Speedo /Taco Error

Hamo42 · 1 Speedo /Taco Error Sat Feb 11, 2023 11:31 pm

Hamo42

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Number of posts : 15

Hi guys & girls, hope you can help. Restored a K5C with K75S final drive. Now taco shows 4750 RPM when at 100 KPH. I know the tyre sizes are different 130/90/17 on mine. 120/80/18 on a normal K75C. Is there anything I can do. It's obviously over reving. Instruments have been calibrated by qualfied shop.

Any assistance appreciated.

Point-Seven-five · 2 Re: Speedo /Taco Error Sun Feb 12, 2023 12:29 am

Point-Seven-five

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By my rough calculations, your tach and speedometer are within the normal speed error of these machines. I am guessing your final drive is 32:10.

My K75RT does 75mph at 5700rpm which is approximately the same ratio as your 100kph(62mph) at 4750rpm.

Hamo42 · 3 Speedo /Taco Error Sun Feb 12, 2023 5:31 am

Hamo42

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Hi, Yes my final drive is a 32/10 but according to the riders manual it should be about 4100rpm at 100KPH.
I am looking at getting a K75C final drive with the brake shoes & the wheel to match. I really don't want to go to that expense.

Still open to sugestions Thanks.

Last edited by Hamo42 on Sun Feb 12, 2023 3:00 pm; edited 1 time in total

Rick G · 4 Re: Speedo /Taco Error Sun Feb 12, 2023 8:22 am

Rick G

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Hamo42 wrote:Hi, Yes my final drive is a 32/10 but according to the riders manual it should be about 4100rpm at !00KPH.
I am looking at getting a K75C final drive with the brake shoes & the wheel to match. I really don't want to go to that expense.

Those figures apply to a K100RT or RS the figures you state are fairly much right for a 32:10 final drive of a K75.
I also removed your other thread on the same subject, there is no purpose to putting it up twice.
Admin

Laitch · 5 Re: Speedo /Taco Error Sun Feb 12, 2023 1:06 pm

Laitch

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For those who might prefer affirmation from a gizmo rather than from experienced riders, there's this from the friendly folks at discount tire.com in Kansas, US of A.

Point-Seven-five · 6 Re: Speedo /Taco Error Sun Feb 12, 2023 3:59 pm

Point-Seven-five

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Hamo42 wrote:Hi, Yes my final drive is a 32/10 but according to the riders manual it should be about 4100rpm at !00KPH.
I am looking at getting a K75C final drive with the brake shoes & the wheel to match. I really don't want to go to that expense.

The MaxBMW parts list shows the K75C having a 32:10 final drive ratio which is what you appear to have now.

I'm getting pretty confused, which is not unusual. Can you clarify what year and model bike you have? that will help identify what the correct final drive ratio is. Then, can you identify what rider's manual you have and where it specifies 4100rpm at 100kph? I have looked at my rider's manual, and all it says is that the final drive is 32:10, there are no engine rpm to speed specifications that I can find.

Last, is your taco beef, chicken or fish? Personally, I prefer burritos, tacos give me a stiff neck.

duck · 7 Re: Speedo /Taco Error Sun Feb 12, 2023 7:54 pm

duck

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Hamo42 wrote:Hi guys & girls, hope you can help. Restored a K5C with K75S final drive. Now taco shows 4750 RPM when at 100 KPH. I know the tyre sizes are different 130/90/17 on mine. 120/80/18 on a normal K75C. Is there anything I can do. It's obviously over reving. Instruments have been calibrated by qualfied shop.

Any assistance appreciated.

The stock tire size for the drum brake wheel is 120/90-18. 130/90-17 for the disc wheel. The disc tire is 1.1% smaller so will need only 1.1% more RPMs for the same ACTUAL speed. And of course the speedo reading will be 1.1% different since the smaller wheel will put out 1.1% more pulses for the same ACTUAL, not indicated, speed.

K75cster · 8 Re: Speedo /Taco Error Sun Feb 12, 2023 8:08 pm

K75cster

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You should have a similar tyre circumference, a 130x90x17 is 2080mm tyre contact with road, so 100kph divided by 2080 x 3.2:1 x 1.67 (K75c gearbox 5th gear) divided by 60mins =4282rpm. A 120x90x18 is also 2080 on the road. So something is amiss with the final drive counter for the speedo. Try doing a speed check with a gps unit. your 4700 rpm is a ways higher in speed best to speed check it before the coppers do it for you.

duck · 9 Re: Speedo /Taco Error Sun Feb 12, 2023 8:31 pm

duck

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K75cster wrote:You should have a similar tyre circumference, a 130x90x17 is 2080mm tyre contact with road, so 100kph divided by 2080 x 3.2:1 x 1.67 (K75c gearbox 5th gear) divided by 60mins =4282rpm. A 120x90x18 is also 2080 on the road. So something is amiss with the final drive counter for the speedo. Try doing a speed check with a gps unit. your 4700 rpm is a ways higher in speed best to speed check it before the coppers do it for you.

