Shadow Aero Speedo

I'm actually amazed. Just tried out my GPS on my bike, took her to an indicated 40 MPH. Actual speed according to GPS? 39 MPH. Both my truck and bike have just a 1 MPH difference between speedo and GPS. Thank goodness I didn't just assume a 7% error for the Aero speedo like I've heard about, I'd have gotten a ticket for sure!

my son in the air force in south dakota trailered his hd street cruiser to marquette. we got to ride together for a short time and compared speedometer readings. they were exactly the same at 30, 45, and 55. I had thought the sw was 10% optimistic, unless the hd also has the same error.
I have trouble believing hd would make a speedo with such an error.
what do you think?,

Have often wondered about all that myself... but have been to cheap to buy a GPS unit to check it out. Although there are many others who have found true discrepancies between their bike and the GPS. Maybe you have to check your own to find out???

What you have to remember with auto speedometers. They are built with up to +/- 10% error - Fact. (Caveat - up to 10 years ago it was certainly the case!)

Unlike the aviation industry, where their air speed indicators, (ASI's), are calibrated and errors indicated next the the gauges.

Certainly in the UK when speeds are monitored by our police, (not the speed camera partnership), the 10% error is allowed when they monitor vehicle speeds. Plus a 2 or 3 mph fudge factor. So if doing 76 mph in a 70 mph sector you could actually NOT be doing over the speed limit.

70mph + 7mph (10%) + 2mph (Fudge factor) = 79mph

Am I correct in thinking, too, that there is a built-in 'margin of error' to account for differences in the rolling radius/profile of different makes of tyre?

MikeO wrote:

Am I correct in thinking, too, that there is a built-in 'margin of error' to account for differences in the rolling radius/profile of different makes of tyre?

That I don't know. It may be part of the 10%!!!!!!

That's sort-of what I meant, Doug.

I know that when I got my truck (04 F150) in 2010, the speedo was off by about 3-5 MPH depending on speed, but when she got her 100,000 service, the computer was updated and the error shrank down to 1 MPH regardless of speed.

I would imagine, (no knowledge of this, just me surmising), that an electronic speedo will be more accurate than the cable driven speedo!

Any one know?

On my 2005 Swing: When I am doing 64 mph I am actually by Garmin GPS doing 60 mph. My Silverwing speedometer is 4 miles "slow." I have checked it on many occasions.
Old Dave

I think DaveR's indication is correct. His discrepancy corresponds with my iPhone GPS application.

We can all do GPS/Scoot comparisons and all it gives is a specific correlation on your machine.

Always good to know.

I ride to my bikes speedo as that is what I will be prosecuted for if exceeding the limits. As a rule if riding to the speedo the margin for error is in my favour as the speedo under reads the true speed.

Alternatively if you ride to GPS speeds your margin for error is almost zero as it is more accurate.

Waspie wrote:

MikeO wrote:

Am I correct in thinking, too, that there is a built-in 'margin of error' to account for differences in the rolling radius/profile of different makes of tyre?

That I don't know. It may be part of the 10%!!!!!!

Manufacturers are more likely to work with their recommended wheel and tyre sizes and makes, inflated to their recommended pressure. Impossible to test every possible combination. UK law allows the speedometer to be up to 10% over, speedos are not allowed to read under by any margin whatsoever.
If you don't know how incorrect you speedo is then allow for a 10% overread, easy calculation to do in your head.
Cheers,

When I went through the GPS/Speedo comparison phase, I found the difference on my Wing was 8%. However as you say TMB2, I use 10% as it's easier to calculate and doesn't require the removal of shoes or socks!!!

I think every Silverwing out there is 4 mph slow on the speedometer. I think they come from the factory that way. I understand you can get this corrected by going to the 'dark side' tire as it will give you a closer and more corrected read.
Old Dave

Or a calculator strapped to your leg!

Cheers,

Dave,

It is impossible for there to be a constant error on the speedo (A 4mph slow as you put it). The error is always going to be a constant percentage, whether is be 5, 10 or 15 percent. Changing the rear tyre may or may not reduce or eliminate the error.
Luckily in the UK we are not allowed to use 'car' tyres on bikes as that would be an MOT failure, or invalidate your insurance.

Cheers,

tarmacburner2 wrote:

Dave,

It is impossible for there to be a constant error on the speedo (A 4mph slow as you put it). The error is always going to be a constant percentage, whether is be 5, 10 or 15 percent. Changing the rear tyre may or may not reduce or eliminate the error.
Luckily in the UK we are not allowed to use 'car' tyres on bikes as that would be an MOT failure, or invalidate your insurance.

Cheers,

Don't go there.

There is a separate section for the car tyre fraternity!!!

Agree on the 'constant error' thing. It's a manufacturing feature due to component/human intervention. No two components or fitters will be the same. Hence the final product will have minute error built in. But we go round in circles again. Shutting up now!