Your GPS versus your Speedometer - need your input.

[Kerry.]

quote:

---

Originally posted by fizzymagic:

.... I know that none of the current crop of Garmin or Magellan units do so.

---

 

Now that's probably putting it a little more in perpective as saying ALL receivers go to the full extent of velocity vectors and freq drift is perhaps over doing things just a tad.

 

Not all recivers out there are the current crop and also there's a few more manufacturers than just the limitations of Magellan and Garmin and even in those camps there can be quite a bit of difference between models.

 

The computing power to determine redunancy in position/time as well as 3 Dimensional velocity vectors plus freq drift for a possible 12 sats as well as do all the other bits and pieces and general house keeping is quite staggering and really in many of the cheaper units there's simply not the capacity.

 

That's not to say that many don't go part of the way to determining velocity vectors using doppler shift principles but there's also other data involved as well to streamline usefull output to accuracy typical of a recreational handheld.

 

But until some of these manufacturers actually make their code available (and that's not going to happen) then just like EPE and FOM, the exact propriety methods used to determine some of these parameters will always be a little subjective and assumptive.

 

Cheers, Kerry.

 

I never get lost everybody keeps telling me where to go

[Richard+Marcy]

To answer the poster's origiinal question:

 

Car: 2003 Honda Accord, with Nav System

GPS: SportTrak Map

 

Nav System, speedometer, and SportTrak (while receiving WAAS) all say the same, within 1 mph. Tested around 65 mph on open road.

[+Cachetrotters]

My Meridian and STPro show 60 mph when my speedos indicate 61 mph in both my Century and Taurus with the cruise on, straight and level, and open ground (so. Cal. desert). This is supported by the roadside radar trailers on several occassions.

 

But, how do I know that I can trust any of it? Fundamental measurements of time and distance, of course. There is a 4-mile measured mile course on a local freeway. That and a nice little digital stopwatch tell me what I need to know.

 

Milage markers are generally accurate anyway, so I use them regularly, but I never trust any single one. I take several readings over several miles at several different locations. Works well.

 

don

[+ottor]

I needed to get a good idea on the actual speed of my Explorer since I put 31" tires on it - I'm about 4 mph faster then what the Speedo shows using the GPS .. Also wanted to test my vette with curious results - 75mph was exactly 75 on the speedometer .. however 100 mph on the speedo showed 104 on the GPS, but 130 on the speedo was exactly 130 on the GPS again.. Guess if I want to be exact, I should drive either 75 or 130 !! ;-)

[+planetrobert]

Test 1 using stock size tires at 50psi

Step 1) measure around the outside of the tire when inflated

step 2) compare speed reading off GPS and Speedo at varying speeds

 

Test 2 using stock size tires at 30psi

Step 1) measure around the outside of the tire when inflated

step 2) compare speed reading off GPS and Speedo at varying speeds

 

Test 3 using oversized tires at 50psi

Step 1) measure around the outside of the tire when inflated

step 2) compare speed reading off GPS and Speedo at varying speeds

 

Test 4 using oversized tires at 30psi

Step 1) measure around the outside of the tire when inflated

step 2) compare speed reading off GPS and Speedo at varying speeds

 

Test 1 Both the speedo and the GPS agreed

Test 2 GPS read 0.4% SLOWER than Speedo

Test 3 GPS read 8% FASTER than Speedo

Test 4 GPS read 7.2% SLOWER than Speedo

 

Test 5 OVER THE HILL

quote:

---

Originally posted by Sheepdog43:

If you travel straight over a mountain, your actual position may only change by 1 mile. Howeve, due to going up and over, you may have actually had to travel 2 miles.

 

What distance will a gps read? I would imagine better models will register 2 miles, but what about cheaper models?

---

I found a hill that i could drive STRAIGHT over which was .3 miles as the crow flies. I drove over and the odometer read 0.45 miles. The GPS read 0.46 miles. This was close enough for me. When i made a route using waypoints along the way it said .3 miles.

 

all tests were done using a Magellan GPS 315 and a Magellan GPS Companion for Palm, The palm GPS using the NavComp Software and Cetus GPS.

 

Now where did I set my GPS??? planetrobert.net

[+NightPilot]

Tire pressure is a big factor in the accuracy of the speedometer. The pressure varies with temperature, speed, and whether any has leaked since the last time you put some air in. Exactly what is the pressure, and what is it supposed to be? The pressure makes more difference than the brand of tire.

 

Regards,

Stan

 

[This message was edited by NightPilot on October 26, 2003 at 08:15 PM.]

[+Volvo Man]

I have to say that I generally trust the GPS. In my works van (which is a testament to budget fleet leasing), I generally show an overreading of 8-10% on the speedo, but in my 87 Jaguar with it's swiss made electronic speedo it's spot on. I've also found this to be the case on most quality luxury vehicles.

 

Although there was this one time that all hell broke loose one morning and my laptop/GPS combo had my Van travelling at mach 6 50 feet above the mediteranean!!

[+Cachetrotters]

quote:

---

Originally posted by SirRalanN:<snip>

 

Test 1 Both the speedo and the GPS agreed

Test 2 GPS read 0.4% SLOWER than Speedo

Test 3 GPS read 8% FASTER than Speedo

Test 4 GPS read 7.2% SLOWER than Speedo

 

<snip>

---

Did you really mean to say 0.4%? 0.4% of what speed--something is needed to make this relative error meaningful. Perhaps you meant 0.4 mph, which would indicate a zero bias and therefor constant error at all speeds.

Otherwise, if the speed is 100 mph, 0.4% is 0.4 mph, or 4 tenths of 1 mph. At 30 mph, 0.4% is 0.12 mph. Perhaps your speedo has a sensitivity error?

But how can you know? Even if a speedo is digital, unless it displays tenths mph the uncertainty is 0.5 mph at all speeds. That means the speedometer display error alone is larger than the stated error at 100 (0.5>0.4), and over 4 times larger than the stated error at 30 (0.5>0.12).

If the speedo is analog, I would expect even larger uncertainties.

 

don

[GSPLover]

So I have a question then. If my car speedo shows me doing 120kph and my GPS shows 115kph then my car odometer will be off by 5 kph over an hour right? For the average driver this wouldn't be a problem but I put A LOT of mileage on my car (its 6 months old and has over 24000 km's on it). This would 'cause' the car to age to fast and lower its value sooner then it should correct?

[+ottor]

No - driving 24000 km in 6 months is what ages your car too fast and lowers it's value.

[GSPLover]

Yes but my point is if the speedos thinking I'm doing 120 per hour when Im really doing 115 then the odometer is tacking on an extra 5 kms an hour.

Thats not right.

[Rabbit 282]

Living (and doing most of my traveling) in the midwest (flat country), I've come to rely on my GPS as an accurate indication of car's speed... for a couple reasons: a) it makes sense in my head and my car's an old pile of crap.

 

Actually, my brother owned my car before me and always blamed his speeding tickets on the car's faulty speedometer. Knowing he was full of crap, I decided to prove him wrong. GPS seemed like a great way to do it. Indeed, he was right, the GPS told us the car speedometer read slower than the actual speed. By about 3mph. That is, until you get over 80mph when this poorly designed speedometer starts reading 5-10mph slower.

There are other factors in your car's speed sensing capabilities: wheel size, cable adjustments, etc. can throw a car- or at least an older car- off. My GPS, however, seems to always be consistent.

 

And really, in my mind, I can't see why it wouldn't be. GPS works on averaging the difference between signals received. We GeoCachers use things for the wrong purpose all together- they're not made to move slowly and in small circles. It's the same reason the compass doesn't point north when you're sitting still- the GPSr isn't receiving enough difference to figure out what's what and what's where.

 

When the GPSr can notice a huge difference (say speeding down a highway), however, it can better average the change in signal reception over a period of time and gauge it with fairly accurate consistence.

 

Then again, I drive a $200 car so I'm obviously not a brilliant physicist.

