Inaccurate speedo

[Charlie]

I bought a new bike not so long ago and the speedo is really inaccurate. When riding next to my brother, my 30mph is actually his 20/25mph. Following cars that have slowed down for 30mph speed cameras mine shows around 35-38mph.

It get's a bit 'wobbly' on the wrong side of 100. Just gets a bit annoying not knowing how fast I am going.

The bike is 1993 vfr750, it was imported from Belgium but has UK clocks on (not sure when they where added).

Any ideas?
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Past: Honda x8rs, Honda City fly, Honda Hornet 250, Honda VFR750, Yamaha xt600e.
Current: Honda CBR929RR & Yamaha XT660Z Tenere

[prawny1]

Buy a sat nav
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[Charlie]

I could use my iPhone for that and just have it on charge all the time while on the bike. But I'd rather not and just use the speedo.
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Past: Honda x8rs, Honda City fly, Honda Hornet 250, Honda VFR750, Yamaha xt600e.
Current: Honda CBR929RR & Yamaha XT660Z Tenere

[Wafer_Thin_Ham]

Where does it read from? Sprocket or front wheel?

If sprocket, are you running standard gearing, if front wheel is that standard?

Fit a speedohealer if your speedo's electronic.

If it's cable driven, I'm not entirely sure.
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[Rogerborg]

Speedos tend to read about 5% over, but yours sounds extreme.

From a quick eBay trawl, it looks like the VFR750 used to use a front wheel drive but changed to an engine drive somewhere before 1992.

[Kickstart]

Hi

It is within the very lax allowance for accuracy (10% +8kmh) but does sound miles out.

As I understand it some Hondas not only had different clocks but also speedo drives between mph and kmh

All the best

Keith
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Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing

[Charlie]

My first thought was different sprockets... I haven't counted them however the receipt for when they where last changed says 16 and 43.

The next thing I thought of was what Keith said, but I dismissed it thinking they'd be the same. Good to know the mph speedo drive may be different to km/h one. I'll give the sprockets a count before I change them over.

Thanks guys
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Past: Honda x8rs, Honda City fly, Honda Hornet 250, Honda VFR750, Yamaha xt600e.
Current: Honda CBR929RR & Yamaha XT660Z Tenere

[Kickstart]

Hi

It doesn't make sense to me either, unless they changed the gearing between markets.

All the best

Keith
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Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing

[Teflon-Mike]

rog's comment about sproket drive speedo is intreguing.
My imediete reaction would have been, that its a front wheel speedo drive, and the same wheel's tyres and brakes as my '93 CB750...
And they have 'uncommon' tyres sizings.
I cant remember what the 'book' sizes are, but they are a tad narrow and not as 'low' profile as more contemprary bikes.
My 750 over reads a little, as it has a 120/60 front tyre on it, where I think it ought to have a 120/70 'or something'.....
Not sure and doesn't matter..... but check the tyre sizes, as 40 & 50 profile tyres are a lot more common and easier to get hold of these days, and MORE than likely, whether the speedo drives from or sproket, a lower profile tyre would reduce the roling diameter of the wheel, hence have to do more turns to cover a mile, hence make teh speedo over read..... and that would work whichever end the speedo takes from.
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[Charlie]

I'll check them to then Mike, I didn't think that wrong sized tyres would change the speedo readings when it is sprocket driven but now you've said it makes sense that it could. Thanks
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Past: Honda x8rs, Honda City fly, Honda Hornet 250, Honda VFR750, Yamaha xt600e.
Current: Honda CBR929RR & Yamaha XT660Z Tenere

[Charlie]

Right I've finally got around to having a look into this.

Front tyre: 120/70 as per handbook
Rear tyre: 170/60 as per handbook

Front sprocket: 16 tooth
Rear sprocket: 43 tooth

So that is all what is expected.

I've emailed David Silver and they say that there doesn't seem to be any differences between different speedo drivers...

I had a look at the speedo driver and it seems fine and spins freely. I am still tempted to try a different speedo driver (eBay for £16 or so), as I cant see what else would be wrong

Anyone got any further ideas?
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Past: Honda x8rs, Honda City fly, Honda Hornet 250, Honda VFR750, Yamaha xt600e.
Current: Honda CBR929RR & Yamaha XT660Z Tenere

[Kickstart]

Hi

Not sure what to suggest. Possibly look for a speedo healer hidden somewhere. Could be that someone has had different gearing on at some point and used a speedo healer to correct the speedo reading, then gone back to stock gearing without removing the speedo healer.

However looking at a parts list it looks like the speedo is cable driven (there is a gearbox for it on the back of the clocks, although the same one for both mph and kmh), but I can't see where it is driven from (and can't find the cable shown anywhere).

All the best

Keith
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Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing

[Charlie]

It's driven from the front sprocket and is cable driven. I've had a look along on the cable and there is no speedo healers fitted.

Having a bit of a read I've found this:

[Charlie]

Although I then find a topic saying this: