Speedo accuracy

MarkB · Post by **MarkB** » 03 Aug 2009 08:22

After a month with my new (1976) 3 1/2 Strada, I'm finding it's an absolute joy to ride. I'm running in a new set of piston rings so I'm taking things fairly easy, but I've noticed that a lot of cars seem to be speeding through speed cameras, Nissan Micras exceed 60mph etc, and I'm wondering how accurate my speedometer is.

Can anyone help translate revs in top gear to actual speed? I've got a 38 tooth rear sprocket and I assume the front sprocket is standard, and I'm using Avon Roadriders, 100/90.18 on the rear.

Thanks, Mark.

robinh44 · Post by **robinh44** » 03 Aug 2009 09:27

Mark

I wondered when you were following me back to stevenage the other week as I was slowing down to 45-50 mph to keep you in my mirrors, I thought it was your tyre issue. I have attached a link to a spreadsheet I use, not mine but widely available on the net. I have populated it with the standard morini 350 gear ratios and rev details. Have a look and see, you can update all the items. Quite useful if you are thinking of playing with sprocket or tyre sizes. I can't comment on speedo accuracy as I use an electronic speedo where you program the front wheel circumference ( I have even added a 5% error to help with speed cameras).:-

Link to excel spreadsheet
http://www.mediafire.com/?uwyzenggmm5

Regards

Robin

MarkB · Post by **MarkB** » 03 Aug 2009 12:19

Thanks Robin, that's really helpful. I'll have to take him out for a ride to compare results. Is that ratio for 6th gear correct at 1.095, as it seems to be lower (?) than 5th, which is 1.047?

robinh44 · Post by **robinh44** » 03 Aug 2009 12:42

Mark

Thats my typo, should be 0.95 for sixth gear. I have corrected the original file and put the new link in the original post.

Robin

boris tarpit · Post by **boris tarpit** » 03 Aug 2009 15:12

That's assuming that the tacho is accurate . . . .

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neil · Post by **neil** » 03 Aug 2009 20:02

Despite what people say about Veglia instruments I find my speedo to be fairly accurate measured against the speeds signs in my area. Speedo indicates about 30-31mph when the signs show 29-30mph. Tacho reads around 5k at about 50mph in top.
Regards, Neil.

danomar · Post by **danomar** » 04 Aug 2009 01:00

The common joke is that Veglia are "vaguelia" accurate. My speedometer and tachometer are both ridiculously inaccurate. Then again, I usually do not look at them when riding my Morini!

3potjohn · Post by **3potjohn** » 04 Aug 2009 05:02

What I've done in the past is to arrange one of my offspring to hold up bits of card in the back window of my car with 10 mph increments.(well it's good to get them involved) And sometimes they get them in the correct sequence.
I think you can count marker posts on the hard shoulder, but I've never made it past about 3 before boredom set in.

EVguru · Post by **EVguru** » 04 Aug 2009 08:10

I'm running in a new set of piston rings so I'm taking things fairly easy

Piston rings should only take 100 miles or so to run in at the most.

You need to keep varying the engine speed and give it occasional short bursts of full throttle from mid revs. In other words DO labour the engine. You need high cylinder pressures to force the rings hard against the bores to get them to seat.

MarkB · Post by **MarkB** » 04 Aug 2009 08:11

It's interesting that Neil's tacho and speedo read 5k and 50mph in top. Mine read 4k and 50mph. If my maths are right, that would give Neil a top speed of 82mph at the red line of 8,200rpm (which seems low) and me slightly over 100mph.

Is there any way of checking the electronic tacho's accuracy as well? The needle's always steady except when changing gear, when it does a quick sweep across the dial.

MarkB · Post by **MarkB** » 04 Aug 2009 08:13

And thanks for the advice EVguru. I ride mainly on country roads, so the revs get plenty of variety, but it's good to know I can give it a quick blast every now and then!

robinh44 · Post by **robinh44** » 04 Aug 2009 08:30

MarkB wrote:It's interesting that Neil's tacho and speedo read 5k and 50mph in top. Mine read 4k and 50mph. If my maths are right, that would give Neil a top speed of 82mph at the red line of 8,200rpm (which seems low) and me slightly over 100mph.

Hi,

On my vapor electronic speedo and tacho in top gear I see approx 70-72 mph at 6000 revs and at 7000 revs about 82-85mph. However I am running a 38 tooth sprocket on a 160/60 17 rear wheel, this is a few mph down on what the spreadsheet indicates. Regarding veglia's I had them on a 1981 Triumph Bonneville, they seemed quite reliable but had the annoying habit of rotating in the rubber holders. They ended up sitting at about the two o'clock position so you got a kink in your neck trying to read them.

Regards

Robin

moriniuk · Post by **moriniuk** » 04 Aug 2009 13:18

3 pot john.

I wonder if other drivers think you're being given marks for your riding style?

H.

ringer · Post by **ringer** » 04 Aug 2009 21:35

Mark,

Have you considered that your speedo drive (in the front wheel) may be geared for kph, so where you think you are reading mph on the speedo, the speed indicated is actually kph? If that is the case, I believe that NLM do a converter. Can you give us an idea of the difference in indicated / actual speed?

MarkB · Post by **MarkB** » 05 Aug 2009 07:41

It's a UK-registered bike (Oct 76) so I'm fairly sure it's mph, and I don't think the discrepancy is enough to be kph vs mph. I'm going to take a ride out with RobinH to check my readings against his hi-tech electronic speedo, which seems a simpler solution than getting my nephew to score my riding out of 60, as 3potjohn suggested!

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Speedo accuracy

Speedo accuracy

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