It wasn’t a “dig.” I was just reading along and liked that he stepped in and shed some light on the issue.
Whenever I come across a flashing road sign saying “your speed is” my 2002 Camrys speedometer reads right on!
Likewise, my outside temp thermometer reads pretty close to actual after the car had been moving for awhile. Within a few degrees anyway. Good enough for me.
Years ago, Car & Driver had an article on this. See the second or third paragraph here: http://tinyurl.com/nvo2gq The web version is a bit less clear than I recall the original magazine version of the article, but I think the point is still made.
The regs for speedometers (at least in Europe) say that they have to be calibrated for the largest possible diameter tire that would come on the car. The stock tires could have a certain diameter, but there may be an option, or even an aftermarket option to go +1 or +2 on the wheel size, and a couple of variants from there on the tires. Whenever any of these options results in a larger tire size than stock, the speedometer will read higher than actual with the stock tires. I don’t know whether Toyota has to comply with a similar standard.
All the discussion about GPS and vectors and such is nice and probably accurate, but probably not applicable to us. I have a hand-held GPS used for flying and hiking that will give BOTH tracking and bearing speed. The nav system in my car does not display a speed, but if it did, it would make sense to show only the speed in the current direction. Anything else would confuse most drivers. At some point, an Interstate may veer widely from the straight line direction you’d want to drive. In such an instance, displaying a speed of 50 mph when the driver knows he’s really going about 70 mph would offer no useful information and may even be confusing. If any GPS’s display that without an option to display the more meaningful information, shame on the designer!
If I had to bet the ranch on the more accurate true speed, I’ll take the GPS over the speedometer whenever the GPS has a good view of the sky. I believe the GPS even accounts for true speed going downhill or uphill, as it can measure that, as well.
H’mmm. My 2005 Scion XB also shows 2 mph more than those radar signs the police put up. I have wondered if its odometer was also “aging” faster than reality? Perhaps to get you in for service that much sooner?
The pilot would have known his ground speed from the GPS, but what’s relevant for stall speeds and airframe stress is the airspeed, which is the speed of the aircraft relative to the air around it.
So, to give an example, if there’s a 50 mph tail wind and the airspeed indicator says 500 mph, the ground speed will actually be 550 mph. Because the airspeed is essentially the air going over the wings and control surfaces of the airplane, this is the more relevant number for the pilot. The pilot needs to keep the airspeed between the stall point where the wings don’t produce enough lift to keep the plane in the air and the point in which the airframe will break up from aerodynamic stress. In a storm, where winds can gust up to hundreds of miles an hour in either direction, the difference between the airspeed and groundspeed can be hundreds of MPH’s in either direction, so if the airspeed indicator isn’t working that is an extremely dangerous situation.
Like meanjoe75fan said, though, there’s no particular reason to think that that was the cause of this crash.
I don’t think terrorism was ever really a reason for GPS scrambling. As far as I can tell, the network was descrambled in most places in 2000, so before terrorism was really on the top of most people’s lists. I also have a hard time thinking of how exactly a somewhat more accurate GPS unit would really be essential to a terrorist plot.
It had more to do with the fact that the system was originally conceived entirely as a military system and originally the US military hadn’t even planned on opening it up to civilian use. The (probably apocryphal) story is that after the KAL 007 crash where an airliner was shot down after wandering into Soviet airspace due to a navigation error, Reagan decided to open it up to civilian use. This also meant that foreign militaries would have access to it, though, so they started scrambling the signal so the US would still have an edge.
So for why they stopped scrambling it, it has to do with the lack of any real adversary with a large conventional military and the increased importance of GPS in the civilian world. Also, the US scrambling was a major reason why the EU and others started making plans for their own SatNav networks at about the same time. Since the US wanted to continue to have a monopoly on satellite navigation (so they can still turn it off for all non US-military users if they ever need to), the unscrambling was also a bit of a political move.
I doubt that even today the thermometer and the speedometer have the accuracy of a fine Swiss watch–even on a Toyota. I remember back in 1950 my Dad traded his 1939 Chevrolet for a 1947 Dodge. We often made a 300 mile trip to visit relatives. He said that the distance was shorter on the Dodge–289 miles as opposed to 301 miles in the Chevrolet.
As far as I know, just about every speedometer is off about 1-2 MPH; depending on speed.
In one of the Subaru factory manuals for instance, there is even a stat published for this.
About 1 MPH error at 30 MPH, 1.5 at 45 MPH, and almost 2 MPH at 60 MPH, etc.
That’s a consistent 3.3% error across the specified range. Often, these type of things are specified with a + or - tolerance so it might read something like 1-100mph (+/-3.3%). So you actually can be from 2mph low to 2mph high at 60mph or a accuracy range of 4mph total. The tire wear allowance has to be included in this top level specification. So an exact speedometer is a transient condition regardless of where it started out. All of my gauges only have 5mph ticks on them anyway so reading any higher resolution is guesswork. Besides, what’s a couple mph between friends anyway?? 
The total speedometer error is the product of the individual errors. For tire wear the error factor ranges from 1.00 for new tires (no error) to maybe 1.02 (bald tire) a change of 2% for just one factor.
Short of using radar as a speedometer monitoring tool, expect error with the exact measure being the exception. Redistribution of weight in a vehicle, tire pressure, wheel alignment etc. all factors though small individually, can cumulatively induce significant error.
I’m sure Toyota stretches the truth about a lot of things relative to their marketing, but intentionally inducing speedo error other than a perceived use of added weight to their trucks and differences in allowed tire sizes and the like, I don’t think so. I feel that applies to ANY auto manufacturer.
Right now without any known margin of error that makers are expected to conform to during state inspections, don’t expect a more “exotic” (expensive) means with fewer error factors to be used. The driver is responsible for imprudent speed, not the car maker.
You have it backwards, egroeG. Using a larger than stock tire will cause your speedo to read low, not high (and vice-versa). Page 1 of the C & D article concurs:
“Normal wear and underinflation reduce the diameter of the tire, causing it to spin faster and produce an artificially high reading.”
Plus/minus tolerances are often different values - in the case of the speedometer, you’re more likely to see something like +3.3%/-0.0%. IOW, the speedo is calibrated so as to never read low.
The Society of Automotive Engineers has a voluntary +/-4% tolerance for odometer accuracy. It is reasonable to assume a similar speedometer accuracy. Honda, not Toyota, was fined for inaccurate odometers (http://www.engadget.com/2007/02/19/honda-paying-out-6-million-for-overclocked-odometers/).
I have an '08 Tundra and the Speedometer also reads about 2mph more than the GPS. I also checked it one night driving home from the airport at 4am. I set the cruise at 65mph and started a stopwatch at a mile marker. Then I drove 20 miles without messing with the cruise. When I got home and did the math, it was about 2mph slower than the cruise was set. Here is the really interesting thing, If I reset the average speed in the truck’s trip computer, it agree’s very well with the GPS unit. I am guessing the hedge the needle a couple mph’s up to keep us out of trouble with the law!
So new test today. My new job requires an 80 mile commute each way. I tested this commute today with my wife’s car and the miles read 78. I am pretty sure that my Camry’s ODO and miles are off a bit. I also realized how uncomfortable the seats are today after trying my wife’s car, but that is a different story!
You are correct. I think I was so happy about finding the C&D article that my brain quit working and I misspoke on that simple concept.
To test this out, compare the speeds shown on your GPS with your speedometer while driving in a large circle