What's More Accurate--Speedometer or GPS?

Period.

A lot of comments on this subject here

http://www.poi-factory.com/node/6695

the GPS! , both Hondas I own report 3-5mph faster on the odometer than the gps, Chevrolet work vehicle spot on!

I'd go with GPS. BMW's shows 3-4 mph faster than you're acutally going. The Chevy's are right on, same for Dodge.

The correct answer should always be: Speedometer

If the speedometer is not accurate, you should get it checked and calibrated. Remember: if you use different size tires than the specs. provided for your vehicle, that will also affect your speedometer reading. The speedometer needs to be calibrated according to tire size.

Also:
1) The GPS ESTIMATES your speed - it is not like a direct calculation that the speedometer/computer performs on a vehicle.
2) Due to delays and time lag in processing GPS signals to/from a satellite, a GPS' estimation of your speed is not synchronized with that EXACT moment in time that you are driving at that speed. The reading on the GPS usually shows the speed from a few seconds earlier.

I would trust the GPS to be more accurate. I believe the speedos come from the factory calibrated 5% faster. That is why a lot of police ignore you if your 5 or 10 over the limit because your not going as fast as you think you are.

While rjak is correct, if your speedometer is indeed incorrect it should be fixed. But, a lot of speedometers are notoriously inaccurate from the factory. The accuracy is improving since almost all speedo's are electronic and driven by encoders on the vehicle transmission(even if your seedo has a needle it is still driven electronically).

As for a GPS making an estimate of your speed rjak is incorrect. They don't send a signal to a satellite as he suggests, but,a GPS processes the incoming satellite signals and calculates the differance in timing and equates that to distance over time which equals speed. I have an older Garmin hand-held GPS that I use on my boat that is very accurate. Speed is displayed in .1 MPH increments. I can take a single step while carring it and will register the position change, compass heading change, and speed in about two seconds. I don't know how fast or often the current automotive GPS units update but just watching ours it is indeed a little bit slower than my hand-held.

I'll trust a GPS dirived speed reading to a vheicle speedometer any day.

@ rajk

As most others here have stated the GPS is always more accurate than the vehicles speedo. My experience over the last 4 years with 5 different GPS units has been that the GPS is extremely accurate while the vehicle speedo is anywhere from 2 to 5 MPH out, depending on speed and whatever other factors you want to throw in, like tire size etc.

IMO delay in the GPS signal is not an issue. We are talking about a 1 second refresh rate for most newer models.

Besides having your speedo calibrated would probably cost more than the GPS these days.

But unfortunately, it almost never is. I have yet to find any speedometer that is correct. Everyone one I've checked has read faster than the vehicles actual speed.

If you are traveling at a constant speed on straight and level roadway (that is important) the GPS will ALWAYS read your exact speed. The speedometer depends on your tires being the exact correct diameter that the speedometer is calibrated for. Any increase or decrease in size (normal wear on the treads) from this diameter will change the speed readout.

Read through the thread that was cited and you'll see plenty of comments on why your speedometer is usually wrong and the GPS is a better gauge of your correct speed.

The GPS system is constantly receiving positional and time data from the constellation of satellites. Each satellite transmits a complete data set every 30 seconds. As the receiver, your car in this case is moving, the point at which the receiver was located at the time the first message was received and the point at which the second message was received can be determined. The difference in location of the receiver in those 30 seconds will allow the speed between the two points to be calculated.

A car traveling at 70 MPH covers 3,080 feet in those 30 seconds. Any variation of that distance will be the result of variation of the speed of the vehicle. Processing delays or delays due to the distance from the satellite to the receiver are virtually immaterial. Averaging the signals from the 4 or more satellites being received by the unit at any one time will further refine the calculations to provide a speed calculation with an error of less than 1/10 of a mile per hour. That's a difference of about 5 feet.

Of course, I may be wrong...

The only time the GPS is "inaccurate" is if the location accuracy changes between the location sampling. And if the speed is updated once a second, you will not notice the problem.

My GPS and my Honda Odyssey have always been the same when I look.

I'm going with my speedometer. The Nuvi 255W shows that I'm going about 1-4mph faster than my speedo. It varies depending if I'm on the freeway or surface streets. I compared my speedo reading to several of those stationary digital speed indicators (the ones that show how fast you're going) and on each one of those units, the digital speed matched my speedo. I believe my speedo over the GPS.

When conditions are reasonable and if the vehicle's speedometer is NOT calibrated or if is not accurate (and the vast majority of car speedos read high by design) the GPS speed indicator is more accurate than that of the vehicular speedometer.

