal to be off a few percent.

On a level stretch of highway, I turned the cruise control on and set the speed so that the speedometer needle was pointing

precisely at the 70 mph mark. The Garmin reads 67 mph. Ok, I am not all that happy with that but blame it on the wheels.

Then I get to thinking about how my odometer must be racking up more miles than I really have. So, I decide to set the trip

and check it with the Garmin miles. When the Garmin showed exactly 100.0 miles travelled, the Lexus odometer showed 100.5 miles.

Pretty darn close. In fact, within 0.5%. Not bad at all.

Then, I decide to check the average speed indicator on my Lexus. I reset that with the cruise still on and it reads 67 mph! Same as the Garmin.

That convinced me that the car speed sensing system knows how fast it is going (and is almost dead on) but the analog needle reading is off.

I don't think there is anyway to correct the needle reading but thought it was interesting. I can live with it.

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My '99 is off too (with stock sized rims and tires), and it drives me batty. The trip computer, GPS, and radar speed signs all show the same 3mph error in my car, so 3 against 1, and LS's speedometer is the loser. It's kind of ironic that Lexus tries to slow you down by 3mph when they make a car that goes 150mph. Some claim that 5%-10% error is "acceptable." Right. I've driven many GMs with perfectly accurate speedometers, so this obviously isn't rocket science.

I've grown to live with it by using the on-board trip computer, Garmin, or judge the speed based on the RPMs. I know for sure that before my next car purchase, the Garmin is going to go on the test drive with me. I took a video of mine a while back...

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I think the "mis-calibration" is intentional on the part of Lexus. They want you to drive slower!

The car knows exactly how fast it's going!

My 2009RX350 is also "off" by 3 mph (at most speeds 60-80mph). I too believe it is intentional to show a little faster speed than actual -- helps prevent tickets. Most cars do the same.

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Oh this is "eye opening" :blink: More than a strange coincidence that all of us have the +3 mph needle increase BUT the car knows the real speed! So yes, I would

agree that it sure looks intentional. We bad boys need some restraint. Wonder if it is gov't mandated? :angry:

I also agree Blake, it is not rocket science. (good video BTW...same thing I did). My Chrysler speedo needle is dead nuts on. (of course they were probably trying

to make it +3 and missed :lol: ).

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My 1998 has stock tires and rims and the Garmin GPS indicates about 2-3 mph less than the speedometer at speeds b/w 60 and 80. I just noticed it two days ago and was wondering about it and then happened to read about it here. Interesting. My 99 LS has the stock Nav system which does not display speed. I might check it at some point against the Garmin.

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Also consider that the road speed indicated by the Garmin GPS is not exact but an approximation. I've had three different Garmin GPS devices and none have been advertised to be perfect.
My Garmin shows exactly the same speed that K and Ka band radar speed signs show, and it shows the same speed as my car's onboard trip computer, so I have 3 different devices that are in agreement that the speedometer shows the wrong speed -- which is enough to convince me. As it was said before, the car knows how fast it is going; the speedometer is where the problem lies.
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