Which MPH is acurate - GPS or Speedo? [Archive]

Mandel

03-25-2007, 10:26 PM

Garmin. Absolutely. Even if your speedo was accurate (which it isn't), there would be variations in the condition of your tires (wear and inflation) that would throw the accuracy off.

On the other hand, a GPS will only sample your location at fixed intervals, so your wheels could travel more distance than the GPS would see. If you look at a GPS track of tight twisties, lt looks like a zig-zag and you don't see the radius of the turns.

GPS is also subject to error when it comes to speed readings. It's not that uncommon to see a top speed reading over 100 when you know you haven't gone a bit over 99.

Mandel

illzoni

03-25-2007, 10:26 PM

That's sounds about opposite most everyone's experience. Usually the more accurate GPS indicates an actual speed almost 10% lower than the ST indicates.

Obviously, YMMV.

Jon

joeca43

03-25-2007, 10:51 PM

rasputin

03-25-2007, 11:07 PM

I'd be keen to hear others experiences with the GPS/Speedo difference.
On my M109R the speedo is about 10% higher than actual..I know with other vehicles I have had the speedos are usually erring high from the manufacturer.
If ST's is reading low..I'd like to have that confirmed as I drive right on the limit of "ticketing"!

HankSTer

03-25-2007, 11:21 PM

Most people (all?) indicate the speedo reads higher then actual. Think about all the law suits Honda would face if it was lower then actual?

Something doesn't seem right there...

Regards,

gnorts

03-25-2007, 11:46 PM

I experianced today exactly what Mandel is stating ocurr's. I saw my max speed was 134 MPH. I know I was never near that today Nor any other day for that matter. I always reset the max speed to avoid an inquisitive :policeST: so it must have lost it's reference at one point today.
My riding buddie stated exactly what Mandel did.
I do take into consideration that the ST speedo is about 5 % off the actual speed. ( slower than actual speed )
Joe
When you're moving, the GPS is constantly running speed calculations based on its most recent info from the satellites, and if you're in an area where you're not getting good steady satellite data, the calculations can get laughably wrong, and the max speed memory will capture them.

When you're making a long steady run across open country on a straight road, the speed your GPS displays will be very accurate. That's the time to compare it to your speedo. My GPS shows 46 when my speedo says I'm doing 50. If the error is linear, that extrapolates to 92 on the GPS when 100 shows on the speedo - an 8% error.

I will leave it up to someone who has done this at 100 to verify my estimate. I don't see triple digits too often.

dduelin

03-26-2007, 05:31 AM

At a tad over 150 indicated my Garmin eMap and later model Garmin 2730 captured 141 as the high speed.

As has been stated GPS units average speed over time. D/T = S. The Garmins update about once per second. If it gets one of those 200 or 300 mph errors from a bad position it goes into the pot to be averaged with all the others that are generally within .01% of dead-on. So if you ride any distance at all with updates 3600 times an hour those spurious high (and low - on occasion I have seen my 2730 read low) numbers don't impact the average very much.

GPS is very accurate. It wasn't always so with early single channel multiplexing receivers in slow moving vehicles but those days are long gone.

SteveST1300

03-26-2007, 06:01 AM

My top speed is indicating 600MPH at the moment. But I find that my speedo reads higher than my GPS indicated speed.:03biker:

Geoff

03-26-2007, 06:31 AM

I just multipy the speed shown on the bike by 7 and thats the mph your over what it is showing..........showing 60... times 7 is 4.2....actual speed is 55.8. At 100 shown you should be doing about 93.

crazykz

03-26-2007, 06:36 AM

About 7% off. The GPS is more accurate. When the GPS is on the bike I never use the Speedo.

Curt

STOnkin'aussie

03-26-2007, 08:03 AM

i have the tom tom rider and dependent upon speedo reading reads bwn 4 and 8% below the indicated speed. i get back into my bmw car and find the same gps is within 1 % all the time. I can only assume old man H is trying to assist us to preserve some points...

dteel

03-26-2007, 08:10 AM

FWIW I have installed the Speedohealer (http://www.speedohealer.com/eng/intro.htm) and have it set at -6.9% and now my GPS and speedometer read identically.

