Well, my '09 Caravan checks out VERY close , I would say it is accurate.
Cars are required by law to read speed high so that no driver can claim speedo error on a speeding ticket. ODOs are acurate.
Odos are no more accurate than their respective speedometers, and there is no law mandatting that speedometers read high.
Regulations prescribing the manufacturing requirements for all over the road vehicles including cars are promulgated through the U.S. Government’s Department Of Transportation
http://www.dot.gov/
If you can find such a law, please link it here. You’ll have my humblest apologies. I don’t think you’ll find one.
Also, if you can find any evidence anywhere that odometers are more accurate than their respective speedos, I’d appreciate seeing that one too.
Speeding tickets, indeed speed limits, are state regulated and controlled, as are all traffic laws. Ther are federal recommendations, and federal controls via the withholding of federal highway traffic funds for states that refuse to comply, but there are no federal traffic laws.
I have to think it is a planned effort by the car companies to get the drivers to think they are going faster than they are. If we’re actually going slower, we will achieve higher MPG and save gas.
I doubt if it’s that sinister, simply because the accuracy of the gages is irrelevant to the EPA testing. I suspect it’s just to give people the perception that their car is a tad faster than irt really is.
My 2004 GMC Yukon XL 2500 is about 5% over across the board. I fully believe they do this to cause extra repairs and trade ins. They do it everywhere else: a really weak alternator, inferior wiring, side terminals on the battery which vibrate loose, plastic sleeves on the bearings so they wear out transmissions in 125,000 miles rather than use metal sleeves that last 275,000 miles, daytime running lights so the lights burn out needing replacement making extra $ for the dealer as well as wearing out the alternator even faster. Daytime lights, by the way, are only negative: it wastes gas, wastes bulbs, wears out everything faster and doesn’t make you safer (and makes it less safe for motorcycles and emergency vehicles).
Occam’s razor, people: the most obviously answer is correct - they really are trying to rip us off on every level.
And of course the GPS and not the vehicle is correct. If you have any doubt, try getting a Magellen, a TomTom, a Garmin and a Motorola Droid on the Verizon network running the vehicle info display. Every single one will be within .1 of a MPH.
You know the obvious solution, right? A simple law mandating that all vehicles have speedomoters and odometers that are GPS drive. It’s a $2 item they put in even the cheapest of digital cameras now. It’s only their desire for fraud and the US Gov’t allowing it that we don’t have 100% to the centimeter accurate speed and distance of all vehicles.
I haven’t seen anyone else post that their speedometer reads slower than they are actually going. My 2006 CTS V reads about 2 mph slower than a GPS when running in the 70 mph speed range. My wife’s Ford Edge reads about 3 mph faster than the GPS in the same speed range. Incidentally, the CTS V’s built in GPS agrees exactly with a portable Garmin unit. I would always bet on the GPS for accuracy.
It’s pretty simple actually. The manufacturer calculates mph from the circumference of the stock tire. This “virtual” tire has no weight upon it thus it is perfectly round. When you mount a tire on any car, you will always have a certain amount of “squat” in the tire. This “squat” decreases effective radius of tire. The more squat, the less diameter, the less circumference, and thus less distance traveled per revolution (i.e. speedo “error”).
The error will always be in the same direction. And it will be more for heavier vehicles.
Nope, tire squat has nothing to do with it. The circumference does not change from that. It does change slightly with inflation pressure.
Yes, um, unless you remove air or rubber, a tire stays the same circumference no matter how it looks otherwise and, um, when you’re moving I can guarantee that tire gets round again. You know how you can tell? If the tire stayed “squat” while you were driving YOU WOULD HAVE A RIDE LIKE JAMIE AND ADAM WHEN THEY TRIED SQUARE TIRES (http://youtu.be/NHpkJeSzYEQ?t=2s)!
Yeesh.
I have driven about 1 million miles including to both coasts. On long drives on the interstates, I check both my odometer and speedometer. Starting where I enter and interstate ora new state, I record the odometer reading to tenths and the mile marker. I record this information every 50 miles. So I this information for 150 to 300 miles at a time. I have done the with several different vehicles and over many states. The results for each vehicle have heen consistent over many trips. For speedometer checking, I use a relatively flat stretch of highway and start my stopwatch at a mile marker. I record the elaspsed time and mile marker every 10 miles. I have done this over stretches of 100 miles or more. Like the odometer checking, I compare the results for each interval with
the results overall. My Escape speedometer is within 0.2% and odometer is 2.5% low. My 1990 Dodge Caravan speedometer is 7.5% high, but the odometer in within 0.2%
Good information, @nucnerd - it shows that the odometer and speedometer are independent measurements in your cars. This was always the case before electronic odometers/speedometers. The odometer ran as a reduction gearing off the speedometer cable, while the speedometer ran from a magnet attached to the end of the speedometer cable rotating in an aluminum housing attached to the speedometer needle. The rotating magnetic field induced a force in the aluminum housing, rotating the speedometer needle against a coil spring.
If I remember correctly…
Edit - yes, here’s more on ‘eddy currents’ and how they make a mechanical speedometer work:
It seems to me that each revolution of the wheels wears down the tire and hence changes the odometer reading. Therefore, the car travels slightly less distance for each new revolution of the wheel. This change can be modeled by a differential equation. With electronic speedometer/odometers, this differential equation could be incorporated into the chip the drives the speedometer/odometer. I really don’t think it is worth the bother, however.
Texases, I rarely disagree but in this case I have to. The squat does make a difference. And if your car has an “indirect” tire pressure monitoring system, if the “squat” grows (pressure drops) too much, the rolling circumference becomes substantially different from the other wheels and the TPMS warning light glows. The difference between the rolling circumference and the tire’s unloaded circumference is the internal and external tire distortion and deflection that causes a low tire to heat up abnormally and fail if left uncorrected.
TSMB - weren’t we here before? The difference in diameter results in shrinkage of the tire (setting off some types of TPMS systems), and squat. But the squat itself changes nothing regarding effective tire circumference. In other words, the effective circumference of a tire on a car held up by a jack is the same when the jack is removed and the tire ‘squats’. Those steel tire cords don’t shrink up in response to taking the car off the jack, right?
Edit - we had two of these discusions, here’s one: https://community.cartalk.com/discussion/2282984/low-tire-smaller-circumference/p7
Yes this was discussed very thoroughly a while back, but there was never any agreement. The guys who say the circumference of a low tire cannot differ from a tire fully loaded may have a tecnical point. But the tire will have to turn faster than the other tire that is up to full pressure. Thus making the speedo read fast.
You know the obvious solution, right? A simple law mandating that all vehicles have
speedomoters and odometers that are GPS drive.
Would this really work?
There are still too many occasions where GPS isn’t available. For example: in cities between tall buildings, and in tunnels. Also heavy cloud cover can (and does) interfere with reliable GPS reception.
I drive a 2009 Toyota Matrix and have been noticing that my speed shown on roadside speed indicators is consistently 3mph slower than the speed indicated on my speedometer. I too was concerned that this was having an impact on my mileage. However, I recently went on a 2,000 mile road trip and my mileage matched that of mapquest as well as my navigation device. I can only surmise that the speedometer and odometer must recieve data independently. Being a loyal Toyota driver I will assume that the issue with the speedometer is simple the manufacturer calibrating on the side of caution…better to think you’re going a little faster than you actually are than the other way around.
I can’t say any of my cars have been off more than 1 or 2 miles per hour, whether using GPS, trip computer, or road side radar devices. Remember the old Ramblers that used to say “police certified” on their speedometers. I always wondered why they did that and no other companies seemed to follow their lead.