A 120/90-18 tire has a circumference of 2,115 mm.

A 130/90-17 is 2,092 mm around.

https://www.calculator.net/tire-size-calculator.html

92KK 84WW Olaf · 10 Re: Speedo /Taco Error Sun Feb 12, 2023 8:49 pm

92KK 84WW Olaf

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I run 140 rear tyres on my Ks. No swing arm clearance issues.

Not difficult to change the final drive to a disc one 31/11 or 32/11.

duck · 11 Re: Speedo /Taco Error Sun Feb 12, 2023 9:06 pm

duck

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92KK 84WW Olaf wrote:I run 140 rear tyres on my Ks. No swing arm clearance issues.

Not difficult to change the final drive to a disc one 31/11 or 32/11.

I run 150/70-17 radials. No clearance issues.

You give up some torque when you go from a 3.2 to a 2.82 or 2.91 final drive. (BMW put 32:10s on K75S for a reason.)

Hamo42 · 12 Speedo /Rev Counter Error Mon Feb 13, 2023 6:04 pm

Hamo42

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Hi Guys, The bike was a bitza. A K75C frame 10/1986 Engine A Jap import K75 with gearbox 32/10 final drive from an K75S
Tyre size is 130 /90/17. Circumference 2080cm. I have had the Speedo/ taco calibrated at a reliable motorcycle instrument shop. I check the speed with a GPS that was almost spot on. Currently I have taken the instruments back to shop to double check.

Hope this all helps.
Starting to pull my hair out now. ( Not much left )

Laitch · 13 Re: Speedo /Taco Error Mon Feb 13, 2023 7:49 pm

Laitch

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Hamo42 wrote:Hi, Yes my final drive is a 32/10 but according to the riders manual it should be about 4100rpm at 100KPH.

Because of your dedication to the data in this manual, it would be helpful to provide the date the manual was published, its title and the page where the data can be found so we can verify it. I haven't found those specs yet in my 1995 Riders Handbook but I realize formats and data in the handbooks have changed over the years.

Better yet would be for you to post a photo of the page.

Hamo42 · 14 Hamo42 Mon Feb 13, 2023 10:50 pm

Hamo42

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Hi Laitch, I have just been posted a copy of the K75 page in BMW-K motorcycle purists on FB. But when I

have ridden it, the speed is ok at 100kph via a GPS but the thing revs at 4750PRM.

Last edited by Hamo42 on Mon Feb 13, 2023 11:01 pm; edited 1 time in total

Point-Seven-five · 15 Re: Speedo /Taco Error Mon Feb 13, 2023 10:56 pm

Point-Seven-five

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I went through my 1994 K75RT/S rider's handbook and couldn't find a reference in it. I also went through my 1990 K75C/S rider's handbook as well.

Perhaps BMW North America doesn't include this information.

robmack · 16 Re: Speedo /Taco Error Tue Feb 14, 2023 3:48 am

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Maybe run some numbers through this K-bike speed to rpm calculator.

daveyson · 17 Speedo/Tacho error Tue Feb 14, 2023 8:38 am

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I think your brick is probably OK, bricks just are high revving.

My K75S is over revving compared with its rider's handbook, but so are my k100's. For some reason the graphs just don't match with reality.

According to the graph on page 87 of my 1991 K75 rider's handbook at 4,000rpm I should be doing 100km/h in fifth, but it does 92km/h, for a K75S. The K75 graph is almost identical.

On my k100RT the tacho and speedo needles are always parallel in fifth, so at 100km/h it does 4,000rpm, but the K100RT graph says it should be about 3,700rpm.

If yours does 92km/h at 4,000rpm (pushing it) it's the same as my K75 and I think OK.

Laitch · 18 Re: Speedo /Taco Error Tue Feb 14, 2023 11:17 am

Laitch

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Hamo42 wrote:I have just been posted a copy of the K75 page in BMW-K motorcycle purists on FB. But when I
have ridden it, the speed is ok at 100kph via a GPS but the thing revs at 4750PRM.

Take a screen shot of that K75 page and post it here, Hamo. I'll help you with that. Send me a pm if you have trouble.

