Miles per Hour - Nuvi Accuracy??

Instead of using a Nuvi, why not get one of those agricultural GPS's. It would be more accurate and it is designed for low speeds. They have tractors now with GPS on board that will drive the tractor while planting or spreading weed killer / fertilizer.

Jack j

At high speed (freeway driving), my Nuvi 760 can't agree with my car speedometer. The GPS is always about 5 MPH slower than the speedometer.

I'd suggest that you check online for the correct tire size for your vehicle and then compare that with the tires that are actually installed. I had a shop install new tires and immediately afterward I discovered that the speed shown (as well as the distance traveled) on my GPS was suddenly in variance with that shown by my car's odometer. I checked the recommended tire size and discovered that the shop had installed tires that were "close" to that size because they were in stock and "nobody would really notice the difference". I sometimes need to record mileage for work and it was not good at all for the mileage to be off - let alone have the speed be off. I made them replace the tires with the proper size and my GPS and my car once again were in close approximation of each other.

Everything is standard (wheels and tires). Yes I have replaced all 4 tires but I got the same size as the original Michelin tires, 195/65-15.

Then there is something wrong with your car's speedometer most likely. Other than when my tires were the wrong size, I've never found the nüvi 760 to show inaccurate speeds. I've tested it against several mileage checks.

It could still be tire size. It is almost certainly tire size if the difference between the GPS and speedometer increases linearly with speed, i.e., if the difference was twice as much at 60 mph as at 30 mph.

FWIW: I am not sure of the decimal point accuracy, but I can tell you when I use my 760 in pedestrian mode it is pretty accurate, because I had my walking pace measured and my nuvi and eTrex Vista CX read the same. Also the eTrex matched my bicycle computer reading. - I use the eTrex on my bicycle too.

I must add that I never really pay continuous attention to the speed. I only took a glance at my GPS while I'm on the freeway and compare that to my car's speedometer. GPS is always slower.

Next time I'm on the road, I'll pay more attention to both car and GPS at different speed from low to high.

I know that my speedometer is reading low by 3 mph from comparing with the occasional warning radar set-ups that some local police use. This agrees with the reading that I get off the GPS which almost always is 3 MPH less than my speedometer.

My speedometer shows 2 mph slower but I know it's because of my tires. They are the right size but I know that my Michelins have a slightly bigger diameter than other brands. I think the Nuvi is fairly accurate only when going at a constant speed. I've checked the speeds with the warning radar boards or whatever that's called and the Nuvi is on spot.

My 2008 Honda CR-V shows 2 MPH faster than my Nuvi 250W. This is at 55 MPH. I had Mom's 200W and it showed the same thing. Maybe it is a conspiracy where the car companies are screwing us and our cars actually have only 90% of the miles they say... Hmm...

my 260W is about 1-2 mph slower than the car's. I think is more to do with the satellite

This topic has been discussed in depth before, and the consensus is that the GPS is typically far more accurate than the speedometer of a car, for many reasons.

My 265 is 1 to 2 mph slow. Not too big of a deal.

and in my vehicle the GPS is always 2-3 MPH slower (verified also by radar) and I have the official OEM tire size. I have heard elsewhere that this is typical of Dodge/Chrysler.

The real problem comes in with how many miles the vehicle actually has...if it is consistently 2MPH faster, that means the odometer will show more miles than it actually has. At 60,000 miles (actual), your odometer will read 61,200. That is around 2,000 mile discrepancy at 100,000 miles which for some vehicles can mean $1,000 or more trade-in/Private owner sale price.

Also, if the car is showing more miles than you actually are driving, you will be paying for more oil changes and other scheduled maintenance if you are referring to your car's odometer.

I noticed that my American branded car seems to be in direct agreement with the GPS but my foreign branded car shows 2-3 MPH faster than the GPS at 70.

Garmin claims the Nuvi is accurate to within 0.5 meters per second (which converts to 1/10 MPH).

This will be printed in your owner's manual in the 'specifications' section. If it isn't there, you can download the full specs for your Nuvi from the Garmin Website.

Note that the 1/10 MPH accuracy is only when you are traveling at a constant speed and have a good satellite fix.

proper tire pressure?

I trust the GPS.

Look here http://www.poi-factory.com/node/6695 for more reading.

You've never tested it against a Honda or Acura!! The discrepency is there and Honda settled a Class Action on it...the speedometer was recording higher than actual speed. A tiny percentage, but higher. Therefore the odometer recorded more miles than actually driven and THEREFORE the WARRANTY expired before actually reaching the maximum milage!!!! And leaseees paid for overages that potentially did not actually occur!!!!

I was thinking the same thing - warranty runs out quicker as well.

My '08 Elantra speedometer consistently shows 2 MPH faster than the Nuvi. I have checked the accuracy using a stopwatch and highway mile marker signs. The Nuvi has it right.

The original post was how accurate. I would say Very accurate when under 10MPH..it is getting thousands of updates per min..so I would say it is More accurate going slow than fast..and it is better than Any Car speedometer at car speeds

My Nuvi 255w is the same as my speedometer and both are very accurate with those portable radar readouts.

My 760 seems to be right on the money.

I agree with everyone else to trust your GPS, however when reading in tenths remember that the current number of satellites being received and the signal strength is a factor. On rare occasions I will see accuracy of 12ft and at times upto 70ft. Normally it will be around 20ft, but it's constantly changing. Next time you pass a radar sign posting your speed, check it with speedometer and GPS.

Sometimes we depend on the wrong thing to do the right job.
The GPS unit states that it should be used as a guideline and not to be depended on for absolute accuracy.
I have noticed that my speedometer is the same as my MPH on the Nuvi until I rotate my tires, then a small variance is noted, but I cannot allow that to keep me from rotation of the tires.
I also noted that the 760 that I had previously that was stolen from my van started lagging when I approached an intersection, It did not show my vehice at the intersection until I had just passed the intersection, So out with the old and in with the new. I now have an 885t with voice recognition and am loving it, but when it does something a little different that it is supposed to do, I'll remember that worrying about small things does not improve my quality of life.
I have too many GOD things to do than to worry about little quirks in my Nuvi.

Apart from rotation of tires, tire size, the inflation of the tires also play a part in the accuracy of the speedometer and fuel consumption of the vehicle

A second GPS (preferably a different model and brand) could confirm a need for the speedometer's calibration. If the two GPS read the same, you have your answer.

We have a German and a Japanese car. The nuvi is 100% dead-on with the Nissan, and the speedo on the other car is always 3 mph higher than the nuvi.

I believe the nuvi is accurate. People on the car forum state that the speedo being over is deliberate on the other car. So if the speedo says 75, the actual is 72....

I have been a dealer automotive technician for 29 years. I have seen more odometers than the average person will ever see in a lifetime. I have dealt with warranty complaints through the years about speeds and mileages not being correct compared to the customers others cars.

The one thing I know is that the automotive speedometer/odometer is not an extremely accurate device. Maybe it has gotten closer lately, but there are many variables that have already been mentioned here that affect this, so I wont re-cover them.

Another one of them to add is that actual tire circumferences do vary between manufacturers, even though the tire sidewall says that they are the same size as the replaced tire.

In this case, the only way to know for certain that you are getting the correct size tire is to measure it's circumference with a tape measure to the tire that was on the vehicle when the speedometer/odometer was certified.

I have seen cases where, because of mismatched brands of same size tires, the ABS/Traction Control systems deploy as it thinks that the car is losing traction. All because the tires are turning at different speeds.

Or.....you can just use your GPS and know that you are going to at least get a 'sane' speed and mileage reading.

I have used both my Garmin 250w and my 60CSx side by side (reading almost the same exact speeds and mileages side by side) to tell me that they are pretty accurate.

Happy GPS-ing!
Vanman

This is the information I needed. I will now go look for my owner's manual. - Thanks