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Posted: Mon Apr 04, 2005 2:45 pm 
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The rolling circumference of a tyre from 15-50psi doesn't appreciably change. It raises the height of the car (rolling radius) more than it changes the circumference.

Think about half tracks and how they work. The deformity of the circle doesn't change its circumference.
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Posted: Mon Apr 04, 2005 5:52 pm 
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jonbays wrote:
The rolling circumference of a tyre from 15-50psi doesn't appreciably change. It raises the height of the car (rolling radius) more than it changes the circumference.


way i see it is if you change the radius, you change the circumference, there's a fair bit of difference in ride height, ie radius, between a tyre at 15psi compared to one at 50, or even a more normal pressure of 32psi, so the circuference would have to be smaller, so of course the wheel would turn more for a given distance

i cbf going outside and letting my tyres down to 15psi to measure it, but i've seen how much a car lifts from a low tyre pressure like 15 to a normal pressure of 32, probably be around 30mm for a 205/60 tyre

that said, under normal pressures, the tyre radius difference at say 32psi and say 28psi wouldn't make too much of a difference to indicated speed, a good check for it would be to check it your cars' speed in one of those roadside radar speedo check setups the cops have, go thru it at normal tyre pressure, then go back tru it again with lower pressure, you could also do that at a cop car speed trap, and ask the cops if they'll check it for you, i've done that in the past, the cop said no problem and told me my true road speed after 2 passes
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Posted: Mon Apr 04, 2005 6:16 pm 
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3.23 diff in 3rd gear at 100kmh = approx 3000 rpm slight less

 

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Posted: Mon Apr 04, 2005 6:26 pm 
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jonbays wrote:
bozz trust my calculations this stuff i am not bad at if i say so myself.

If you know your diff ratio and tyre sizw i can calc out a spreadsheet and graphs with your speed in gears and email it to you.

PM me if you are interested


Sounds cool, I can provide soem hosting space for it, pm me.

 

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Posted: Mon Apr 04, 2005 7:06 pm 
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Just in case anyone wants to calculate anything to do with driveline, here is an excellent calculator
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Posted: Mon Apr 04, 2005 8:43 pm 
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don't know my ratio, but i doubt it's anything other than a standard one,
3rd gear with econ mode

70kph = 1750rpm
80kph = 2050rpm
90kph = 2300rpm
100kph = 2600rpm
4th gear 100kph = 1700rpm
4th gear 120kph = 2100rpm ish

so it's 250rpm ish for each 10kph in 3rd gear
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Posted: Mon Apr 04, 2005 8:48 pm 
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Dellboy999 wrote:
3.23 diff in 3rd gear at 100kmh = approx 3000 rpm slight less


bit different to yours, mines 100kph = 2600rpm, quite a bit less than 3000rpm, you have an ed tho, maybe my ratios different to yours?
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Posted: Tue Apr 05, 2005 8:43 am 
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dddp! wrote:
, you could also do that at a cop car speed trap, and ask the cops if they'll check it for you, i've done that in the past, the cop said no problem and told me my true road speed after 2 passes


s**t must have good Cops around your way. I would imagine 99% of the Cops over here would immediately say after that request would be,
" SCRAM, BEAT IT, get the f**k out of here. Cant you see we are loosing revenue with your antics. "

 

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