GPS vs. Automobile Mileage

Which one do you think is true?

I would doubt the odometer reading since it was not calibrated.

Is it possible that you've changed your tire size or even tire pressure? I would think that your odometer and speedometer make an assumption of some circumference in one revolution.

Good question though. Makes me go "Hmmm"

Bob
Ottawa, Canada

When you put the trip into your GPSr, what distance did it predict?

Speedometers/Odometers are known for not being very accurate. I believe Honda just lost a class action lawsuit because their odometers were off by a significant amount. This was causing people who had leases on these vehicles to rack up miles faster than they actually were, thereby incurring mileage overage fees. I think another manufacturer has a suit in progress for the same thing as well.
Also, if you have changed the tire size to something other than what the manufacturer installed/recommends, that will throw off the speedo/odo. The computer is programmed for a certain size diameter tire, and computes speed/distance based on that size tire (based on RPM's from the VSS (vehicle speed sensor) or RWSS (rear wheel speed sensor) and a known diameter). I put larger tires on my truck and had to have the computer reprogrammed for the larger diameter tires. (I have a programmer that lets me do it myself via the OBDII port, by inputting the diameter of the tires installed).

Here is a link to the Honda suit:
http://www.hondaodometerclassaction.com/

~Rob

Trust the GPS IF it is on and locked up with satellites before you move.
Accuracy of the GPS Satellite is in feet !
As you drive your car - the tires heat up and expand
giving you errors in mileage recorded on odometer.
In the case of my van - at 60 miles per hour I have speedometer error of about 4 % . I have checked this several times with roadsite mile markers, GPS Reciever and GPS Navigator !
Just my observations.

The GPS was on and locked up before movement. I always keep the tire pressure at 32 psi, and tire size same as recommended by Toyota.
of course there is expansion and contraction of the tires. I sometimes check the 2nd screen and the speedometer and GPS are exactly together at 70 MPH. Highway Patrols were really picking people up in Miss. and La. The GPS predicted 1220 miles, so it's actual was off by 6 miles. Taking into account Rest Areas, and fuel stops there could be a slight difference. I think I'll stick with the GPS at 1214. The speedometer could be off one or two tenths, and the accumulation could be the cause.

I have been reading a lot on GPS the past month, so I can't remember where I read it, but the article said the GPS would always be more accurate than the gauges on your car. The only thing it said was the GPS could be off by one second when you are looking at your miles per hour, but if you kept at a steady speed, it would be accurate.

Rick, I noticed that. I locked my cruise control down at 70 and the GPS was right on. Bottom line? I'll take the GPS readings. It didn't falter one time, and always gave me my exits in due time to make lane changes. It made a believer out of my wife. She said that was the neatest gadget she ever saw. and really came in handy.

Robert:

The difference between the two is only about 2% or looking at it the other way around it's 98% accurate!

I will take the GPS over the speedometer any day. I was quite surprised to find that the speedometer in my Ford Expedition shows me going about 3 to 4 miles an hour slower than the GPS! No wonder those state troopers on the interstates are always stopping me to say hi!

Stan

May be worth a shot going back to the 'pre gps' days and check it against several mile markers. I'm guessing you'll find the odometer off.

RT

I agree with everyone and trust the GPSr more so then my gauges. However, my speedometer is exactly the same as my GPSr and RADAR gun.

I have noticed lately in rental cars (brand new or close to it) that they are a bit fast. My GPSr says 73 and their speedometer says 75 or 76. I wonder if manufacturers are doing that on purpose.

I don't know which is correct, but when my Explorer (original size and new tires) is showing 100km/hr. the GPS shows 102 to 103 so this works for me, at my normal driving at 10 to 15 over the speed limits, if I get stopped it is usually just a slap on the wrist...

Car manufacturers are required to have a speedometer accuracy within 10% so yours may be better than average. My experience is that most read a higher speed than actual. As someone earlier mentioned, a quick check can be made with the highway mile markers over about 10 miles. My Toyota gives estimated mileage which is always off by about 10% which I'm sure is no accident that it reads high.

FMCSA 393.82 Speedometer

Each bus, truck, and truck-tractor must be equipped with a speedometer indicating vehicle speed in miles per hour and/or kilometers per hour. The speedometer must be accurate to within plus or minus 8 km/hr (5 mph) at a speed of 80 km/hr (50 mph).
[70 FR 48054, Aug 15, 2005]

DOT Interpretation - 393.82

Question 1: What does the phrase "reasonable accuracy" mean?
Guidance: "Reasonable accuracy" in interpreted to mean accuracy to within plus or minus 5 mph at a speed of 50 mph.

