toneman

My guess is that the odometer reading is not based off the tire diameter...cuz otherwise it would be too easy to trick the odometer reading so that it can read higher or lower than actual. Probably either off the wheel hub or some gear inside the tranny is where the odometer reading is taking place...

chuckb

I don't know about that, but actually I believe honda just recently got in trouble for this same thing and had to extend warranties on some models.

http://www.usatoday.com/money/autos/...ter-usat_x.htm

dreyfus

Actually, no matter where the reading is taken it is affected by the wheel diameter. Think of it, hub or transmission, the movement of the measuring point is ultimately decided by the revolution of the tire. If your tire has a smaller diameter than the stock tire you will have more revolutions than the speedometer programing expects for a given distance and it will read faster. If you have an oversized tire then the speedometer will read slower than your actual speed. The odometer is similarly affected.

This is well known to people who upsize their wheels. Calculations to determine your speedometer error based on a change in tire size are automatically done at this site: http://www.miata.net/garage/tirecalc.html. I found it very helpful. Check it out.

toneman

Okay that makes sense...however, would you know how many modern motorcycles can apparently record fairly-accurate odometer readings? As you may or may not know, speedometers on many motorcycles are notorious for reading quite a bit higher than actual, even at speeds as "low" as 50 mph. However, on the bike I used to own ('03 GSXR 750) I did a short test where I hooked up my portable GPS to it, set the tripmeter on both the bike and the GPS to 0, and then rode it for 30 miles (in an area w/ very little overhead obstruction) at a minimum 65 mph (at that point the speedo on my bike reads 5 mph higher, according to my GPS, and 7 mph higher at 75 mph). At the end of my 30 mile ride, the tripmeter on both the bike and GPS were dead-on, even down to the last tenth of a mile. Given that, how ould you be able to explain why, even though the bike speedometer is clearly not displaying actual speed, the bike tripmeter was able to accurately record the number of miles travelled? Yes, I'm making an assumption that the GPS is correctly recording mileage/speed data...but even if it wasn't...what are the odds that its tripmeter reading happened to exactly match that of the bike tripmeter?

dreyfus

There's no reason a speedometer/odometer can't be accurate. Assuming you have stock tires then the calculations are straightforward and simple. As the tires wear down the accuracy will also degrade (the speedometer/odometer will read slightly faster/longer than the actual values), but this shouldn't be a significant change (comparted to going to 18" or 19" rims, for example). I would expect the manufacturers to calibrate their systems to be mildly inaccurate for brand new and well worn tires, and very accurate for tires with a little wear.

To hear that Lexus is purposely giving false information for any reason, let alone to make you think you're getting better gas mileage than you really are is something I find detestable. My guages should give me accurate information, not propaganda.

jbusch

My Garmin Streetpilot III shows my car reads 2 mph faster then the GPS indicates. Also just returned from a trip where mileage post were posted along the roadside. I checked this twice but over a 40 mile span the odometer showed 38 miles covered. Means I am getting better gas mileage when you do the calculations at the gas pump. {(Odometer reading/gallons purchased)/.95} I am not complaining. Doesn't this also mean that when my car shows 60000 miles on the odometer and the warranty is up, I really have driven 63157 miles. Does Lexus know about this?

350Dave

I plan to test mine vs my laptop GPS, and if it is off as I suspect (~2mph) I will request it be calibrated at the 5K service. I will let you all know what happens...

350Dave

The speedometer in the service menu reads accurately. It showed about a 3 mph difference at 35 mph. I need someone who takes a long stretch of Interstate Highway to see which 'reading' the odometer uses - if we are getting cheated out of 6 - 9 % of our true miles, then our car depreciates faster, our service intervals are wrong, warranty is only about 46 -47 K. I won't rest until I know!!!

dunnojack

interesting

350Dave

Another thought, I am tired after a long week so please excuse me. If the speedometer reads 6- 9% lower than actual speed, then my MPG readout is actually correspondingly lower than actual MPG, correct?

My 2007ES is showing 25.8 after 3000 miles, my 2002ES300 averaged about 20 under the same conditions. So, the proof is in the meter reading at the pump when I fill up. I will compare the next couple of fill ups to see the results.

RobF350

car magazines used to include speedo error in the testing, its super dooper common to be off by a mph or 2

350Dave

I expect more from Lexus, and my 2 previous Lexii had accurate speedometers.

toneman

If you think about it, just about any vehicle speedometer won't be 100% accurate, 100% of the time; as your tires wear down, the overall circumference gets smaller and smaller each hour/day/week/month/whatever interval floats your boat...thus (theoretically) affecting the speedometer (and possibly) odometer readings.

Now if you've managed to accurately determine that your speedo is continuously off by 10% or more, while rolling on tires w/ just about all of its tread intact at the time of testing--then maybe you'd have an argument.

AlexusAnja

Hey, if the speed radars are accurate, then you're below what you think so it's good. Don't want to be thinking you're doing 65 when it's actually 72...

toneman

"...reads accurately", based on/against what? I assume that it's based against whatever portable GPS unit you were using inside your vehicle; in any case, I will offer the following:

1) That finding (service menu speedo accuracy) in itself doesn't necessarily--if not absolutely--prove that the odometer is any more or less accurate;

2) If the speedometer reading in the nav service menu matches the reading of the portable GPS unit, that doesn't necessarily reveal that the service menu speedo is calculating/displaying the speed based (solely) on its' internal GPS unit. For all we know, it could be calculating/displaying the speed based on the number of pulses and the pulse rate of the speed sensor...in which case, see #1 above. I say this because FWIW the nav service menu speedo in my GMC Yukon Denali reads 1 mph higher than the speed readout on my Garmin iQue 3600 (which I have an external high-gain GPS antenna attached to it) at freeway speeds.

FWIW--at 45 mph, the service menu speedo in my ES reads roughly 2 mph (give or take a few tenths, as it fluctuates between 2-3 mph difference) lower than the cluster speedo. I hadn't had a chance to check the accuracy at freeway speeds (I did this on my way to work this morning--my work commute doesn't require me to drive on any freeways), but I'll provide that info when the opportunity to do so arises (if anyone else is interested).