Should I trust my speedometer or my GPS?
My GPS lists my driving speed in the corner, but it lists it as 2 mph faster than my speedometer shows. The tires are the correct size, but I think I should probably get this checked out... Any comments as to which you would trust more?
P.S. The GPS is a Garmin Nuvi, the car is a 2003 Toyota Corolla
by
tossthedice
asked 6 months ago
I'm no expert in this matter but I'd probably trust the gps. My gps says 2mph more also its an Ease. You have to think it's tracking your location and pinpointing you from one lat/long to the next. In theory the gps should be 100% accurate. It's satellite tacking you as you move judging by how fast you least point 1 to point 2, from mile to mile as it updates it know you went from mile marker 12 to 13 in .... amount of time making your speed ....mph. It should be accurate to the tenth, but then again it's technology and technology is flawed from time to time. But that's my 2 cents.
I have one of those GPS systems and it works very accurately.
To make sure that there's not a wrong sized tire on, do you notice a difference in the gap when you speed up or slow down?
If not I would take it to someone who knows what they are doing to do a thorough checkup to make sure nothing serious is going on.
The government will not allow civilian GPS to be 100% accurate as to your exact location so I am not sure I would trust it to be 100% for your speed. I can say that my GPS and speedometer do appear to read the same.
Traveling at a constant speed, the GPS is going to be extremely accurate. Trust it.
If the GPS reads faster than your speedometer, and the speed gap increases the faster you go, then your tires are smaller than what your speedometer is calibrated to. But that doesn't mean you have the wrong size tires; read below.
Right now I'm running 195/60's when my car should have 195/65's on it. If you work the math, its not a huge difference in physical size. The difference between the two is less than 1 cm from rim to top of tread. However with my tires that difference means the circumference is 6cm smaller, which adds up at higher speed. When my speedometer reads 70, I'm actually going 65 per the GPS.
My point is, even small changes in tire size will change your speedometer's accuracy. Small changes like: normal wear. If you've had your tires for a while, its possible you've lost 2-3 mm's of tread, and that would be enough to throw your speedometer off a bit.
I always was told that a car's speedo would be off, perhaps a tad fast at slow speeds and a bit slow at fast speeds. Plus tire size (and tire wear) all come into play on linear travel.
Does the 2 mph variation happen always (i.e. above 5 or 10 mph)?
I have my car hooked up through my phone (bluetooth OBDII + Torque) and have one screen with phone GPS speed and car computer speed side by side, it's fun to watch the variation.
Guess I actually have no idea then :) but it's fun to watch.
@wadekind: Actually that is no longer true. You are referring to Selective Availability, where civilian GPS receivers could only receive a degraded signal. That program ended in May 2000, and civilian GPSs are extremely accurate because of it.
I have a an app on my Android phones called Speed (or something like that) which shows your speed and appears to be dead accurate. I've tested it with the cruise control keeping the car's speed steady using measured miles (or highway mile markers) and a stop watch from 35 to 75 MPH.
Even though consumer GPS is less accurate than military, it doesn't follow that speed measurements using a consumer GPS would be less accurate.
I'd say to trust the GPS.
My car's speedometer shows 2 MPH less from 55 to 75 MPH (though it is analog, so it could be 1.5 to 3 less).
I have the correct sized tires on the car and they are fairly new. My assumption was that the speedometer is calibrated for the smallest size the tires should get as they wear.
Compare your odometer to mile markers on the freeway. If your odometer is off, then your speedometer will be be off, too. This is at least true in both my car and my father's car
Can you lend your GPS to someone and test it?
My guess is GPS=Greatest Perceptual Speed, which is to say I would trust the GPS. This would make you safe ticket wise.
use the force........let go......
@baqui63: Consumer GPS is just as accurate as military GPS, and has been for over 10 years, see my comment directly above yours.
Consumer GPS is extremely accurate. For example, when I run I wear a Garmin Forerunner 305 (its a wrist-worn GPS with a tiny antenna.) The trail I run is not a loop, so I run the same path in two directions. GPS is so accurate that when I view my tracks on a map, I can see two distinct paths (my outgoing and return paths are not stacked on top of each other) and they accurately show which side of the footpath I was running on. That means my tiny little GPS is accurate to at least 2 feet.
Hiking and auto GPSs have bigger and better antennas and can track more satellites making them even more accurate than mine.
Car speedometers are legally required (in the U.S.) to be accurate to within ±4% of indicated speed (or of actual speed…I don't remember which way it goes). I would expect any GPS receiver with a decent antenna to be nearly an order of magnitude more accurate than that (or ±0.4%), barring severe weather or other poor-reception conditions.
You likely have a different sized rim/tire outer diameter than what is stock from the factory. Even a small amount could equate to 3+% difference between the speedometer readout and actual speed (who knows how accurate these consumer grade gps' are). 3% at 70mph is 2.1mph.
Trust the GPS because if you are using "Snapshot" from Progressive or anything like that, it will be based on the GPS.
Yep. Trust the GPS, @firebirdude is right. If your tires have the wrong PSI or are the incorrect size then your speedometer will not read the true speed.
I've always noticed GPS systems to be very accurate in reading speed... mine annoyingly yells "YOU ARE OVER THE SPEED-LIMIT" once you reach 1 mph over the limit lol
You know, I knew that already, but it didn't click when I was reading last night. (I was zoned in on a Heisenberg Uncertainty Principle joke that I decided not to use. Probably the gin.)
The only thing I can say is that if your GPS is giving you whole number readouts it could be rounding things up a bit. You know, you are doing 1.4mph faster than your cars speedometer and it is displaying 2mph.
My car has a digital speedometer without a decimal place so the car could be rounding it down while the GPS is rounding it up. (e.g. My car thinks I'm doing 50.4 so it says 50, GPS think 50.5 so it shows 51.) Even then I do not know what each devices threshold for showing the next increment is maybe 50.9 is still displayed 50mpg.
Just saying depending on how things are displayed and other variables they could both be reporting speed slightly off.
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