MPH... not even close

[Session101]

So something that i've been thinking a lot about lately is how accurate is my speedometer?

well i don't talk on the phone while riding and mainly forget to ask a friend to drive 65 and stay there so i can see what im doing. Just today i find one of those radar signs and a large open road with no cars

so guess what my numbers were? My speedo said 70
actual speed 58....

so im thinking tires right? mine are 100/90/18 front and 110/90/18 rear, i did try to use a tire calculator against a 120/90/18 and its only a 2% loss. what is going on?

so that's about 5700rpm and only doing 65... pretty disappointed
the stamp on the rear for the final ratio is 31/9

[georgesgiralt]

Hello !
At first you should see if the odometer is right or wrong. If it is right, this means you've the good indicator for the bike's rear drive and mph.
To measure speed mechanical devices use a spinning disk with a magnet and all of this very close to another disk.
This create a torque in the non driven disk due to electro-magnetic forces. This torque is proportional to the speed of the incoming disk.
This torque is measured against a spring.
Now, you know the theory.
So what could be wrong ?
Spring had weakened so it takes less torque to move the dial indicator
Wear has taken it's toll and something is not as it used to be resulting in a wrong indicator.
 Indicator was not well calibrated from beginning..
IMHO, what you see is a combination of the 3 factors.
Now, a couple of solutions :
Borrow a GPS and calibrate your speedo and put sticker on the glass. Be careful not to kill yourself on the road doing this. You can preset the sticker with a calculation of the speed given the RPM the gear ratio and the tire perimeter.
Search the web for calibration methods for Motometer dials and try to open the "thing" and calibrate it. Try not to destroy a now rare instrument.
I would opt for the first calibration method. After all you need to know is that you're UNDER the legal speed limit, not the actual speed....

[Barry]

BMW had the speedo read deliberately fast. Maximum error is I think 10% + 3 mph.  There is a service bulletin on this somewhere that would confirm that. The 110 tire will typically be approx. 655 mm in diameter vs up to 675 for the stock 4.00 which will add another 3 % error.  

So 70mph -13% - 3 mph = 58mph.  According to BMW that's within spec and acceptable  but I agree it's awful and I would want to recalibrate the speedo.

Odometers are much more accurate more like +- 2% so if your odometer is OK you probably have the correct speedo fitted but to check here are the final drive speedo combinations and also the theoretical mph/1000rpm assuming a 655mm diameter tire:-

Edit: Here is alink to a previous thread that includes the service bulletin. It was 10% + 2.4 mph http://www.bmwr65.org/cgi-bin/yabb2/YaBB.pl?num=1337321174/1#1

[Bob_Roller]

It sounds like your speedometer needs a calibration .

Those informational radar signs, are pretty accurate .

If you want any more info on how speedometers work, look up magnetic drag cup speedometer, indicator .

Mechanically driven tachometers work the same way .

[Session101]

so basically i need to get the right rear tire size to fix all of this would be easiest, both now and in the long run, saving gas milage and possibly being able to drive on the freeway for longer...

[Bob_Roller]

I don't think a different size rear tire will make much of a difference at all, I've used the 4.00x18 and 120x18 on my '81 R65 and didn't see any noticeable difference, not enough to cause the error you have .

[Barry]

o basically i need to get the right rear tire size to fix all of this would be easiest, both now and in the long run, saving gas milage and possibly being able to drive on the freeway for longer...  

The correct size tire will make a difference in the right direction but only by 3%. That's not much you'll only drop approx.150 RPM at 70mph. Given the choice of tires I like to take into account the difference in diameter but it's only 3% maximum from the smallest to the largest. Still, all other things being equal I'm going to pick the largest I can.,

  

[tvrla]

I think it's all pretty well been said. Not much to add.

And I agree - tire sizes don't seem to make much difference.

Agreed - 70 indicated is way way too high for 58 actual!

Hopefully your speedo is a W 1.259 to match the final drive. Let us know whatcha got!

[Session101]

it is 1276 0-120mph speedo

basically now my concern isn't the speedo so much, its the RPMs
if changing the tire to a correct size will make these values true?

i should be given 65mph  at just over 4050rpm, correct?
or at least be at 70mph @ 4592

this is according to snowbums website with R65
http://bmwmotorcycletech.info/ringgears.htm

so what is the best way to make this true? That way i know for a fact how fast i am going given the Tach reading and the speedo will just be off a bit, but the concern for milage is priority.