However the GPS tends to update the display once per second and thus it is possible for the analog speedometer to be more 'current' than that of the GPS and where the GPS goes weird or the signals are poor, you *can* get serious issues, though these are usually fairly obvious and are characterised by the map (and speed) jumping around a lot - like in the RF canyons of a metropolitan core area, surrounded by office towers.

Having said that, this is not the first thread on the topic. I, for one hope that 250 people do NOT publish their speedo/GPS relationships again.

When conditions are reasonable and if the vehicle's speedometer is NOT calibrated or if is not accurate (and the vast majority of car speedos read high by design) the GPS speed indicator is more accurate than that of the vehicular speedometer.

However the GPS tends to update the display once per second and thus it is possible for the analog speedometer to be more 'current' than that of the GPS and where the GPS goes weird or the signals are poor, you *can* get serious issues, though these are usually fairly obvious and are characterised by the map (and speed) jumping around a lot - like in the RF canyons of a metropolitan core area, surrounded by office towers.

Having said that, this is not the first thread on the topic. I, for one hope that 250 people do NOT publish their speedo/GPS relationships again.

With my Honda CRV and a Garmin 885t, I passed one of those radar speed display. Setting my speed control to the speedometer to the speed limit I passed by the radar speed display and the GPS was right on with the radar and the speedometer was off by -3 mph.

very good question ORnonprophet!

gotta go with the satellites man, aka GPS

I believe at one time, Automakers were required to have +/- 10% accuracy on a vehicle speedometer.

I was involved in the automotive business for over 30 years and worked in the sales, service and parts departments at a Chevy/Buick dealer. I never saw a new GM vehicle with a spedometer that read lower than the actual speed. But the judges often saw a "Calibration Certificate" that said the speedo read low We sold a lot of speedometer gears in the Parts Dept.

I think the automakers allow them to err on the high side, rather than the low side.

There are 2 advantages to this:

1. No lawsuits because of speedo error. If your speedo reads higher than the actual speed, then it's not their fault. I think this is the main reason for most (if not all) to read high.

2. More miles are racked up on the Odometer and your warranty runs out before it should. 5% error means 5 additional miles every 100 miles - 50 additional miles every 1000 miles and so on.

No speedo gear in today's cars. In a modern car, the electronic fuel injection system is a Digital system and it reads the pulses from a Hall Effect sensor that senses the rotating splines on a drive or axle shaft. (Vehicle speed is a very important factor for emissions control.) There is a sub-program in the computer that divides the the number of pulses in a specific time period by a number to determine the Mph/Kph to display. You may have an analog speedo like I do, but the speed is determined digitally.

The number that the computer uses to divide the pulses by can be changed - if you have the right widget!

The number of turns in a given distance is still dependent on the tire rotating, so even though much of the measurement is digital, the basic measurement, the rotation of the tire as it travels down the road, is analog. Since the tire rotates slower for a given distance when new (larger diameter) your speedo will read a lower speed when you have new tires on the car and higher as they wear. Of course the amount of change is small, but over the life of the tires, it adds up on the odometer.

Sorry, but I didn't state anything about the outside diameter of the tire affecting the speedo, I thought that was understood to be a fact.

And I wasn't correcting, just clarifying in case someone else might be confused.

Most modern cars digitally count the number of turns made by a specific shaft.
Knowing the gearing and the tire size, the computer calculates the distance travelled. odometer errors are created by either changes in gearing, or tire size.

The computer also calculates the number of turns per unit time. This is then displayed as either miles or Km per hour. Speed errors Will be due to errors in calculating distance and any inaccuracies in the speedometer circuitry. Most manufacturers now use a double coil system in the speedometer, much more accurate than the older designs.

There are no analog measurements. Every measurement is digital.

So, doesn't the accuracy of your odometer give you a good measurement of your speedometer accuracy?

Get a stop watch and find a known set distance of one mile. Set the vehicle to 60 MPH and set the cruise. Now when you pass the first marker hit the button. Hit the button again when you pass the 1 mile marker. If you are at 60 seconds and 60MPH you are set.

Of course by doing this you have the inaccuracies of the timing of pushing the button at the correct timing.

Either way I'll stick with the GPS as more accurate.

If you are wearing a speedo while you drive and look at your unit, I'm staying in my lane

Have we ever seen any information that says the digital read out signs are as accurate as the GPS reading?

This weekend I'll throw some fresh batteries in my hand-held Garmin marine GPS and take a drive with our 765T and compare the speed readings. Will report back next week.

As for road side construction area speed warnings, I wouldn't trust them to be accurate. They're not calibrated like the police units are. If you stumble across one of the police photo radar units, then they should be accurate.

In any of the three cars I have driven with my GPSr, there has been no difference that I could observe. PT Cruiser, and 2 Chrysler minivans.