CTtech

Just curious if anyone else has had this issue. I have the factory 18" wheels on and my MPH seems to be off by 2-3 MPH. I figured my actual MPH by timing between 3 separate mile markers on the highway with cruise set at 75MPH and took the average witch was @50 seconds. I then divided 360 (Minutes in an hour) by the 50 (average time to travel 1 mile) this shows me traveling at @72MPH. I did the same for 70MPH and 65MPH and the difference was roughly 2-3 MPH each time. Is this normal? Can this be adjusted in some way? It's not that it's the worst thing in the world to adjust in my head, but I don't feel I should have to on a brand new vehicle.

Thanks in advance for any assistance,
CTtech

RX330inFL

Most all vehicle speedometers are off, just varies between makes and models. As the RX comes with 17" and 18" factory wheel configurations the speedometer has been set to be a compromise between the two.

Here is an experiment for those who care:

During the average life of a tire you will wear off 8/32" worth of tread, or 1/4 ". What affect will that have on your speedometer reading between new tires and well worn tires? Clue: By reducing the tire depth 1/4" you are reducing the overall diameter of the tire 1/2"

I have one of those automated speed signs I pass on the way to the office. It shows me doing 42 when my speedo shows 45. Yes, I have the 18" wheel package -- originally with Michelins now with Bridgestones.

There is no way to adjust.

I have driven cars which cost many times more than my RX and the speedos were off far more. At least yours reads low so you will be going slower than you think, if you only go by the speedo reading, when you go through those speed traps.

Grumpa72

I would venture a guess that being off by 2-3 mph is well within the tolerances of the manufacturer. In addition, while timing of mile markers is fairly accurate, I would bet that a 1-2 mph error can be estimated on your part.

Fwiw, I have a GPS unit in my BMW and their speedo reads 2 mph fast. So, in actuality, I am driving slower than BMW says and, on their discussion board, I find that I am not alone on this.

Have you tried independent GPS verification?

Gary

shikbas

i have this digital cruise screen comes with laser cruise control,the speedometer reads 2-3 miles less than the speed on the digital display.

salimshah

Another pursuit of accuracy ....

One needs to understand how each system works and how the measured to displayed information is shown. Each system suffers from tolerances and some are governed by laws.

Lexus calculates distance by rotation (requiring you to drive in a perfect straight line) and circumference (tread + air pressure) contribute to deviation . Crystal vibrations determine time and there is lack of precision there. Once all the math is done it is converted in to appropriate voltage/current [may further be inaccurate based on voltage stability. Then you have deflection variance of the needle and even placement of the gauge. For a mass produced vehicle you can never expect accurate reading and calibration makes little sense when the circumference is a changing number.

My understanding is that the speedo is required to be +/-5% till 45mph (based on memory ... I wont be surprised if it is a different number). Another tidbit is that in GDR the requirement is +5/-2% (again not sure about the exact number) but that makes the manufacturer's to skew the center to show faster mph/kmph. ... I infer that is the reason that you can contest a ticket if you are within 5% of the posted speed.


GPS also has it limitations (some intentionally introduced by DOD)

Don't let the drive for accuracy spoil your enjoyment of driving. Estimation is a wonderful thing. Enjoy the pumpkin pie instead of calculating 22/7. Or figuring out how to precisely split the pie in equal thirds.



Salim

CTtech

First off, let me say thanks for all of the responses. Some interesting thoughts. I plan on bringing it up with my dealer regardless. If they say there is nothing they can do, then I can live with it. At least it is to my advantage as far as I am actually traveling slower than my speedo shows. It surprises me as the only other time I have seen this issue is when the wrong tires were put on my last vehicle, and this was corrected by swapping the tires back. All in all I am more than pleased with the RX and wouldn't hold this minor annoyance against it.
Thanks again,
CTtech