If your instrument's speed display is the same as your ground speed then I see two strategies ahead. The first strategy is to check for clutch slippage. The clutch could be slipping at high speed but the slippage isn't noticeable at low speed. Goose it at high speed and observe if the revs jump faster than your neck snaps. If it's slipping, check the clutch adjustment. I rode with worn clutch disc that acted like that for 10,000 miles. I replaced it when fuel prices rose. Laughing

If the clutch is ok, the second strategy is to calculate your fuel economy over two or three fill-ups. If fuel economy isn't good, a compression test and general system check could reveal why it takes more revs to go the speed that your mysterious data source indicates it should. If it is good, there you go; cover the tach dial with an pleasant image or a symbol of spiritual well-being. Shift your preoccupation to personal health, politics or end-of-the-climate-as-we-know-it adaptation strategies. One of those should fill the void.

Laitch · 19 Re: Speedo /Taco Error Tue Feb 14, 2023 4:38 pm

Laitch

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daveyson wrote:I think your brick is probably OK, bricks just are high revving.

You two should ride downhill on slab with tailwinds. That should bring them both into spec. Smile

Laitch · 20 Re: Speedo /Taco Error Tue Feb 14, 2023 4:39 pm

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daveyson wrote:I think your brick is probably OK, bricks just are high revving.

Thanks for posting that fable table, daveyson.

Last edited by Laitch on Wed May 03, 2023 9:21 pm; edited 1 time in total

daveyson · 21 Speedo/Tacho error Tue Feb 14, 2023 7:12 pm

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I'm thinking the books might be wrong, I think that might be at least a part of the reason.

Hamo42 · 22 Speedo /Rev Counter Error Tue Feb 14, 2023 7:22 pm

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Hi Laitch This is what I was meaning that graph Davidson Posted is what I mean. The one I have is for K75C
And you guys crack me up.
Also I can't seem to post the Image I have stored on my desk top it say post to long ?

Although when riding it really seems its reving its guts out compared to my K75 S I just sold.

92KK 84WW Olaf · 23 Re: Speedo /Taco Error Wed Feb 15, 2023 2:07 pm

92KK 84WW Olaf

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A point not mentioned and one that does happen.....the speedo needle not moving fully due to the backing coming loose and obstructing the needle causing it to read slow......making the revs look higher. A gps check on the speedo will reveal this.

Hamo42 · 24 Speedo /Rev Counter Error Wed Feb 15, 2023 2:31 pm

Hamo42

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92KK 84WW Olaf wrote:A point not mentioned and one that does happen.....the speedo needle not moving fully due to the backing coming loose and obstructing the needle causing it to read slow......making the revs look higher. A gps check on the speedo will reveal this.

Hi Olaf, I have checked with GPS & a different Instrument assembly, Still 100kph /4750 revs Thanks anyway.

Regards Geoff

MartinW · 25 Re: Speedo /Taco Error Thu Feb 16, 2023 1:28 am

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I've just come back from a highway run and the speed of 100KPH on the speedo correlates with the GPS at a 100KPH and the tachometer shows 4700 RPM + or -.
Regards Martin.

Laitch · 26 Re: Speedo /Taco Error Thu Feb 16, 2023 2:04 am

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OK! One more and we can start a support group.

TacKler · 27 Re: Speedo /Taco Error Thu Feb 16, 2023 4:40 am

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Laitch wrote:OK! One more and we can start a support group.

As one who has no interest in joining any more groups, if for no other reason than having met enough reincarnations of a one Mr Hitler, late of Berlin.

My current speedo sits around 4,500 to 4,550 RPM for 100km/hr.

Last edited by TacKler on Sun Feb 19, 2023 10:09 pm; edited 1 time in total

Laitch · 28 Re: Speedo /Taco Error Thu Feb 16, 2023 11:20 am

Laitch

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TacKler wrote:As one who has no interest in joining any more groups, if for no other reason than having met enough reincarnations of a one Mr Hitler, late of Berlin.

Maybe you'll be invited on as board chairman solely to direct the search for empathetic leadership from your saddle. Smile

K75cster · 29 Re: Speedo /Taco Error Thu Feb 16, 2023 8:46 pm

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the 3.36:1 f/d would do 48k at 100kph or 4th gear does about that on a 3.2:1 f/d. Is it definitely going into 5th?

Hamo42 · 30 Speedo /Rev Counter Error Fri Feb 17, 2023 2:37 pm

Hamo42

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K75cster wrote:the 3.36:1 f/d would do 48k at 100kph or 4th gear does about that on a 3.2:1 f/d. Is it definitely going into 5th?

Yep

duck · 31 Re: Speedo /Taco Error Fri Feb 17, 2023 8:39 pm

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I have a taco error too!