 

/Benjamin

[+NightPilot]

I've been doing some observing lately, and it seems the software makes a difference. I have Mapopolis, for navigating in the car, and a Garmin eTrex Legend, mostly for geocaching, but for other things, too. I've used both the Garmin and a Radio Shack Digitraveler with Mapopolis, and Mapopolis consistently reports my speed as 1-2 mph slower than the Garmin, even when I have the Garmin connected to the Palm and using Mapopolis. I can run the Garmin, and read its speed display while simultaneously having the Garmin provide input to the Palm via Mapopolis, and Mapopolis and Garmin disagree by about 1 mph, assuming Mapopolis always rounds down to the lower integer mph.

 

I also just bought a new Toyota pickup. I had been running the GPS primarily in our Honda CRV. The CRV consistently reads 3 mph higher than the GPS, while the pickup seems to agree almost exactly with the GPS, allowing for lag in presentation on the GPS. I'd be willing to bet that the GPS is consistent, and is giving more nearly correct speeds than either speedometer. As the tires wear down on the new pickup, it will probably diverge from the GPS in speed, as would be expected. The speedometer only knows how many times the wheels turn, not how far they travel on each turn.

[EnglishBob]

Well this got my interest.. so yesterday armed with my pocketpc and an Ambisoft CF GPS card I tired a few cars on the same 3 mile stretch of road...

 

New Honda Pilot/S2000/CR-V/Element: All 4 of these read 1 MPH higher than the GPS at 65 mph on flat open ground.

 

99 CR-V: Read 3 mph higher than GPS.

 

2001 Ford F150: 2 MPH lower than GPS.

 

2002 Saturn: Read 1 MPH higher then GPS.

 

2003 Volkswagon Passat: Read 5 MPH higher than GPS.

 

The Passat was the only European car available for me to try. No I don't own a fleet, I work at a car dealers and was passing a VERY boring sunday afternoon.

[+Team GPSaxophone]

2003 Volkswagon Passat: Read 5 MPH higher than GPS.

Funny you should mention the Passat. My Mom followed me from California to my house in New Mexico. I set the cruise control in my 1993 Chevy Suburban on 73mph. My GPS showed I was travelling at 75mph. My Mom was pushing 80mph according to the speedometer in her 1999 Passat.

[+RobAGD]

Jeep #1 - It was dead on from 40-75 mph

Jeep #2 - at 40 is a tad slow GPS shows 43mph, @ 65 its showing 71mph

Suburban Rental - It was within a mph or 2

Durango Rental - at 70 gps said 78 this was the worst I had seen in a newer car

 

I trust the GPS more than the Speedometers in most cars.

 

-Robert

[+Hunt]

well my Meri. Platinum has always been acurate when i have a decent satalite reading except for the tenth of mph.

[Cracker.]

About a year ago, my Merigreen and Sportrak Map agreed exactly with the speedo in my Toyota Tacoma at 65MPH...(That is with oversized tires - about 3" bigger than what the truck came with). Now, after my tires have worn down to the wear indicators, at an indicated speed of 65MPH on my speedo, my GPS reads about 62.5MPH.

 

Usually with speedos, the error is not a constant, linear error with changes in speed, but varies over different speeds.

 

Also, (since I am into off-roading, I have some knowledge of over-sized/undersized tires), not only do car manufacturers have specific standards and toleranes they must adhere to.

 

Tire manufacturers are allowed a +0/+7% tolerance on tire size....

 

A tires' weight and load limit is largely determined by tire size.

If a tire is slightly undersized (due to manufacturing proccesses, there can be some difference), from the stated size, and is determined to be the cause of a blow out or accident, then the tire manufacturer can be liable.

SO, they also manufacture tire size exactly to specifications, or slightly over to cover their asses.

 

This leads to slighly differing tire sizes causing variances in speedometer readings, also. Larger tires will make your speedo read slower at faster speeds, and smaller tires will make the speedo read faster than what you are travelling.

 

Between the tolerances factored in one way by the auto manufacturers, and the tolerances going in the other direction by the tire manufacturers, and over time the changes caused by tires wearing (essentially getting smaller), its hard to get speedos to read accurately.

 

There are other factors included also, as mentioned above....

 

Basically I would trust the GPS over the speedo...

[+PastorDIC]

My GPS shows a few miles slower than my speedometer. Nice when I am passing near the speed limit and my wife has the GPS and is back seat driving