GPS vs indicated speed

First topic message reminder :

I recently upgraded from the Reflex and my GPS always indicated 2-5 mph less than my indicated speed. So I got used to traveling at 60 believing I was going just above 55. Now on my Swing I also travel at indicated 60 and I find my GPS is also reading 60. Is the indicated speed on the Swing correct?

Keith
Iowa

Waspie wrote:

At that speed - who cares!! I'd be too busy just hangin' on.

Waspie wrote:

honda_silver wrote:

Waspie wrote:

Silverwing with JATO bottles and afterburners fully on!!! =

Would the GPS display the correct MPH?

At that speed - who cares!! I'd be too busy just hangin' on.

In this situation wouldn't the GPS be measuring ground speed
when it should be measuring air speed?

I've touched on this before but can't seem to find it when I search for it.

When I bought my 2010 Silverwing in October of last year I measured the speedometer to if it was off the usual 10% or so. I found by doing several checks that it was acturate and not off again. I've riden 12,000 miles since then.

Well today a friend of mine bought a new Victory mc, and I think he said it was a 1800 cc. He wanted to check his speedometer and wanted me to ride side by side for a 2 mile stretch to see how it did compared to mine. Well we took off and when the run ended and I pulled up beside him he was thrilled that we were running neck to neck at 100 mph, only I didn't have the guts to bust his bubble and tell him mine was only indicating 88 mph.

Keith

Seems it would have been better to do what he asked for and give him an honest answer.

dspevack wrote:

Waspie wrote:

honda_silver wrote:

Waspie wrote:

Silverwing with JATO bottles and afterburners fully on!!! =

Would the GPS display the correct MPH?

At that speed - who cares!! I'd be too busy just hangin' on.

In this situation wouldn't the GPS be measuring ground speed
when it should be measuring air speed?

You are correct though with such a low glide ratio, I'm not sure it would mattter much...

Just to throw another spanner in the works!

Off to France next week so needed some means of having kph displayed (the kilometer speed markings on the speedo are poor to say the least).
Solution was to fit a bicycle computer. Measured the distance each revolution of the front wheel travelled and hey presto a speed reading that is accurate to 0.1 of a kilometer, so hopefully no fines from the French gendarmes.
Cheers,

exavid wrote:

Seems it would have been better to do what he asked for and give him an honest answer.

Not in this case. Right now he really isn't into riding and if I told him his mc was not telling him the truth he might give it up altogether. After all his mc was telling him he was going 100 so to most people that would be truth. So far if he rides 50 miles in a year that's good enough.

Keith

When I first got my Swing and hadn't put the GPS on it yet I drove down 55mph roads and people would tailgate me and pass me and I wondered why everyone was driving like a maniac that day.

When I learned that I was actually driving 49-50, things started to make sense.

Rumor has it that Honda builds in a 10% speedo difference. That would equate to 55 - 5.5 = 49.5 mpg. An interesting discovery I have made with the trike kit... I am actually going 2 to 3 mph faster than the speedo shows now. I have adjusted my driving habit to reflect that discovery.

I think that's common here; our VW is roughly 10% over (or under?) to assist drivers to stay under the speed-limits.

kbcmdba wrote:

dspevack wrote:

Waspie wrote:

honda_silver wrote:

Waspie wrote:

Silverwing with JATO bottles and afterburners fully on!!! =

Would the GPS display the correct MPH?

At that speed - who cares!! I'd be too busy just hangin' on.

In this situation wouldn't the GPS be measuring ground speed
when it should be measuring air speed?

You are correct though with such a low glide ratio, I'm not sure it would mattter much...

So would that be 'true', 'rectified' or 'indicated airspeed.'

Either way my original response still applies!! " Who cares"

Right on Doug!

You left out Mach number....

YA know, I was just thinking.

When i ride I keep up with traffic and rarely look at the speedo.

I know its off by GPS.
The group I ride with some know their speedo is off and adjust accordingly. Others in the group say we ae speeding and slow down.
Last summer I got on the CB and would call out the speed of the GPS and have them look at their speedos at that time.

Some still said how do you know the GPS is right? I said that is is more accurate that all the speedos, since it was accurate to within 12 ft. So at 60 mph it could be off by .05%.

So now when traveling the lead calls out for speed reference and I tell him.
Everyone then knows we are at the true speed limit.

When alone I do what traffic does or a little faster.

All things are relevant to the moment. I find that rock and roll is for faster speeds and ballads/love songs are for toodling down country lanes.

MaxB (who really doesnt care what the speedo says)

Isn't air speed given in knots?????

I don't think my GPS has that setting.

I did knots in boy scouts long ago.

just saying.

MaxB

The only important thing about a speedo, is that I'm not wearing one.

Dan

Another bit of gas to add to the fire...