Speedo /Taco Error GWAuDfd

Point-Seven-five · 32 Re: Speedo /Taco Error Fri Feb 17, 2023 9:46 pm

Point-Seven-five

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You don't have that problem with burritos. Nor any chance of getting "taco neck".

...and at my age, Speedos are ALWAYS AN ERROR.

Hamo42 · 33 Speedo/Tacho error Sun Feb 19, 2023 6:18 am

Hamo42

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Laitch wrote:

daveyson wrote:I think your brick is probably OK, bricks just are high revving.

You two should ride downhill on slab with tailwinds. That should bring them both into spec.

I am not pursuing it any more. Just want to get my speedo back & fixed.

The instruments I borrowed the speedo now doesn't work What a pain in the B____. Speedo /Taco Error 8157

Laitch · 34 Re: Speedo /Taco Error Sun Feb 19, 2023 7:42 am

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Hamo42 wrote:I am not pursuing it any more.

Hamo42 · 35 Speedo/Tacho error Mon Feb 20, 2023 3:28 am

Hamo42

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daveyson wrote:I'm thinking the books might be wrong, I think that might be at least a part of the reason.

Well back to square one. I tested it with the K75C final drive & basically its the same. So I'm putting my Disc

final drive back on with the 130/90/17 tyre & I will put it down as a learning experience.

Thank you all again for your help & hints.

Cheers from Brisvegas

daveyson · 36 Re: Speedo /Taco Error Mon Feb 20, 2023 3:43 am

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Hamo42 wrote:
Although when riding it really seems its reving its guts out compared to my K75 S I just sold.

Haven't checked bricks, but every car I checked at those speed checkers along the freeways, when the speedo said 100km/h they were actually doing 95 or 96km/h. Since your latest brick has been calibrated for accuracy, that might be why it's revving higher.

My handbook doesn't have a graph for K75C bricks, I think they were no longer in production then, but there's a graph for the K75 (page 85) I think it might be the same, since you say at 100km/h it looks about 4,100rpm.

Laitch · 37 Re: Speedo /Taco Error Mon Feb 20, 2023 9:45 am

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Hamo42 wrote:Well back to square one . . .

Viva Brisvegas!

Dai · 38 Re: Speedo /Taco Error Mon Feb 20, 2023 10:13 am

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daveyson wrote:every car I checked at those speed checkers along the freeways, when the speedo said 100km/h they were actually doing 95 or 96km/h

If you're talking about modern stuff i.e. < 20 years old, they're all calibrated to read fast. In the UK the speedo reads 2mph faster than you're actually doing. It's ~~supposed~~ to ~~help~~ stop you speeding.

Point-Seven-five · 39 Re: Speedo /Taco Error Mon Feb 20, 2023 10:48 am

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Low reading speedometers must be a European thing.

Having done years of timed rally and enduro events I am really picky about speedo and odometer accuracy especially since I used to do 50-60,000 miles a year as an industrial sales rep and didn't need speeding tickets in strange towns.

I periodically test my cruise control and speedo accuracy by recording the time it takes to cover a marked distance on the interstate with a stop watch. On a long flat section I would set the cruise for 10mph over the posted limit and watch how much time it would cover a distance of 5 or more miles. Seconds per mile is calculated by dividing 3600 by the speed in mph. For example, at 75mph it takes 48 seconds to cover a mile.

My experience with Honda and GM vehicles over the past 30 years is that with the OEM tire sizes the speedometers are accurate to less than 1mph error, which is probably more a function of how accurately I can read the speed on the analog meter.

The low reading speedos on my bricks are annoying because I like to run at the limit of what the police will let you get by. I also keep a running schedule in my head when touring, and the speedo error forces a lot of fudging. I usually go by engine rpm now when I'm on the interstate and eating lots of miles. I have a pretty good idea of where the tach needle is at 65 and 75mph and that is what I watch.

I did do a calibration with Karamba on my RT, but even with the adjustment max'd out, the speedo still read a little low.

davemadsen · 40 Over/under revving Wed Feb 22, 2023 4:31 pm

davemadsen

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If you feel the tach is reading higher or lower than it should (or than you would like), a simple solution is to adjust the “pot” (potentiometer) behind the tach. In this way, you can have the tach show whatever you want it to show.

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31 Re: Speedo /Taco Error Fri Feb 17, 2023 8:39 pm

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35 Speedo/Tacho error Mon Feb 20, 2023 3:28 am

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36 Re: Speedo /Taco Error Mon Feb 20, 2023 3:43 am

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37 Re: Speedo /Taco Error Mon Feb 20, 2023 9:45 am

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38 Re: Speedo /Taco Error Mon Feb 20, 2023 10:13 am

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39 Re: Speedo /Taco Error Mon Feb 20, 2023 10:48 am

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