Speedo Error???

[jchantzWR400F]

I just bought a Speedo off of an old street bike. Yamaha XS500 I believe. The front tire size stock for the XS500 was 19". The speedo mounted up very well, but now I'm wondering about how off will it be with my 21" front.

Can I expect my actual speed to be faster or slower than what the speedo reads.

Thanks in advance.

[jim-in-so-ore]

If you have the speedo drive on the front hub your speedo should be acurate. The drive is where the accuracy comes from, not the speedo head. ?

Jim

[dirtboxboy]

Hey there.

Pretty sure that with your new speedo you will be travelling fatster than what you read on the speedo.

Because the speedo was set with the 19" wheel and now you are using it with the 21", you travel a greater distance for every revolution of your tire. So you go further, travel faster, than what the speedo thinks.

Anyone want to back me up on this. I would hate to be giving out wrong information.

Working out how much is another question. I'll think about it and let you know

[Indy_WR450]

Hey Chantz what is the speedo for? ?

[jchantzWR400F]

Just for my entertainment really. I put a set of street tires on the old WR. Got a plate for it and am having a blast on it around town (back and forth to my softball games, stuff like that). Always curious to know about how fast I'm moving. Plus I like doing 50 MPH wheelies, but sometimes it's hard to tell when I'm doing 50....or 70.....or ???

Smoked a dude on a Harley the other day. Man, you should have seen the look on his face when he caught me at the next stop light. These things are a hoot. ?

[Indy_WR450]

Good to see you are still the local terrorist in Bloomington! ?

[Gordingull]

Hey there.

Pretty sure that with your new speedo you will be travelling fatster than what you read on the speedo.

Because the speedo was set with the 19" wheel and now you are using it with the 21", you travel a greater distance for every revolution of your tire. So you go further, travel faster, than what the speedo thinks.

Anyone want to back me up on this. I would hate to be giving out wrong information.

Working out how much is another question. I'll think about it and let you know

I'll back you up ?

[jim-in-so-ore]

Sorry to argue the point guys, but when I had my WR 400 I had a Honda road bike speedo on it and it was accurate. It depends on the speedo drive on the front wheel.

Jim

[jchantzWR400F]

Good to see you are still the local terrorist in Bloomington! ?

Oh, I'm sure the local environmentalists love me around there.

[jchantzWR400F]

Sorry to argue the point guys, but when I had my WR 400 I had a Honda road bike speedo on it and it was accurate. It depends on the speedo drive on the front wheel.

Jim

I see your point Jim, unfortunately, I've not had a chance to test the accuracy yet. If the speedo drive is where the accuaracy comes from, then, I should be in good shape. I just hooked the speedo up to my stock odometer cable. The drive was not changed.

[gomopar440]

Simplest way to test it is to use a GPS and check the speed as you go... but if you have a GPS already... ?

[jchantzWR400F]

Thought about that....Don't have one.

[gomopar440]

It was worth a shot ?

Anyone got one you could borrow? It wouldn't need to be mounted. First clear the recorded top speed in the GPS memory. Put the GPS in your pocket and make a speed run over a short stretch and make a mental note of your top speed. When you pull over look at the measured top speed on the GPS and compare. It's the easiest way I can think of.

But if you can't locate a GPS to use try getting someone to pace you with another bike that has a speedometer. Your speedometer will only be as accurate as his bike though. My R6 (and most other street bikes) measured 7% slower than displayed on the speedometer when checked against a GPS.

[ZEKEDAWG]

Tell you what, I have Jim's old speedo and I have no use for it. If you want it pay the shipping and you can swich them out and see what is what. Otherwise get a trail tech. way more functions and it is cool to see what you did during your ride.

[Bamster]

Sorry to argue the point guys, but when I had my WR 400 I had a Honda road bike speedo on it and it was accurate. It depends on the speedo drive on the front wheel.

