Lies, Damn Lies, and Speed

"Sir, do you know how fast you were going?"

Sound familiar?  Sound unpleasant?  Well if you're riding a Ducati you can truthfully answer "No, officer, I don't."

Even if you're watching your speedometer all the time (which I don't recommend) you still don't know how fast you were going.  For the simple reason that the Ducati speedometer may not be telling you the truth.

When I first started converting DDA files into CSVs, I noticed that the speeds didn't match the ones shown in the Prosa software.  They were usually 6-8% lower.  I wasn't worried about it at the time, I wrote it off to some legal liability decision made by Ducati and moved on.

But now every time I convert a DDA file for someone I have to explain this difference so I wanted to be sure it was real.  I decided to take a closer look.  I took the bike to the track along with a video camera and an independent GPS.  Here's a video snippet that shows a bit of what I'm talking about (the video may be clearer on youtube):

The video shows 3 measurements of speed:

• The dashboard (sorry about the glare).  That's what the bike is telling the rider in real time.
• The independent GPS.  (The phone right below the dash.)  That's my speed according to some billion dollar satellites.  (I'm using a QStarz 818XT GPS receiver.)
• The data from the DDA file after it's converter to a CSV.  That's shown by the overlay speedo on the left.

(There's a 4th reported speed that isn't included here because I can't add it to the video easily, and that's the speed shown when you open the same DDA file in the Prosa software.  You'll have to trust me when I say that Prosa very closely matches the Ducati dashboard. If anything Prosa shows speeds a bit higher than the dash.)

It's not perfect but it's clear that the speed reported by the GPS matches the CSV and both are lower than what's shown on the dashboard (and in Prosa).  At lower speeds (<40mph) the difference is pretty small but at 90+ the difference is significant.  In the clip the dashboard shows as much as 127 while the GPS peaks at 118.7 and the CSV touches 120 for a single frame.

So, no officer, I don't really know how fast I was going.

(But I'm inclined to believe the billion dollar satellites.)