YMMV

RideOn

03-26-2007, 08:34 AM

That's sounds about opposite most everyone's experience. Usually the more accurate GPS indicates an actual speed almost 10% lower than the ST indicates.

Obviously, YMMV.

Jon

Oops. In the original post I meant to say that my GPS reads 60 when the speedo reads 65. That is more in line with other's experiences. Thanks.

joeca43

03-26-2007, 09:02 AM

My comment about lower speed was the same as Ride On . When the States 65 I'm really ding about 58 or 59. I believe Curt has the factor pretty close at 7 % difference.
Joe

Pat Thompson

03-26-2007, 11:16 AM

On the other hand, a GPS will only sample your location at fixed intervals, so your wheels could travel more distance than the GPS would see. If you look at a GPS track of tight twisties, lt looks like a zig-zag and you don't see the radius of the turns.

What you're seeing here is a function of the limited memory for saving tracks that these small units have. The GPS is taking in far more data than is saved in a track. Would not be practical/possible to save it all hence the zig-zag twisties. You'll notice that more points are saved in the twisties than on a long straight. If direction is changing more points are saved. Less otherwise.

GPS is also subject to error when it comes to speed readings. It's not that uncommon to see a top speed reading over 100 when you know you haven't gone a bit over 99.

A spurious reflection of a satellite signal can cause those bizarre speed numbers. I was coming out of a parking structure among highrises and had a signal bounce me to a point out in the ocean 30 miles away. I had a track leg that had me going 1500 some odd mph for a few seconds. :eek: Delete that one point from the track and you're back to normal. :|

GPS speed is very accurate. Moreso than our bike speedos.

jahoobob

03-26-2007, 12:37 PM

I depend on my GPS for accurate speed on my bike and in my cage (Acura - Honda product too.) Both vehicles are about the same off. My friends FJR seems to be much closer to actual and I believe that this is a Honda feature:( , not necessarily and Japanese MC problem.

crazykz

03-26-2007, 01:00 PM

I believe Curt has the factor pretty close at 7 % difference.
Joe

Yes, as soon as everyone has come to the understanding that I am always right we'll all get along much better. :D

Actually this has been an issue for me but I don't care if the bike is right since I run GPS. For me I did to have accurate information for rallies so I've done a bit of comparisons to check this. YMMV

Curt

c-lapier

03-26-2007, 04:06 PM

Many posts have pegged the speedo error at 7%. I checked the odometer against mile markers on I-75 for 200mi from the Mackinaw bridge and it was 2.25% over.

BlaSTr

03-26-2007, 05:43 PM

The GPS, if it is tracking enough satellites for a 3D display, will always be more accurate since the error in position is less (I typically get 6-8' when the 276C indicates WAAS and Differential and I'm tracking 8+ satellites). Most GPSs default to displaying data either once per second or Auto. Calculations are done far more frequently than that. On many GPSs you can tell the unit to display speed data more often.

As for inaccuracies in the twisties, since the GPS is recalculating far faster than the normal curve is, it will be more accurate. The display, if set for 1 sec. updates, may indicate a different reading as this is an average of the calculated speeds over that time. Even so, it'll still be more accurate than the speedo. You can set it to continuous, 1sec intervals, or auto. This is why you may see different speeds than the speedo. They're not synched and the default (and only option) on the speedo is continuous.

The next time you ride a challenging road, set the track resolution to something around 10 feet (5.28 would be perfect), run the route, save the track, reset the resolution to something more reasonable and download the track when you get home. The "pips" will indicate the exact position (depending on the resolution of the signal) and you can see how your speed varies in the tight turns. And how many calculations were made.

jahoobob

03-26-2007, 07:27 PM

Many posts have pegged the speedo error at 7%. I checked the odometer against mile markers on I-75 for 200mi from the Mackinaw bridge and it was 2.25% over.

Odometer and speedometer are not necessarily tied together as many with GPSs can testify.

crazykz

03-26-2007, 07:32 PM

Many posts have pegged the speedo error at 7%. I checked the odometer against mile markers on I-75 for 200mi from the Mackinaw bridge and it was 2.25% over.