Jim

If the gear was for a 19 inch wheel and he has a 21 inch

wheel ,the wheel will make less rotations in a given distance. This will in turn give an incorrect reading on the speedo and odometer. It's simple geometry. ?

I used a speedometer from a DT.

[jchantzWR400F]

Sorry to argue the point guys, but when I had my WR 400 I had a Honda road bike speedo on it and it was accurate. It depends on the speedo drive on the front wheel.

Jim

If the gear was for a 19 inch wheel and he has a 21 inch

wheel ,the wheel will make less rotations in a given distance. This will in turn give an incorrect reading on the speedo and odometer. It's simple geometry. ?

I used a speedometer from a DT.

You raise a good point, don't know if this clear's the water but all I did was unhook my odometer and replace it with the speedo. I did not touch the cable or gear at the wheel. So, the question is, where does the accuracy lie. In the gear at the wheel, or with the gears in the speedo?

[The_Missile]

whatever the mechanism is, it counts/measures wheel revolutions.

The crux of the biscuit however is wheel CIRCUMFERENCE. A speedo is matched to the wheel diameter installed on the vehicle it is originally installed on. If you have the same wheel circumference when swapping speedos good. If not, well you are buggered!!

One important piece of information which is required is the number of revolutions it takes your speedometer to register one mile. This may be found on the bottom (or elsewhere) of the speedometer face, usually in very small numbers. Look for something like 940, 960, 1000, or 1020, or something close. This number is the number of speedometer cable revolutions (or equivalent in the case of electronic sender units) to increment the odometer one mile. Most speedometers (modern ones) are 1000 revs/mile

Math follows:

Circumference is calculated as Pi x Diameter

PiD for 21 inch wheel = 3.14*21 = 65.95 inches so

1000 revolutions in 1 hour = 65,950 inches = 1.04 mph (63,360 inches in a mile!!!)

PiD for 19 inch wheel = 3.14*19 = 59.66 inches so

1000 revolutions in 1 hour = 59660 inches 0.941 mph

Multiply that by 60,000 revs and you get one measurement of 62.4 mph vs 56.5 mph = 10% error

In conclusion - same wheel diameter - speedo accurate

So to answer the original question : the speedo thinks it has a 19inch wheel but you have a 21 inch on, so it will show 56.5 mph when you are in fact going 62.4 mph

[jchantzWR400F]

whatever the mechanism is, it counts/measures wheel revolutions.

The crux of the biscuit however is wheel CIRCUMFERENCE. A speedo is matched to the wheel diameter installed on the vehicle it is originally installed on. If you have the same wheel circumference when swapping speedos good. If not, well you are buggered!!

One important piece of information which is required is the number of revolutions it takes your speedometer to register one mile. This may be found on the bottom (or elsewhere) of the speedometer face, usually in very small numbers. Look for something like 940, 960, 1000, or 1020, or something close. This number is the number of speedometer cable revolutions (or equivalent in the case of electronic sender units) to increment the odometer one mile. Most speedometers (modern ones) are 1000 revs/mile

Math follows:

Circumference is calculated as Pi x Diameter

PiD for 21 inch wheel = 3.14*21 = 65.95 inches so

1000 revolutions in 1 hour = 65,950 inches = 1.04 mph (63,360 inches in a mile!!!)

PiD for 19 inch wheel = 3.14*19 = 59.66 inches so

1000 revolutions in 1 hour = 59660 inches 0.941 mph

Multiply that by 60,000 revs and you get one measurement of 62.4 mph vs 56.5 mph = 10% error

In conclusion - same wheel diameter - speedo accurate

So to answer the original question : the speedo thinks it has a 19inch wheel but you have a 21 inch on, so it will show 56.5 mph when you are in fact going 62.4 mph

Well, I was hoping I would make it through life without being buggered ?

Anyway, your information seems reasonable and very helpful. Thanks

Of course if most motorcycyles have terribly inaccurate speedo's to begin with, I'm not sure what to think. I guess I'll have to get someone to pace me in a car, or break down and by a GPS.