Tyre profile/speedo accuracy !!

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davidbarr

davidbarr

Joined
Jan 27, 2010
Messages
850
Location
south west uk
just ridden back from frasers of gloucester where i had a service and 2 new michelin power pures fitted.
Followed my good lady all the way back in the car at what she said was a steady 90 but i was showing 95 !
clocked her at 108 at one point though she swears she NEVER went over the ton....
This got me thinking ( and yes, it does hurt ) , upon checking the rear tyre i find that they have fitted a 50 profile tyre whereas before it was a 55 profile, has this affected the speedo accuracy.
We all know the speedo runs off the disc bolts, but does changing the tyre profile affect speedo accuracy ??
I think it must do but am quite happy for someone much cleverer than me to put me right.......what do we think ??
 
Dave , im sure it wont make any difference mate , because of it reading off of the disk bolts , if it was reading the speed off of the output shaft like some then yes it might make some difference , bit like changing the gearing on some bikes as this doesnt affect our bikes but affects others and you end up fitting a speedo healer , bloody speedos always read way off , makes me laugh when i hear guys with 600s saying ..... yeh i was doing 185mph ..... more like 165mph .
If in doubt fit a speedo healer , bout £80 i think .





Wot do i know , im stupid .... dont listen to me !
 
know what your saying Sean, but its starting to play with my head. I know it shouldnt make any difference,the speedo only shows what speed the bolts are rotating at regardless of the profile of the tyre so......just to play devils advocate, if you fitted say, a 200 profile ( ridiculous i know ) at an indicated 100 mph, would you ACTUALLY be going faster ??? Somebody tell me before my head explodes....
 
yes you would go faster with say a 200 , but the difference between a 50 and 55 ...... cant make that much difference surely ........ **** , im questioning myself now !
Will have to clock you on your bike with mine and see if theres any difference !
 
i know it makes sod all difference really, just wondered if there was anything in the theory of it ??
 
Dave , im off up to Fowlers tomorrow to meet my mate for breakfast , come along and i will clock your speed !
 
that's just made me check mine (they are 180/55)

I've got the power pures as well. What do you reckon to em so far?
 
the speedo picks up on revolutions on a known fixed centre.

ergo - if this stays the same, but the circle (wheel) becomes greater, then the surface further from the fixed centre, will be faster...

put the bike on a paddock stand, start it up, go through the gears, speedo will read your speed.

but the wheel isnt even on the ground.

the bigger the wheel, the faster the outer surface speed, but the fixed centre remains the same.

make sense ?
 
Last edited:
All car speedos overread - they are not allowed to underread for obvious reasons and at those speeds will be way off. I've had a few GPS devices now in my last few cars that tell your speed very accurately & speedo speed is off by 5ish mph doing about 70-80.
 
if the tyre is bigger then yes, the speedo would read slower than your travelling.
the bolts always read the same but the outer edge of the tyre is bigger therefore travelling a greater distance and greater speed.
not sure it makes that much difference though, tyre would need to be 10% bigger to make any real difference.
and changing tyre from a 50 profile to a 55 would only change the speedo by 2%

50 profile circumference is 167.04cm
55 profile circumference is 170.188cm



ive had a few scotches but this is all i can come up with
 
ive had a few scotches but this is all i can come up with..


i,ve had a shedfull S pal, and i was strugglin..


well done.
 
I agree with swordfish, it does make a small difference. I am on the brandy and could not put it as well as swordfish.
 
I'm on glenmorangie and can only agree, well put swordfish.
 
cheers guys, i knew there were some clever buggers out there ! You've put my mind at rest......i think !!
 
rusty76 said:
that's just made me check mine (they are 180/55)

I've got the power pures as well. What do you reckon to em so far?
Click to expand...
Bit early to tell Rusty, only ridden em down the M5 so far, did a few twisties on the last stretch just to scrub em in a bit. Seem good though, turn in is pretty quick but that could just be down to being used to the old worn out supercorsas..I always liked the 2ct's so im expecting good things, might be coming to Snetterton so that'll be the real test !!
 
shotgun said:
Dave , im off up to Fowlers tomorrow to meet my mate for breakfast , come along and i will clock your speed !
Click to expand...
Would have done that Sean if i hadn't just got up nursing a big hangover....shouldnt have opened that bottle of champagne after a skinfull of beer !!
 
DAVE B said:
Would have done that Sean if i hadn't just got up nursing a big hangover....shouldnt have opened that bottle of champagne after a skinfull of beer !!
Click to expand...

OOOOOO get you with your Champagne , bit posh aint it Dave , bet you were sipping it in your smoking jacket and cravat in the library while the butler was buffing the silverware ! :rolleyes
 
how very dare you......one doesnt drink champagne in the library, thats for plebs, one should only ever drink shampers in the drawing room !!! lol
Seriously though, not saying we were desperate but it's all we had left in the house !!
 

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