[nhmaf]

I don't know, I have found some of the construction zone radar speed signs to be in accurate, too.

[nhmaf]

I don't know, I have found some of the construction zone radar speed signs to be in accurate, too.

[Barry]

it is 1276 0-120mph speedo

basically now my concern isn't the speedo so much, its the RPMs
if changing the tire to a correct size will make these values true?

i should be given 65mphat just over 4050rpm, correct?
or at least be at 70mph @ 4592

this is according to snowbums website with R65
http://bmwmotorcycletech.info/ringgears.htm

so what is the best way to make this true? That way i know for a fact how fast i am going given the Tach reading and the speedo will just be off a bit, but the concern for milage is priority.  

Taking each of your questions in turn:-

Your 1276 speedo ratio is correct for the 31/9 final drive that you have.

You cannot correct a speedo error of 13% just by changing the rear tire. You can only correct 3% of that error at most.

Not sure what you are looking at on Snowbums site but according to my table above at 4500 rpm you should be doing 66.9 mph. That assumes a typical 110 tire.

Nobody's stock 120mph speedo is accurate. They all over read,   typically by at least 5%. BMW were happy that they over read by up to 10% + 2.4 mph.  If you want to improve on that you have no option but to have the speedo recalibrated.

[Session101]

 Â [/quote]


Taking each of your questions in turn:-

Your 1276 speedo ratio is correct for the 31/9 final drive that you have.

You cannot correct a speedo error of 13% just by changing the rear tire. You can only correct 3% of that error at most.

Not sure what you are looking at on Snowbums site but according to my table above at 4500 rpm you should be doing 66.9 mph. That assumes a typical 110 tire.

Nobody's stock 120mph speedo is accurate. They all over read,   typically by at least 5%. BMW were happy that they over read by up to 10% + 2.4 mph.  If you want to improve on that you have no option but to have the speedo recalibrated.

[/quote]

well in your response i see that i have to make sure of something, to disreguard the speedometer altogether.
My focus is now on the Tach and that i am driving the correct speed given the RPM on your table, i will have to find a way to test that while in 5th gear at 4500rpm i should be going 66.9mph, if that isn't true with my bike then there itself lies the problem that i would like to be fixed. I will have to rig up a GPS mount of some sort and a nice long flat road to run test, but if my bike is only able to get 66.9mph while doing 5500rpm then what are my options is what im asking.

[Barry]

I realise we haven't solved the problem for you but we have tried to put it in context. The thing is even if your speedo was in perfect working order it would still read high typically by 5 % plus any error from using a non-stock tire. No manufacturer designs a speedo to be dead accurate although the 80MPH speedo's fitted to some bikes are reckoned to be very close. The 120 mph all deliberately read high for legal reasons but They are not usually as bad as yours. I wouldn't be happy with that amount of error either and if the tests show that your tacho is accurate then your only option is to have the speedo recalibrated by a specialist. If you want a dead accurate speedo you could try compensating for the deliberate error by using one that had the wrong ratio. For example  a 1318 speedo would read 3.3% lower than a 1276 but bear in mind that would also introduce an error into the odometer so it's not really an ideal solution.

If you could find an 80MPH speedo of the correct ratio that might be another option.

[tvrla]

Since it's been determined your speedo is correct for the final drive, we can assume the mileage readings are also correct. Our odometers tend to be very close to spot-on.

Don't worry about the tire size, it won't make much difference in either the speedo or fuel consumption.

What I used to do when I got a 'new' bike was to discover the difference between indicated and actual speedo readings. If it says 60 and I'm actually going 55 - I'd know that 65 was really 60. I'd extrapolate my real speed from the indicated.

Some speedos have a linear error - 5mph high all through the range, and others may read high down low and low up high. So watch the speed signs and figure it out for yourself. For instance - if one of those speed signs says your doing 30 and the speedo says 35, you've got it figured out.

Maybe you could borrow a GPS and figure it out that way.

As for the tach - they're usually pretty accurate, but extrapolating speed from a tach reading isn't something I've done except for a quick check.