Comicwoman

I set the car cruise control at 70 mph. I glanced over at the GPS and it registed a speed of 67 mph. I shared this odd fact with my sister and she said she noticed that her speedometer usually registers higher than those radar "You are Going XX mph" signs along some roads.

So, which one do you think is right?

bdjohns1

On any modern GPS, the speed reading should be more accurate than the speedometer.

Steph3n

My speedometer is slow by 2MPH I have a lock on 12 sats I know for a fact that it is more accurate than the speedometer. If I have 2-3 locks it jumps around too much to trust it fully.

Big_Dutch

Speedometers are not totally accurate, and most speedometers have tolerances of some 10% plus or minus due to wear on tires as it occurs. Additional sources of error are tire diameter variations due to temperature, pressure, vehicle load, and nominal tire size.
Modern speedometers are said to be accurate within 10% but as this is legislated accuracy, this may not be entirely correct. This can make it difficult to accurately stay on the speed limits imposed; most countries allow for this known variance when using RADAR to measure speed, although levels of some 3 km/h or 3% are also used in areas of tough enforcement.

As of 1997, Federal standards in the United States allowed a maximum 5% error on speedometer readings (per "Auto Tutor", American Automobile Association of California magazine, Oct. 17, 1997). Aftermarket modifications, such as different tire and wheel sizes or different differential gearing, can cause speedometer inaccuracy.

GPS devices are capable of showing speed readings based on change in position between measurements (usually taken at one-second intervals). As the GPS is an independent* system, its speed calculations are not subject to the same sources of error as the vehicle's speedometer. Instead, the GPS's positional accuracy, and therefore the accuracy of its calculated speed, is dependent on the satellite signal quality at the time. Speed calculations will be more accurate at higher speeds, when the ratio of positional error to positional change is lower. The GPS software may also use a moving average calculation to reduce error.

cordelli

There was an excellent thread not too long ago about the accuracy of speedometers and several people cited the actual standards they have for accuracy. It's pretty lenient in terms of how much they can be off.

mee

My speedo is pretty consistent with my gps, until I hit about 50mph. Above 50, the speedo under-reports by about 10%, so on a motorway with a limit of 70, I typically keep the needle a bit under 80, and know I am safe!

-- Mike

SouthsideJAX

I believe my GPS is very accurate when it comes my MPH, but I have noticed that there is a bit of lag in reporting the data. For instance, the speedometer will drop first, then the GPS will show that I am slowing down one or two seconds later. Nothing much, but there is latency in there.

slawecki

the odometer part of the speedometer is usually quite accurate, once calibrated. a GPS is no as accurate a distance measuring device.

CPRich

My speedometer is 3.5mph fast at 60-70mph. This gap is consistent with multiple GPS's and just about every rader-based "your speed is" sign I pass on the highway. I think the radar/GPS technology is much more accurate.

I disagree that speedometers are "very accurate". Many manufacturers intentionally design them to report high. They never want to read slow, putting a customer in position of a ticket when the speedo says you're OK. So they stack all of the tolerances available, and assume full tread, max inflated tires. Then throw in a safety factor.

Yes, when calibrated, a police speedo is accurate and can be used to issue tickets (yes, I know from experience). But they must be calibrated on an individual and very frequent basis to account for individual variances and assorted wear items. The average car on the road doesn't have this level of attention.

There was an article in one of the car magazines a few years ago. I believe one of the regulators (DOT, European?) has +0/-10% requirements - you couldn't read slower than actual but had tolerance to read higher. I think BMW has actually published specs that their standard is something like -3mph, +/-5% at 60mph - aiming for 57, allowing for 54-60. My 56.5 would fit right in that target.

Many of the mags also report on actual/indicated speed and they are almost always 1-3mph high.

TMOliver

The "Odometer" is no more accurate than the speedometer, since they function using the same input, a mechanical "counter" of wheel revolutions. Changing your car's wheels to a different size, or even mounting tires of different size/specs will change the speedometer's and the odometer's readouts, both of which are set for the tires/wheels with which the car was originally equipped.

The GPS on the other hand will be far more accurate at measuring distance, although depending upon its software, will have a pronounced delay in displaying speed changes and a lesser lag time in reporting distance traveled. Depending on the type and sophistication of the GPS, its monitoring of rate of change of satellite reference points is almost constant.

Even in the days when the technology was young, and the US Navy's GPSs were big and cumbersome, they were far more accurate when used for traveling in confined waters than the best calibrated pit log (a mechanical device which measures hull speed through water) and certainly as good as the best navigation team translating optical bearings to chart positions. Back when the technology was new, surveying groups using GPS equipment discovered countless "landmarks" which were inaccurately located on maps and very precise surveys.

The current models are amazing bits of electronics (but in map display mode, entirely dependent upon the accuracy - and age - of the electronic maps with which they are "loaded" or updated).

SJUAMMF

Many people change to a different size tires/wheels combination either as new or as replacement. I am using a set of alternate OEM size on my car that is larger than what came on the car by 4 percent but never bothered to recalibrate the speedo. So the speedo is under reporting the actual speed.

On the other hand, the GPS reports the true speed.

SRQ Guy

Almost certainly the GPS. There are several factors that can cause your speedometer to be off by a few mph.

N965VJ

I have a pickup truck with large off-road tires, so my speedometer in that vehicle is ~4 MPH slower than the GPS. That’s important to know if you’re like me and like to cruise at 5+ over the speed limit.

HereAndThereSC

That's what one would think. However, my 2000 Jetta always displays the speed as high (70mph when the GPS says 67), however it is dead-on accurate (0.1 mile difference over a 600+ mile tank of fuel, e.g.) when registering miles on the odometer. I have no idea why that is, except that the speed needle isn't precise.
JP

slawecki

the speedometer is driven by some electro or electromagnetic system, with a needle that is spring loaded.

almost all odometers are directly geared to the tire on the road. once it is CALIBRATED it is extremely accurate.(there are a couple of very strange exceptions)

GPS do not compute linear mileage, but use a delta and sum system. a point can be accurately determined with a gps, but the distance calculation getting from one point to another is not.