Difference in speed between Seca Atari and GPS Speedo

I've got an HTC Android phone with a speedometer app that I thought I would test on Monday. So I mounted the phone with the app active in the middle of the Atari and set off on the highway.

Almost in all speeds, the GPS measured my speed to 10 km/t lower than what the stock Atari did.

Any thoughts to why this is the case? Don't really think that the speedo gears are that worn.... or can they be?

 

I think that the manufacturers deliberately make speedos read a bit high to be on the safe side. After all, you'll always have some amount of speedometer error; if it showed lower than actual speed, then they'd be opening themselves up to all sorts of lawsuits from people who crashed or got a ticket when, "My speedometer told me I was only doing 65, not 70!"

This is a guess for our older mechanical setups, but my brother-in-law has a modern Kawasaki, and there are actually aftermarket boxes to fix the wheel speed pickup count rate on those and show actual MPH instead of a slightly exaggerated number. The fact that the odometer on those bikes reads correctly but the speedo reads high shows that this is a deliberate effort by the manufacturer.

 

http://www.youtube.com/watch?v=EoTkZGleYbM

Calling Mr BigFitz - - here's a test rig for calibrating and adjusting these old speedos, that uses a cheep bicycle speedometer as the "test standard".
The info is in this guy's book.

 

My 750 seca atari speedometer is dead on with my gps speed ( well, within 1 +- kph). One thing that could effect the differential in speed readings would most likely be tire size, put on a larger diameter tire, you would travel any given distance in less tire revolutions, meaning a lower speedometer reading.

 

Riiggght. And how accurate is said "cheap bicycle speedometer?"

A proper rig would just have a very accurate electronic RPM readout, and be fine-tuneable for speed. That's all that's needed for STOCK calibration.

Here's the deal. X number of RPM's =60 mph. Yamaha used the same silly speedo drive ratio as every other Jap bike of the era.

Virtually every Jap bike (every bike, actually, except maybe BMW) of the era used a 19" front wheel, with tires that were "close enough" in circumference so that nobody cared. Speedos of the era were KNOWN to be off; most of the motorcycle mags published a "speedometer error at 60mph" figure in their road tests. Most bikes were 5-7mph "optimistic" at 60, back when new.

Think of your original speedo as a "guideline" not a "precision instrument."

 

Hehehe... Thanks for the info Fitz.
The speedo error I can correct by riding 10 over. But before I determined that there was an issue with the speedo, I thought I was going 50 when in reality I was doing 40... imagine the annoyed looks I got in the rear view mirror

 

The GPS systems are designed as primarily "mapping applications" they are not speed reading devices. In Ontario they are not recognized, in law, as being an accurate device for measuring speed. The defence that the GPS read the one speed and the speedometer must be inaccurate won't hold up. So, don't be led astray by GPS even if they are probably more accurate than the speedometer.

 

but a cheep two dollar calculator is just as accurate as an expensive scientific calculator when it comes to adding and multiplying. It's a programmable counter - as long as the magnet is set-up correctly . . .

It would appear this guy has figured out HOW to turn 40 year old tachs and speedos into "precision instruments". Within 2% would be fantastic !!

 

I shall investigate.

 

my guess the factory speedo is off. Most factory speedos are off up to 10percent read higher then you are actually going to keep law suites down from speeders wanting to sue the manufacturer saying it said i was going xx mph. I am not sure how to explain it. The speedo is like a magnet if they sit for a long time or as they get older wear ect they do not work as well