85mph speedo guys...how fast am I going??

[tkent02]

56-57mph at 4K in fifth....so X2 = 112-114....

OK...that leads to the next question...how accurate are the speedos on these things...

+ or - 10% to 15% would be a fair guess if it's working correctly.
Probably 10% is more likely. I have ridden a few GSes that were dead nuts, but most read 5 or 7 mph fast at normal speeds.

Test yours with a stopwatch or alongside a car with a GPS. :-)

[Dogma]

Centrifugal tire swell may be significant at that speed. I dunno about traction. Maybe Lorentz contraction.

[Calvin Blackmore]

http://www.car-videos.net/tools/speedrpm.asp

might also be helpful

[mriddle]

Centrifugal tire swell may be significant at that speed. I dunno about traction. Maybe Lorentz contraction.

I hadn't considered that.

[tkent02]

Maybe Lorentz contraction.

We're going to need a bigger motorcycle. Much bigger.

[Baatfam]

How about I just find a 140mph speedo???

[Steve]

OK...that leads to the next question...how accurate are the speedos on these things...

In the case of my wife's 850L (with 100/90-19 tire on the front), it's rather accurate. 70 mph on the speedo shows as about 68.7 on the GPS.
My Wing is off just a bit in the other direction. 70 mph on the speedo is about 70.5 on the GPS.
My Kawasaki Voyager 1300 was WAY off, though, 70 on the speedo was about 62 on the GPS.

How about I just find a 140mph speedo???

No, that would be FAR too easy.

.

[Rudeman]

How many decimals does the speed readout go to?
Tire slippage is insignificant unless doing a burnout. It's really more tire squirm than real slippage, it's not much.
Clutch slipping? Then you are not going very fast at all.
Transmission slipping? What, broken gears or something? Again, it won't go very fast like that.

By the way, I use GPS all the time at work. Good stuff. With only three satellites the position is not all that accurate, the speed is an averaged estimation. With more satellites being received the position resolution gets much better, hence the speed readout becomes more accurate. But the GPS display does not tell you how many satellites it is receiving at the time unless you ask it, nor does it tell you how accurate the speed readout is at the moment.

It seems to me if he had a GPS he wouldn't have asked the question.

My GPS units readout to tenths of a MPH and they are all 12 channel receivers. My speedometers are only graduated to one whole MPH. Transmission slippage is a reference to automatics used on autos. There is also propeller slippage on watercraft.

How accurate is your speedo if you change tires and decide to run a different width/aspect ratio. Do you then have the speedo recalibrated? I use GPS on my boat because speedometers on boats are notoriously inaccurate since they rely on pitot tubes that measure hydrodynamic pressure to determine speed. GPS is the standard for measuring speed on water craft.

I think we got on this tangent trying to help a guy to figure out how fast someone was going once he pegs his 85MPH speedo??

[Steve]

I think we got on this tangent trying to help a guy to figure out how fast someone was going once he pegs his 85MPH speedo??

Yup, that we did. It's been kinda fun, too.

.

[tkent02]

How accurate is your speedo if you change tires and decide to run a different width/aspect ratio. Do you then have the speedo recalibrated?

Absolutely I do, in my head. Tested by stopwatch when the tires go on. Every car, every bike, the exact error correction is ready to be instantly applied any time revenue enhancement is in progress.

The rest of the time I go however fast I feel like, given the time space continuum in use at the time.

[littleroot]

Borrow a GPS and find out

On my Garmin Nuvi 350 GPS I can reset the MPH counter, put it in my pocket and go ride to the speed I want to check. Later I can review the GPSD and see what the top speed was. I do this to check the speed on my scooters and cars - I have yet to do it on my GS1100GL. I'd like to know what 5th gear at 7,000 RPM is, but I probably do not want to know!

[crapwacker]

Can he just switch the speedo?