AV8R_BOB

Good idea, but I think I'm only looking for about a 1% increase in dia, about a quarter inch. The tires suggested above would give 1" to 2" dia change so the speedometer would be off way more than it is now.

When we need new tires I'll talk to the tire experts and see what they can do, and we'll live with it til then.

AV8R_BOB

How about THREE GPS's?

I used a portable Lowrance unit, plus my Blackberry's GPS, plus the built in Lexus GPS. They all agree. I'll believe them over the speedometer.

Then there's mile markers. They don't lie.

In case you're wondering, no the Lexus GPS does not have a current speed display (that we have been able to find anyway). However it does give Average Trip Speed. If you set your cruise control on the freeway, then press the "Reset" button on the trip data screen, it zeros the GPS average speed and then gives you the speed data from the point you hit reset. As long as you keep the vehicle speed constant it gives you your current ground speed. That number agrees with the other two GPS's.

By the way, the GPS satellite system is US military operated for the purpose of navigating cruise missiles to a specific building from a thousand miles away, and for navigating aircraft, ships, and troops. It's also used for civilian aircraft navigation including precision instrument landings. I have three GPS receivers used in my two aircraft. What I'm saying is the GPS system is pretty darn accurate.

Regarding the other gauges being inaccurate also, you're probably right. But you refer to the speedometer a lot more than those other ones, and its accuracy can affect stuff like your drivers license, insurance rates, etc.

jfelbab

I was on I43 today west of Milwaukee and the freeway has a 10 mile stretch marked by mile markers. After 10 miles on the odometer I had passed the ten mile marker on the freeway. Odd, as I'd have expected the odometer to be off based on this discussion but it was dead on.

I recall a class action suit being filed over this issue with another car maker I believe it was Honda/Acura.

http://www.cbsnews.com/stories/2007/...e=search_story

herbvdh

I checked my speedometer with a scangauge (www.scangauge.com) which is digital and with a radar trap displaying your speed. It took me several attempts to read all three fast enough but I found that all three were dead on. The scangauge and the radar trap exactly the same with the analog speedometer. The radar trap data does get published and they tell you the fastest speed recorded through the trap.

triplecore

I think only police cars have perfect speedos as they are calibrated annually and the dash states its calibration.

AV8R_BOB

I live W of Milw too. I think mile markers are all along the entire I system.

Thanks for the article. I had heard about that Honda suit but didn't look for info.

Honda and other Jap motorcycles are notorious for innaccurate speedometers. Every bike I've ever owned has read about 7% high, hold 65 to go 60 actual. A Honda rep suggested that might be to make riders go slower. One of these days I'll check to see if the odometer is off on my current bike, a 2006 Honda ST1300.

Lil4X

I have GPS mapping software on my laptop and a separate antenna that I tuck into the moonroof shade (www.delorme.com). I used it on long trips in my RX 300 before trading for my RX 330 with nav. The software now needs an update, but it is far more useful than the one built into the RX, particularly when my wife and I travel together, the "navigator" can actually do the route planning and a dozen other operations like checking out scenic route options, phoning ahead for a hotel, making dinner reservations, etc. while underway. You know, all of the things that you see in various ads for GPS systems that most lock you out of when underway.

One of the neat features of the software is a "real-time" mode that gives you a running readout on speed, compass heading, even altitude. Because the GPS data is processed quickly, you can see variations of one-tenth MPH in your speed within a second or two, if you have a solid lock on three satellites. Using the GPS, I've found my groundspeed indicated on the screen is within a mile per hour of my speedometer - at least since I put on the Bridgestone Alenzas. I suspect that my nearly-bald Michelins were showing a bit higher reading on the speedometer since the wheels were turning faster with the smaller diameter (almost treadless) tires. If so, that difference in rolling circumfrence should affect the odometer as well, indicating I had covered more ground than that actually traveled.

As an aside, the altitude reading provided by the laptop, even with a 3-D fix, is a bit wonky. I've been clocked traveling east on I-10 toward the Louisiana state line at an altitude of -45 feet (below sea level). OK, the Gulf of Mexico is a few miles to my right, and I haven't encountered a fish yet. Maybe the speed and heading information are accurate, but if I were using this software to navigate a private aircraft, I'd rather depend on my altimeter or just looking out the window for altitude data. That scraping sound at negative altitude is SO annoying.

jfelbab

In this section of I-43 (between Waukesha and Mukwonago) markers are at every tenth of a mile intervals. Good for more precise calibration I guess.

I live out near North Prairie, BTW. If you are ever planning to be out in the area let me know and we can have a beer or coffee and compare info on car care or the like.

salimshah

For accuracy junkies ..

Before, any one bets their farm on the accuracy of GPS, they need to know how it works. The GPS signal has a "intended" drift and is kept inaccurate, to prevent "others" from using it for wrong purposes. Over time the "x,y" drift has been reduced to help the commercial use but the, altitude has kept loose. The military GPS receivers can decode the drift and ascertain more accurate position.

Most of the GPS receivers have an averaging system built in which helps in predicting a reasonable location and then the software also helps in snapping the position on the road. This becomes more muddy as realtime information has to be displayed (averaging works better if you sit at the same spot).

The GPS transmits "time" and every thing is calculated based on this signal (position, speed, heading etc) by the receiver.

Salim

AV8R_BOB

It depends on how many satellites you are locked onto providing your position. The more the better. But even with a good fix it's still not accurate enough for precision approaches, so aircraft GPS receivers use WAAS, an augmentation system to enhance accuracy... I think to about 2 ft or so.

I just talked to a surveyor the ohter day and he was using a similar augmentaiton system that uses the section markers as a fixed, known location. That gives his GPS an accuracy of 1/4 INCH in X,Y, and Z.

Who_Me

When I used hand-held doppler radar guns a few years ago, we had to calibrate them every time we took them out of the case to use them. They are calibrated with a tuning fork, that you whack on the dash and the radar synchs with the vibrations. Your calibration log could be called into evidence if the speed infraction went to trial.

I don't personally know any law enforcement officers that don't give at least 10% tolerance above the posted limit, except in school zones, with children present. There probably are some, but I just haven't met any. Speedometer error in the amounts mentioned in this thread just aren't that important to me because I drive close enough to the posted speed to allow for the tolerance level. I would be more concerned with an odometer error, since that could affect my mileage against warranty. Then, what if the error gave me an extra few thousand miles?

silver2000

Im surprised by the reactions here but assuming you were BENT about it why not change the tire size in the Nav settings to "fix" the speed ?

Im pretty sure my GS350 has a tire size adjustment section in the touch screen - so do some math and try "adjusting " your tire size to see if it helps

Since as it was stated the GPS signals have intentional errors in them per the defense dept, how can you be sure your car isnt correct and the GPSs drift is 3pm at 70 ? or 4 %