Speedo vs. Odo, linked together???

rusty

Turbocharged
Joined
Aug 23, 2006
Messages
735
Location
Northwest, MO.
Ride
2005 Rocket III
I rode to Forrest City. Arkansas (just west of Memphis, Tenn, about an hour) from St. Joseph, MO. this past weekend to visit my sister. Great weekend for riding, both the trip down & the return. A buddy loaned me his GPS, which I placed on the gas tank (it has a suction cup mounting base) and relied on it for both the time & my speed. By the GPS, my speedometer is off by 5 mph when running 75 by the speedo (GPS stated I was doing 70 mph). Now, with this in mind, driving 507 miles would put the odometer off by about 35 miles at the end of the ride down. But, to my surprise, when I arrived I found the difference between the GPS (507.1 miles) and the speedometer (507.7 miles) to be only .6 mile. Pretty darn close after 507 miles, ea?

So, this question comes to mind. When we know the speedo is off by some amount, does that mean the odometer is "also" off because it is driven by the speedo? I find it hard to believe that, with a 5 mph difference while out on the road running that far that the odometer is off by such an insignificant amount (when compared to the GPS). Anyone else made a similar comparison?

When I figured the mileage of the Rocket, I got 36.2 mpg on the ride down, bucking a steady southern head wind most of the way. The ride home was also pretty windy, mostly cross winds but some head wind as well. As I figured mileage for the total trip, it figured out somewhere in the 38 mpg range.

9 ½ hours of rumble from the newly installed TORs was making me wish I had left the stockers on for this ride. I do like the sound they put out while on the road. It took me a while to put it together but I think the sound, from sitting in the captain’s seat, resembles the prop wash from a ‘copter flying overhead. Pretty neat, but not for 9+ hours.

The route I took was a nice ride too. Lots of 4 lane but enough 2 lane that I was able to test the passing properties of the Rocket. I took 5 vehicles at one point. I believe the Rocket said something nasty to all 5 of them as we went by. Just into Arkansas the roads get really twisty & that is were the best part of the trip took place. I was really looking forward to that stretch on the return trip. Just as fun as the first time through.

See ya.
 
Last edited:

Toystoretom

Living Legend
Joined
Feb 25, 2006
Messages
2,449
Just a theory..:D... but both the speedo and the odo are driven by the number of revolutions by the back wheel. However... the two might not be related. The odo may be driven exactly by the number of rear wheel revolutions x wheel diameter. We know that the speedo is a calculated value that is off by 7% and can be corrected by using Tuneboy. The odo function may not be a calculated value, if it were, it would probably violate Federal Odometer laws, because you could use something like Tuneboy to lessen the amount of actual mileage shown. Therefore, no matter what the speedo says, the odo probably chugs along as an actual number that can't be tampered with.

The only factors that may affect the odo would be a worn or wrong size rear tire. Also, if you think about it, the GPS, being based off of a satelite, is probably more of a line of sight calculation. As the crow flies, so to speak. But, as you are riding your bike, you are going around countless turns and curves within any given distance, that may add or subtract miles from the actual distance between point A and B. If you draw a bunch of squiggles between two points and had to follow those you can see it would be a lot longer distance than just a straight line. It may be just a freaky coincidence that the two mile figures where so close.
 

HeR3tic

Living Legend
Joined
Nov 25, 2006
Messages
3,454
Location
Heart of Dixie (Alabama)
Tomo - EXCELLENT reasoning! I'll buy two. The GPS unit used however is likely registering position data near every fraction per second, whatever fraction that is? The 5/10th mile difference was the diversion to places unseen by normal mortals:eek:

Rusty - Glad you had such a good time. Yep, you get what you pay for. TORs = Noise, fact. Ear plugs...that's the way to go. I like the soft yellow sponge type. Cheep and disposable.