This actually applies to all motor vehicles.
I drive a truck for a living and some of the truck speedometers are off 5-7 mph. So, I use the GPS reading when I travel. Haven't gotten a speeding ticket yet. Even when the speedometer shows 77 mph at a 70 mph speed.

My recommendation is to trust the GPS except in a city with tall buildings. Yesterday, in New York City, it showed I was going 8.3 mph when I was sitting at a red light.

Hope this info is useful.

Good information EagleOne. This may be the reason State Troopers very seldom make speeding traffic stops when going 3 or 4 MPH over the posted 70 MPH speed limit. I was running at 70, but cars passed me I know had to be going 73, would pass a side of the road trooper and not get stopped. I guess there is a tolerance allowed for. My Avalon is right on the money with the GPS though.

A lot of troopers allow 9 over.

I would venture to say the Satellite is far more accurate then your Odometer

You Got Longitude & latitude for these Troopers I would like to know where these Officers are at LOL

I asked my brother. He is an engineer and says that GPS is more accurate if it samples the location often enough. If it only samples every 30 seconds, it will indicate a lower mileage than actual. He also said that the wrong size tires or tires that are worn out will cause the wrong mileage reading.
Jen

I think GPS lies sometimes.

Hotdram,

I would like to understand how you reprogrammed for the larger tires and if it is possible to reprogram the average mileage calculation for a Toyota 4Runner. Mine consistently read 2 mpg greater than a manual calculation. It would be nice to have it read more accurately. I assumed you would need the source code and a flash memory programmer which are not available to normal users.
Mike

Mike,
I have a programmer called "Smarty" from a company called MADS Electronics. With the programmer, I can change fueling delivery curves/timing (from 10 preset levels available on the programmer), check and reset OBDII diagnostic trouble codes and reprogram my tire size. MADS only makes programmers for Dodges with Cummins diesel engines.
However, there are other companies out there that make programmers for other vehicles. One of the bigger ones is Superchips. Edge is another company that I think is getting into the gas engine market (they started out in the diesel performance world).
There are more, but I can't think of them right now. Another possibility is to check with the dealer, they might be able to reprogram the tire size with their OBDII diagnostic tool.

Good luck,
~Rob

Thanks for the information. I'll have to look and see what I can find. I knew there were ways to reprogram the engine but didn't know there were other functions that could be changed.
Mike

Here's an interesting link regarding odometer accuracy.
http://tinyurl.com/6pp4a4

Does it mean that if I put over-sized tires on my car the warranty will be longer, or will it void my warranty?

That is a 2% difference which is exactly the difference I have found in speed, using any GPS in any car I driven. The GPS is accurate for speed and distance while car speedometers and odometers show 2% faster and greater mileage.

I know my Ford speedometer reads a bit high, which I expects also means the odometer runs a bit high and shows more miles than are really on the car. I do expect my gps is much more accurate. But I also expect that the gps is measuring mileage without regard to changes in elevation. If you are in an area where you drive up and down hills a lot, you may well find that the gps only records the "horizontal" distance between points and does not take into account the vertical component. You are actually driving on the hypotenuse of many different triangles, but the gps measures the x parts of those triangles.

Of course, in the flat mid-west this isn't likely much of an issue, but in some parts of the country it does add up.

Yes, it does not account for the elevation and computes a speed based on a flat earth. However, even inclines of 8º or so are pretty rare so the difference is really negligible and GPS is still more accurate than a car speedometer. The spedometer could be more accurate, but they are programmed to read a bit fast and therefore, rack up about 2% more indicated miles than reality.

The GPS is accurate between points, but consider the entire trip is a series of discrete points. It calculates the distance between each point as a straight line. Since roadways are rarely straight over long distances, that is a significant source of error that adds up if the points are not very close together.

In addition, the Garmin software will align each points to a roadway in navigation mode. Sometimes it makes a mistake and plots a point at a place you did not actually drive, especially if roads are close together.

Look at the route you took in Map Source or Base Camp and you will likely see places where your route distance was rounded between two points, even though the road was on a curve that did not take a straight course between the same two points.

Not necessarily. Ford programs its speedometers to indicate 2-3 mph higher than actual. But this in the programming for displaying speed only.

If you put the car in DEBUG mode (on my car, hold down reset button until car starts), you can view the actual precise speed on the odometer display.

You can also obtain the true speed via OBD II software or mobile device software that use OBD II/BlueTooth transmitters.

My KIA speedometer was out, 5-6% and as was indicated previously, Honda had to settle in a class action, because it affected people's warranty due to the inaccuracies.

My mechanic didn't believe me, but then he also noted that the people that complained all had GPS'.

On the other end, GM speedometer accuracy seems to be right on, and tracks the GPS. I don't have experience with other North American Cars.