I've clocked the odo at 2% off.

Curt

RibsST1300

03-26-2007, 08:01 PM

So if Honda can offer a "calibrated Speedo with 2 mph increments" for the LEO's-why can't we have the same accuracy?? They have proven that they can offer true speed readings. they chose not to.

We should demand better-
HONDA Give me an acurate Speedo!

BlaSTr

03-26-2007, 08:46 PM

Mandel

03-26-2007, 09:05 PM

I think someone posted a table before, showing the difference between the speedo readout and the actual speed at various speeds. I don't remember exactly what the numbers showed, but what I took away from that post was a general sense of how to "discount" the speed shown on the speedo. If it says 75, I know I'm doing close to 70, probably a bit less. If it says 85 I know I;m doing about 78.5. When it shows 90 I know I'm in the low 80s.

That's all I really need to know. Does it matter whether I'm going 77 or 78? Although I ride with a gps and gather all relevant info in its trip computer, I rarely have it display my speed in real time.

What I have done is check my speed against my customary riding speeds indicated on the speedo so I get a feel for reading the speedo and adjusting it downward.

I do believe that as between the gps and the speedo, it's a no-brainer: The gps is far more accurate.

joeca43

03-27-2007, 12:29 AM

Yes, as soon as everyone has come to the understanding that I am always right we'll all get along much better. :D

Actually this has been an issue for me but I don't care if the bike is right since I run GPS. For me I did to have accurate information for rallies so I've done a bit of comparisons to check this. YMMV

Curt

I would never question "THE CURT MAN " in the sense of being right or wrong he might put a hex on one of my farkles. :eek:

crazykz

03-27-2007, 06:23 AM

I would never question "THE CURT MAN " in the sense of being right or wrong he might put a hex on one of my farkles. :eek:

See.... Joe knows. :p:

Curt

CaST

03-28-2007, 12:30 AM

My Garmin 2610 reads 65 mph when my ST1100 speedometer reads 60 mph. Seems like a pretty big difference. Which should I believe? I tend to believe the Garmin.

My F250 speedo matches my Garmin and my Toyota Sienna but not the ST so I believe the Garmin.

Brighteyesue

03-28-2007, 10:44 AM

Just 2 days ago, by speedo I was travleing at 70 when Zumo said I was going 65. Huge difference in my opinion! I know I will lawyer up if I get pulled over for speeding.

crazykz

03-28-2007, 10:47 AM

Just 2 days ago, by speedo I was travleing at 70 when Zumo said I was going 65. Huge difference in my opinion! I know I will lawyer up if I get pulled over for speeding.

7.1428571428571428571428571428571%

Thanks for the data point Sue. ;) Just proving me right. :crackup :D

Curt

Brighteyesue

03-28-2007, 11:08 AM

Curt,

Glad I could help.

I think I will have lunch at Waffle House today :)

crazykz

03-28-2007, 11:16 AM

Curt,
Glad I could help.
I think I will have lunch at Waffle House today :)

Damn! I should have seen that one coming. Well have an extra couple for me. :)

Curt

Brighteyesue

03-28-2007, 11:19 AM

Damn! I should have seen that one coming. Well have an extra couple for me. :)

Curt

I like the pecan waffle - yummylicious. Hey man, I was just thinkin of you :D

crazykz

03-28-2007, 12:34 PM

I like the pecan waffle - yummylicious. Hey man, I was just thinkin of you :D

Well I am on the minds of many women so that's no surprise. Of course most of them are behind bars with a different motive than what you might normally think. :rolleyes:

Curt

Brighteyesue

03-28-2007, 12:39 PM

Well I am on the minds of many women so that's no surprise. Of course most of them are behind bars with a different motive than what you might normally think. :rolleyes:

Curt

ROTFLMAO And I was just thinkin of your love of waffles, Oh Boy!

STeve1300

03-28-2007, 12:43 PM

Well I am on the minds of many women... Of course most of them are behind bars ... :rolleyes: Curt

I think Curt means bars like this (http://www.innofer.com/images/cage.jpg)....

crazykz

03-28-2007, 12:45 PM

I think Curt means bars like this (http://www.innofer.com/images/cage.jpg)....