Because speed is computed based on a set radius of the tire, it is expected that indicated speed will change as tire temperature changes because the change in temperature will change air pressure and as a result, change the circumference of the tire. While this change may not be significant (unless your tire is grossly under-inflated), it does make a difference as speed goes higher. The only way to truly tell speed is to compare to an object is to compare it relative to something deemed stationary. We consider the ground to be stationary usually (except maybe in earthquakes).

All in all, however, if I get pulled over, I'm sure I'm likely to tell the officer that I am just trying to be safe keeping up with the traffic around me. It's one of those catch-22's. You can be arrested for speeding or you can be arrested for impeding traffic. Which is safer for the biker? You decide.

I generally don't care what my speedo says at that time (within reason) because speed doesn't kill. Difference in speed is what kills (for our NASA folk, it's not velocity that kills, it's delta-velocity that kills...) :-). Of course, delta-v (speed) relative to the ground gets higher as the speedo indicates faster whether or not it's accurate.

Skydivers and base jumpers all understand this: "Jumping off the (tall structure) doesn't kill. It's a sudden stop at the end that could kill you. Check your parachutes!"

Maybe I should start wearing a parachute in case I see an impending crash... :-) Yeah - that's it - put an ejection seat on my SWing :-)

... :-)

If I put an ejection seat on my SWing, I'll also put a bumper sticker on it...

"In event of crash, this vehicle will no longer be occupied."

DH said that every Honda he's ever owned showed approximately 5 mph faster than actual highway speed (considering 55-60 as "normal" highway speed) which would be approximately 10%. He put the GPS on mine and it was exactly that. Explains why I wasn't pulled over the other day by a policeman doing radar from a side street as I doing 46 in a 40 mph zone--per a later GPS run, I was actually doing about 42 and that probably wasn't worth the effort to pull me over (thank goodness). I mentally make note of that 10% and as another response indicates...explains why some guy was tailgating me across a bridge where the speed limit was 40 and I was going 42...was actually going about 36-38

Waspie wrote:

kbcmdba wrote:

dspevack wrote:

Waspie wrote:

honda_silver wrote:

Waspie wrote:

Silverwing with JATO bottles and afterburners fully on!!! =

Would the GPS display the correct MPH?

At that speed - who cares!! I'd be too busy just hangin' on.

In this situation wouldn't the GPS be measuring ground speed
when it should be measuring air speed?

You are correct though with such a low glide ratio, I'm not sure it would mattter much...

So would that be 'true', 'rectified' or 'indicated airspeed.'

Either way my original response still applies!! " Who cares"

+1

They make speedo healers, but like Waspie said . . . . . . .

scootingranny wrote:

DH said that every Honda he's ever owned showed approximately 5 mph faster than actual highway speed (considering 55-60 as "normal" highway speed) which would be approximately 10%. He put the GPS on mine and it was exactly that. Explains why I wasn't pulled over the other day by a policeman doing radar from a side street as I doing 46 in a 40 mph zone--per a later GPS run, I was actually doing about 42 and that probably wasn't worth the effort to pull me over (thank goodness). I mentally make note of that 10% and as another response indicates...explains why some guy was tailgating me across a bridge where the speed limit was 40 and I was going 42...was actually going about 36-38

Here in the UK the police, (real walking - talking police officers,) have a little bit of discretion over when the 'pull' you for a speeding offence.

They 'normally' use the speed limit add 10% for guage error and add a further 2 mph to that figure, however add to that bad day and you may not get the same response!!!!

So;

If the speed limit was 70 mph, add 10% (7mph) = 77 mph then add a further 2 mph giving a total of 79 mph before they would pull you over. (On a good day!!)

There is also the road side so called 'Safety Camera' vans. They have no lee way at all. If the speed limit is 30 mph and you do 31 you will receive a ticket through the post!

In bright sunlight it's hard enough to see the map on my GPS so I'm not going to take my eyes off the road to peer down at the screen at a tiny MPH display to see if it matches the speedometer.

Just think after all the posts about wearing hi-viz gear if you got clattered by a car because you were too busy squinting down at your sat nav screen.

After over 12,000 miles a recheck confirms that my speedometer is accurate.

compguy wrote:

After over 12,000 miles a recheck confirms that my speedometer is acturate.

Unlike your spelling of accurate!

Meldrew wrote:

compguy wrote:

After over 12,000 miles a recheck confirms that my speedometer is acturate.

Unlike your spelling of accurate!

Thanks

I just purchased a new 2011 Swing. I have to agree with Compguy about GPS speeds. My Swing is dead on with my GPS at any speed.... My 07 VTX1300R is not. It is off by what most of you experence. Example: On the X at 35 mph the gps is reads only 30. AT 40 the gps is only 35 etc. To cruse at 65 mph on my VTX I have to go 72 mph indicated. The new Swings just may be dead on. To cruse at 65 mph on the swing I travel at 65 indicated. Just my 2 cents. The most important thing , its a great ride no mater the speed you travel at. Have a wonderful day.

My GPS also reads lower than my speedometer. It's close to a 10% difference but it's no big deal.

just sing louder !!!!!!!!!!!!