Try checking between the mileage markers on the highway. In Ontario, I found that these are quite accurate. They show the km to a single decimal. When the show a marker as 356.0 km, instead of just 356 km, it implies that it's accurate. I don't know about the USA.

Although this discussion is resurrected from a 7 year old thread, it is a topic that comes up from time to time on the forum. Here is a post I made in another thread that describes how to use mile markers and a stopwatch to check the accuracy of your speedometer:
http://www.poi-factory.com/node/34140?page=0#comment-255438

A better test is to check over 3 mi/km to really see the accuracy.

On most newer Nuvis you can see the speed you are travelling on the map page. Simply look at that speed then look at your speedometer to compare the two readings. My old Ford Explorer shows 2 MPH faster than the GPS speed.

Possibly car manufacturers calibrated the speedometers higher to protect drivers from speeding tickets that may be borderline.

I find it interesting that this message thread started so long ago!

On the subject of the original post, I agree with the response immediately above this one. A lawyer told me over 50 years ago that auto manufacturers have historically calibrated speedometers to read higher than the actual speed, to protect against class-action lawsuits over speeding tickets.

If the design point were dead-accurate speedometers, the unavoidable car-to-car variation would result in some that read below the actual speed, leaving the drivers open to tickets even when they thought they were below the limit. Setting the speedometer design point to read high would protect against this scenario, and against the manufacturers getting hit with large judgements.

I cannot guarantee that the lawyer knew what he was talking about, but it sounds believable to me.

- Tom -

My experience the only time lawyers tell the truth is when their lips aren't moving

If you get a ticket, neither the GPS info (which is probably the most accurate) nor the speedometer will get you out of what the radar said. They actually usually give you a bit of a margin to allow for the error of the speedometer.

Obvious is tire size and calibration, but your GPS only see course movement of GPS, not the little turns and wiggles of the car. Try a footage wheel on a straight line then wee-whaa it and see the difference, trenching contractors do this to get more money on footage all the time!

As far as mileage is concerned, I find the GPS consistantly records more mile than the vehicle does. I have a 2008 Canyon and the speedometer is at least 3 mph fast using the stock programming and the stock recommended tire. (31 inch diameter) The problem is the rolling diameter at proper inflation is considerably less.

You can find your approximate rolling diameter by measuring from the ground up to the center of the wheel, then multiply by 2. You will see this is less than the manufacturers stated diameter.

So, I have a programmer that allows me to enter the corrected diameter, as confirmed by checking the speedometer against the GPS at 6mph. While the speed is corrected, this is where I get the mileage difference.

Typically speedometers have some error. One day speed won't be based on wheel rotation speed, but until then we have to deal with what we have. The speedometer/odometer on my modern car is exceptionally accurate. My classic car registers 3 mph less then actual speed and has the old cable running off wheel drive which is seldom exactly accurate.

Motorcycle speedometers are designed to read high at the factory. Especially the Japanese bikes. If it read low and you got a ticket you could sue the manufacturer and they are allowed to read 10% high so they design for 8% high. HD speedos are not as bad. My Kawasaki was right at 8% high. My HD is only about 2% high.

Again, as I previously noted, Ford does program its speedometers to display a higher indicated speed than actual. The car computer does operate on the actual speed. You can view the actual speed (as seen by the computer) by putting the display in debug mode; it is then displayed (along with many other parameters) on the odometer or multifunction display.

You can also use an OBDII tool to obtain the same result.

Little turns and wiggles will rarely end up on a ticket though...

very interesting

I previously owned a VW Golf TDI and it showed about a 10% difference between the speedometer and my actual speed as displayed on my Nuvi's. I was told by the dealer that all VW's were built with this discrepancy build in.

I now own a 2014 Chevrolet Equinox and the speedo dial reading, the speed shown on the trip computer and the speed reading on my GPS are always exactly the same.

If you look closely at your GPS track file, you will see that it is composed entirely of straight lines. When you go on a road that curves, there will be multiple track points along the curve, each with a straight line joining them, approximating the actual curve of the road. These straight lines are shorter than the actual curve. On a long trip, it will add up. Or in this case, add down.

I'm a little surprised at the difference myself, though.

my GPS reads more miles than the odometer...I'd not worry about a 1.15% difference

When I replaced the tires on my work car the mileage was suddenly quite a bit off from the GPS. I checked the size and discovered the shop had put on tires that were "close enough" to the original size. I took them back. The shop figured nobody would notice and they got rid of tires that were hanging around. Needless to say I never bought tires at that shop again.

The odometer in several vehicles have had has always been less than the GPS.

A Speedo? Really?

Chuckle, chuckle.