18" front wheel on where 19" should be.

I didn't realise this until I had a slow leak fixed at the tyre shop and was told that the wheel on the front of my XJ was not the original wheel but an 18" instead of the 19"

It would explain why I get people passing me slowly though... and here i thought everyone was just speeding!

What do you think of these calculations?

Metres(metric) and Feet(imperial)

19" vs 18" revolutions per Km or Mile
NB:19" rotations / 18" rotations = ratio (not taking any differences in tyre height into consideration)

Metric
(1000/(0.4826 x Pi)) /(1000/(0.4572 x Pi)) = 0.947368421052631
Imperial
(5280/(1.5833 x Pi)) /(5280/(1.5 x Pi)) = 0.947388366070865

So in a
60Km/h zone it should be 60/0.947368421052631 = 63.33333333333337
80Km/h zone it should be 80/0.947368421052631 = 84.4444444444445
100Km/h zone it should be 100/0.947368421052631 = 105.5555555555556

Seem right or am I backwards somewhere?
Thanks!

 

Just realised... everything cancels everything else out... damn that algebra so just back to the original issue 18 compared to 19.

18/19 = 0.9473684210526316

 

Yes your numbers look correct although I think you will find it hard to match their perfection while smiling and dealing with those damn earth bound variables. Smaller diameter or radius for geometric freaks will cause increased revolutions vs. time or distance and therefore increased indicated speed.

Tire size could be a small factor but theoretically an 18" wheel with an equivalent ratio tire (100/90-19 vs 100/90-18) should be ~1" less in diameter. Of course as your tread wears your radius will decrease as well. So we all need to ride faster when the tread is gone right? On a dry day I suppose! :->

 

I checked out the pic in your gallery,very tidy lookin bike by the way,and I wouldn't bet that is the wrong size wheel.
Going by year my 81 seca has a 19'' wheel but those wheels have a swirly wheel pattern for want of a better description.
But my 83 750 and 900 and the spare set of wheels off an 86 900 have a similiar pattern wheel to the ones in your photo and they are all 18's.
Now yours is an 85 if I remember correctly so it makes sense it would have the later wheels on it.
Also dont the police bikes have a properly calibrated speedo so they can catch the evil speed demons?
Borrow a gps and do a speedo check that would be the simplest option.
Swirly wheels.
http://www.motorcyclespecs.co.za/Galler ... 20Sega.jpg
18 inch wheel pattern
http://home.kpn.nl/g.roest/yamaha20xj750.jpg
BUT after looking through google is it me or do your wheels look like the turbo's?
http://classic-motorbikes.net/images/gallery/xj750d.jpg
But when looking at the specs the turbo has 19'' front wheel so got me beat.
And just to add a little bit more confusion the antidive forks on yours look like the ones on my 83' 900 the secas had the anti dive mounted differently.
Were the p's a parts bin special?

 

I got a sweet deal on a dunlop 401F 90/90-19 (one of those
harley brands that run an extra $100 for the name :roll: ).

Usually I run a matching (to the rear) 404 100/90-19.

The first thing I noticed was a 5k increase on the tack between
4.8 and 6k. 8O

I'll prob stick with the 100/90 since I like to memorize my relative
tack to speedo position in case the speedo go's out....as It has in the past.

 

Yes the wheels are the same as the XJ650Turbo and I thought the same thing about the spokes and didn't realise there is a couple of other bikes with the same spoke pattern.
I'm thinking the wheel could be from a XS400R or something similar as it seems to match the spokes and is 18".

The bike in my gallery is the first XJ750 and it does have the 19" front wheel.
This is the second one Here with the 18" wheel.

That GPS idea is great! I'll get that damn thing out of the car glovebox and strap it down somewhere to test the speedo!

The XJ750P is definitely a weird one in Western Australia. The front forks are altogether foreign as far as i can tell from parts lists but the inners are the same diameter as the 900 forks.