speedometer relation to odometer

[snicrep5]

After getting my bike on the road I was at first ecstatic that I was going so fast with such ease. However I soom noticed that when at 90 indicated, utility truck pulling trailers, and women in ford focuses were passing me. So now I know the spedometer is optimistic. Im still trying to gain a reference point as to actual mph. Im waiting to pull along side a car with a digital speedo. I could go 60 ind and count off between mile markers but going that slow is no fun and im probably going more like 50. I did observe my odometer between 2 known mile markers and it was spot on. Am i to assume that the odometer and speedometer arent running off of separate mechanisms? Id like to think my speedometer is relatively correct and there are just alot of lead foots out on the highway. thanks.
btw when you "metric people" discuss speed, can you translate it to mph for us bacward americans please?

[Skaat]

I've had the same issue/question as well. anybody?

[Steve]

Don't automatically trust any vehicle's speedometer, digital or analog.

Do you have a GPS? It's easy enough to strap it or tape it somewhere on the bike where you can see it to check your speed.

I have found all of our speedometers to be reasonably accurate. At an indicated 70 mph, my GPS shows 69 on my GS, 68 on my wife's and 71 on my Wing.
My van is also within 1 mph, but my wife's car is off by about 2 or 3 at 70.

.

[duaneage]

I notice the speedo becomes optimistic and reads higher as the front tire wears. At higher speeds it's more pronounced, but mine is always off by 2 or 3 mph.
If you need an accurate reading the cop that gives you a ticket usually has a written result to give you,

[snicrep5]

only gps i have is in my phone and its not a "smart phone".
i suspect its off by 9 or 10 mph. On the MOTORCYCLE SPECS site, which is apparently british, they have a review of an 83 gs1100ez, wherein its stated that at 5000 rpm its going 75mph. on my 83 gs 1100 ed, it indicates 85 at 5000.

[SqDancerLynn1]

What size tire is on the front ? My 850 calls for a 100/90/19 I am using a 110/90/19 and the speedo is more accurate

[Normk]

Bike speedometers are typically optimistic. Assuming 10% is usually not too far off but as most of us like to know where the magic limit is so as to avoid official sanction checking your own machine with GPS is usually the best. When touring, I go exclusively by the GPS speed although have noted the main speed differences. I have recalibrated many bike and automotive speedos so if it really annoys you could consider doing that.

The odometer is usually more accurate than is the speedo but more difficult to adjust calibration because it is a direct geared drive. The most practical means of adjusting that function is to change the drive gear but that is not simple in theory or practice unless mass produced parts are available.

Here's a photo of a GS speedo with housing removed. One can see the odometer section with the disk shaped needle drive drum to the right. The drive inserts into the spindle on the further right. I can describe some of the procedures required if someone has interest.

[hampshirehog]

Here's a photo of a GS speedo with housing removed. One can see the odometer section with the disk shaped needle drive drum to the right. The drive inserts into the spindle on the further right. I can describe some of the procedures required if someone has interest.

Norm, I've rebuilt dozens of these to get them working smoothly again but there would be a lot of interest if you know how to calibrate the speedo accurately.

I know Ashley Pople (the guy who makes a living restoring old British instruments) and he won't touch these. I can't contemplate the level of skill he uses to do his work and it would be sweet to pop in to his workshop and have boast.

Incidentally a plug for Ashley. If you have a knackered speedo or tach off any British he can make it waaay better than new at a decent price.

[FiremanBob]

I used my GPS. The 550 was spot-on up to 50 mph, then 2 mph optimistic at 60 and 70. The speedo on my old V65 Honda was 10% optimistic.

[Nessism]

Yes, as Norm stated, the odometer is directly geared to the speedo cable drive. The speedometer needle on the other hand moves in relation to the rotating magnetic dome mechanism which is not direct acting. As the magnetic field between the drive changes, the accuracy of the speedometer changes.

I've been inside a few Suzuki speedometers and have no clue how to calibrate them or how to reduce the needle oscillation which is so common. I'd also love to learn...

[duaneage]

only gps i have is in my phone and its not a "smart phone".
i suspect its off by 9 or 10 mph. On the MOTORCYCLE SPECS site, which is apparently british, they have a review of an 83 gs1100ez, wherein its stated that at 5000 rpm its going 75mph. on my 83 gs 1100 ed, it indicates 85 at 5000.

If the rear tire is bigger the bike will go faster at 5000 RPM. I went down to a 110 tire and saw my RPMS go up almost 750 at speed.

[Nessism]

If the rear tire is bigger the bike will go faster at 5000 RPM. I went down to a 110 tire and saw my RPMS go up almost 750 at speed.

Yes, the tire diameter will have an effect; larger tires will reduce the rpm's for any given speed. Changes like this are pretty subtle though; you won't see the rpm's drop 750 just by moving a tire size or two.

[cowboyup3371]

How would you compare the speedometer against one of those radar speed trailers you see the cops put out then? I ran by one and saw what it said matched my speedometer but I don't know if it is always right.

[JEEPRUSTY]

What is the wheel diameter on the front wheel. It is where the drive for the speedo is located.

Is it possible some well meaning person traded a small wheel for a larger one?

[Normk]

The oscillation is usually caused by binding of the speedometer cable with the housing. In a few cases we came to the conclusion that the issue was a cable and housing which were worn so that the cable whipped from side to side within the housing and so apparently created differences in rotation. There may be other causes unknown to me but those are the ones with which I recall dealing.

One aspect of speedo operation which someone may be able to offer illumination is that of slow reaction to speed changes. I came to the conclusion that a sticky needle shaft is not the cause of a few old ones from elderly low mileage bikes. My own 1979 GS850G is around 34 years of age with only 31,000 km (18,600 miles) which is around 900 km (550 miles) per year. Obviously, the old girl spent 90-something% of the time parked as the average speed is only about 1/10 mph and it couldn't have been moving that slowly.

I'm wondering if the rotor magnet has caused magnetizing of the drum? Didn't think of that until reinstalled the speedo after servicing and re-lubing the mechanism. We used to de-gauss vehicle roofs, boats and other stuff because of compass interference. I'm wondering.....

Wish someone had suggested that as it would have been interesting to try an AC field from a solder gun to see if it would de-gauss the drum....wondering about affecting the magnet rotor but that should be shielded by the drum.....???....

Any thoughts?