I pass by one of those police radar stations that they usually put around construction sites almost every day and it always seems to show that I'm doing 2MPH less than what my speedometer is showing me. It doesn't seem to scale either as speed goes up. If I'm doing 45 on my speedo, the radar says I'm doing 43. If I'm doing 55, it says I'm doing 53. Is this normal? I have an '04 RX330 with the stock 18" rims/tires.

Pete

 

You have to wonder how often they calibrate those portable radar units, but I must admit that I've had the same experience.
I've found that the RX330 (also w/18" wheels) seems to be a bit closer than my 300's were, but even the 330 reads 1-2 MPH high according to those radar signs.

Let's see... speedo reads high... fuel gauge reads low. The people who designed the RX clearly choose to err on the side of caution!

 

Due to variations in the diameter of different brands and sizes of tires--even the OEM tires--the speedometer isn't likely to be dead-on. I asked about this on www.lexus.com a few years ago and got this reply:

I am following up with you regarding the speedometer accuracy on you 1999 GS 400. The following are the specifications for a Lexus dealer to perform a speedometer accuracy test. If the vehicle does not fall within specification, it may be due to non-OEM wheels/tires, tires are under inflated, or tires are worn, etc.

Speedometer Tester Reading/Speedometer Reading
20 mph/18-24 mph
40 mph/38-44 mph
60 mph/56-66 mph
80 mph/78-88 mph

This is pretty typical. I've seen ranges like this in the Toyota/Lexus shop manuals.

Comparing my speedometer with my GPS, my winter tires cause my speedometer to read dead-on. But my summer tires cause my speedometer to read about 1.5 mph high.

 

Interesting.

I wonder how much auto makers put into this. I can't see them making speedo's that display speeds slower than you're going for fear of making warrantees longer than they would otherwise be but I can seem them doing the opposite.

 

You will find that almost all new vehicles with an analog speedometer will be inaccurate and err on the side of caution. Meaning, they will read a few mph faster than you are actually going. You will notice a greater error at higher speeds. This is normal and done on purpose. I remember reading somewhere that some countries pose a fine on vehicles with a speedometer that reads slower than you are actually going thus the benefit to having a safe margin of error. The same article sited GM as having, on average, the most accurate speedometers.

Now, I specifically said that "analog" speedometers are inaccurate (and purposely slow) because in my experience digital speedometers are highly accurate. I have measured my digital speedo on my Nissan Z and have noticed that those on C4 Corvettes are 100% accurate with police radar guns. While the Z's digital speedo is perfectly accurate, it's analog speedo has the similar margain of error as you see on you Lexus.

Finally, while your speed is not 100% accurate to your real-time speed, this does not affect your odometer, trip meter, or digital computer readings. You can test this by resetting your trip computer's average mph while setting the cruise control on the highway (on a relatively flat section of highway). You'll notice a difference between your analog speedo's readout and the average mph displayed by your computer. The average displayed will be higher.

If you're interesting in more info, subjects like this one have been discussed before on ClubLexus with regards to the speedometer and others with regards to tire sizes and how it affects the speedo. You can try a general search in the RX or other forums but there isn't much more to it than what you've read here

 

Car and Driver did an interesting story about the speedometer a while back. They found that most auto makers made the speedometers look "faster" than the car's actual speed. On average, Toyota is 1 MPH over the actual speed, BMW is 3 MPH over, etc.

 

HarrierAWD that's where I read about it at, Car & Driver. I just couln't remember where the article had come from. I have that article somewhere in my pile of C&D magazines, do you have any idea on the date of the magazine?

 

I have an Acura TL (I know boo-hiss). I found the same problem with it. It reads about 4% high. Unfortunately, so does the odometer. I used one of the handheld GPS units on a reasonable length trip to get it sorted out. Of course the dealer played completely dumb. They didn't grasp the concept of using the handheld GPS to compare to the speedo/odometer. What a way to shorten your warranty by a few thousand miles. Our Dodge Ram was so close we called it perfect.

I'm trolling this board to learn a little more about the RX330. Considering it and a few other SUV's and cars for a potential purchase down the road. But I had to sign up and respond to this speedometer issue because I vehemently disagree with the acceptable speedo ranges earlier in the thread. That would probably mean the odometer is very inaccurate also and you are being cheated out of warranty and value with the excess reported miles. If mine was 10-15% high, I certainly would be more aggressive in getting it corrected.

 

Now you've got me curious - I have a Garmin GPS V and will test the RX330's speedometer and odometer with it.

 

It's in the April 2002, quite an interesting article.