I was responsible for passing out pink wax/cotton ear plugs to the aircraft mechanics, the newby butter bar aircrew members, and a few passengers from time to time, aboard the WC-130 I flew with in the late 70s. the majority of the young mechanics and all the zoomies saw what looked to them like a can of bazzoka bubble gum. I wasn't there to tell them the difference :D

Then there's split pea soup...but, that's another story. And the WC-130 honey bucket story...still another good fish story.
 

Gunshots

Nitrous
Joined
Dec 10, 2006
Messages
1,017
Location
Gun Lake, MI
I did a little reasearch on this with my BMW with the BMW/Garmin GPS. It seems that the speed indicated on a gps is very accurate, however the miles traveled is not so accurate. The way it was discribed to me was that the gps "drops a marker" at time intervals and uses calculations to process distance traveled. Fine if you are on the straight and narrow but move into the twisties or even gentle curves and it starts cutting corners. Mile marker posts on the freeway are still the best way to check ODO. GPS will verify actual speed. All this didn't help me much, BMW speddo's suck worse than Triumphs
 

HeR3tic

Living Legend
Joined
Nov 25, 2006
Messages
3,454
Location
Heart of Dixie (Alabama)
...the speed indicated on a gps is very accurate, however the miles traveled is not so accurate. The way it was discribed to me was that the gps "drops a marker" at time intervals and uses calculations to process distance traveled. Fine if you are on the straight and narrow but move into the twisties or even gentle curves and it starts cutting corners....

I work daily with GPS equip'd atmospheric sampling packages. I was earlier thinking the unit transmitted position data on a fractional second basis. It is instead every second. That would jive with what you're saying about dropping markers. And we all know how quickly the twisties can be handled.
 

Sidecar Flip

Living Legend
Joined
Jul 16, 2006
Messages
6,358
Location
20150 Mc Carty Rd. Deerfield, Michigan 49238
between the lines........

I work daily with GPS equip'd atmospheric sampling packages. I was earlier thinking the unit transmitted position data on a fractional second basis. It is instead every second. That would jive with what you're saying about dropping markers. And we all know how quickly the twisties can be handled.

Does that mean in essence you are a high tech 'balloon twister'?
 

Toystoretom

Living Legend
Joined
Feb 25, 2006
Messages
2,449
If Rusty could duplicate that kind of accuracy for distance traveled several times over that would be a great testimony to the accuracy of both the GPS and the Rocket's odometer.

If you have TuneBoy... you can really make your speedo much more accurate. I don't know why motorsickle companies deflate the MPH readings... most do it about 7%, ie if your at an indicated 100 mph you are actually going 93. I guess they want to inflate the actual performance of the bike or maybe they want to try and keep you out of trouble... who knows. :confused:
 

Sidecar Flip

Living Legend
Joined
Jul 16, 2006
Messages
6,358
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20150 Mc Carty Rd. Deerfield, Michigan 49238
Tomo:

I suspect the latter. If you are running an indicated 80 here in Michigan, you are actually 3-4 over the limit, well within the safe zone of the MSP officers and your insurance premiums. It's an ego thing. My Bonnie exhibits the same error and it's a purely mechanical speedometer.

Are automobiles off to the same degree ? You should know that being a automotive technician. Big trucks are right on or just a tad light because as a rule, the man is less tolerant of truck speeding than cages or bikes.
 

HeR3tic

Living Legend
Joined
Nov 25, 2006
Messages
3,454
Location
Heart of Dixie (Alabama)
Does that mean in essence you are a high tech 'balloon twister'?

Yes, that can be inferred.

...Are automobiles off to the same degree ?...
The several GOVs I roam around in frequently, figuratively speaking, are all GPS equipped. Which are the correct readouts??? Good question; but, the GPS units uniformily read, just under, 10% faster than the speedometers. I trust the GPS units. I don't believe GSA special orders these vehicles with higher calibrated speed-os. It's got to be a conspiracy:eek: at higher levels. Oops, now I've done it: I'm on the Watch List.
 
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