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Old 01-18-2012, 08:31 AM   #1 (permalink)
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Compensating for New tyres? Calculating the % out my milometer will be..

Hi,

I've just replaced 2x 205/60/15's with 1.6mm of tread

for 2 new tyres of the same size with appox 9mm of tread.

My BMW takes its milometer feed from the diff, so the increased circumference of the new tyres will change that..

whats the best way to factor in the % difference, or does anyone know a way to calibrate an electronic speedo/milometer?

thanks

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Old 01-18-2012, 03:00 PM   #2 (permalink)
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...does Scotland have "mile-post" markers? If so, simply drive at steady 60 mph (using cruise-control helps) and measure the time (stop watch) between 1 and 2 mile posts.

MPH(x) = 3600 / sec for "1-mile" marker

MPH(X) = 7200 / SEC for "2-mile" marker

...the "correction" factor (%) will be:

% = MPH(x) / 60 mph or % = MPH(X) / 60 mph

Last edited by gone-ot; 01-18-2012 at 07:19 PM..
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Old 01-19-2012, 01:21 PM   #3 (permalink)
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Quote:
Originally Posted by 320touring View Post
I've just replaced 2x 205/60/15's with 1.6mm of tread for 2 new tyres of the same size with appox 9mm of tread.

My BMW takes its milometer feed from the diff, so the increased circumference of the new tyres will change that..
You could use a GPS to calibrate the meter.

BMW likely calibrated it with fresh tyres though.
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Old 01-19-2012, 03:41 PM   #4 (permalink)
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Quote:
Originally Posted by Old Tele man View Post
...does Scotland have "mile-post" markers? If so, simply drive at steady 60 mph (using cruise-control helps) and measure the time (stop watch) between 1 and 2 mile posts.

MPH(x) = 3600 / sec for "1-mile" marker

MPH(X) = 7200 / SEC for "2-mile" marker

...the "correction" factor (%) will be:

% = MPH(x) / 60 mph or % = MPH(X) / 60 mph
cruise control?unfortunately not...

cheers for the formula though


Quote:
Originally Posted by euromodder View Post
You could use a GPS to calibrate the meter.

BMW likely calibrated it with fresh tyres though.
Fair shout- I may try to get my dad's one..
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Old 01-21-2012, 04:28 AM   #5 (permalink)
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The previous owner of my Mazda Astina (323F in Britain) fitted smaller profile tyres (195/50 15 verses 195/55 15).

I checked the speedo against my GPS on the motorway and it reads about 4.5% fast, as expected.

I checked the odometer against a measured 5 kilometer "speedometer test" sign posted on the motorway and found it to be accurate.....Huh?

I checked the odometer against the GPS driving to work. The GPS said my commute is 51.9km, my odometer said 52.0km, close enough to perfect. Again I say.......Huh?

My point is, don't assume the speedo accuracy reflects the odometer accuracy, as I always did in the past.


If you really have to adjust your speedo, this might help.

Speedo Corrector MkII Kit "Kit Back Catalogue" - Jaycar Electronics

I haven't tried this kit myself, but it sounds OK.

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