So Steve does still exist. I'm thinking more like this: http://www.babecage.com/index.htm

Curt

Brighteyesue

03-28-2007, 08:12 PM

I think Curt means bars like this (http://www.innofer.com/images/cage.jpg)....

Good one.

So, today 120 on the speedo equaled 108 on the GPS. Not that I was driving that fast. It was a friend of mine.

dduelin

03-28-2007, 08:44 PM

7% error at 120 indicated would be 112 by GPS.

dduelin

10-02-2007, 06:11 PM

Our commercially available GPS units have a built in INACCURACY of at least 5%. This, coupled with the variables of weather, terrain and cover, can make the speed readings off considerably.
Motorcycle speedos have never been known for being very accurate (and I have no idea why they aren't)- with some being off by as much as 10%.

Best way to get a 'read' on accuracy- do a measured mile at 60 mph indicated (on either the speedo or the GPS)- then you'll know for certain.
Your watch should be pretty accurate, unless it's a Rolex (which is always off by the same amount- it's built in to the things!)

I was traveling back to my home from Washington when I was stopped in Virginia for a suspected radar device seen on the dash by a State Police officer.
It was a handheld Garmin III.
The officer was quite familiar with these things and asked if he could take a look at it.
He immediatley paged to the 'history' one- and noticed that my recorded Top Speed was 694 MPH.
"Care to tell me where you did this one, sir?" he asked.
"Not on any roads around here, sir." was my answer.

[I'd taken a flight in the jump seat on a fighter jet during a demo - and flicked the unit on when we were about to hit Mach out over the Atlantic somewhere!]
i'd like to see your support for the contention that commercially available GPS is off by at least 5%. +/- .01% is closer to it given a few miles to average.

Max Speed is often very inaccurate - I have seen several of my units register Max Speeds in cars, sailboats, and on motorcycles in the 300 and 400 mpg range but those are 'snapshots' and with units today updating once per second the average over any appreciable time/distance is quite accurate. Even the ancient Garmin III was accurate to .01 knot in a steady state (read your manual) and that was old pre-WAAS DGPS technology.

BlaSTr

10-02-2007, 08:06 PM

:BDH:

Doug

illzoni

10-02-2007, 09:30 PM

From the info I've seen, consumer-grade GPS devices are accurate to with .1 knot at steady speed.

Where you getting the 5% figure, Dave?

saaz

10-03-2007, 07:05 AM

I can't see why a calibrated police spec speedo would not fit straight in a standard bike. I have the police ST11 and speedo looks the same, apart from the 2kph increments. The speedo drive is the same as the civilian version. Speedos can be calibrated, but the cost is probably prohibitive.

UNTMatt

10-03-2007, 08:09 AM

We should demand better-
HONDA Give me an acurate Speedo!

I've yet to see an accurate OEM speedo on a bike unless it has a speedohealer.

It still blows me away that so many blame Honda when it seems to be more of an industry standard to set the speedo at about 8% off. Get over it. Buy a speedohealer or GPS and go ride. :rolleyes:

STNY

10-13-2007, 09:35 PM

I know this thread has been beaten like a dead horse, but...

Honda Speedo 110 MPH - Garmin Zumo 550 = 100 MPH
Honda Speedo 100 MPH - Garmin Zumo 550 = 91 MPH
Honda Speedo 90 MPH - Garmin Zumo 550 = 82 MPH

THATS ALL I WANT SAY :) :confused:

OH Yea... :wasntme

That's it...

illzoni

10-30-2007, 07:57 AM

04ST1300TX

11-13-2007, 07:45 AM

www.speedohealer.com

Subdivided

12-08-2007, 05:14 PM

I have verified that my ST speedometer is off 8% by riding next to two different new Honda Civics. The new Civic has a large digital display of the MPH which I believe to be very accurate. This display can easly be seen while riding next to the car.

Every motorcycle I've owned has about this same speedometer error. Very disconcerting to be to always have to recalculate the actual speed. I am disappointed at this trait in the ST. The temprature gauge seems accurate to within one degree, why can't the speedometer do likewise? (are you listening Honda = is there a fix for this?)