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Ok I just can't work this out I've never been good at maths, I've got an artistic brain.
I've just put bigger rims and tyres on my Lib from 195/65/14 to 205/70/15 which has increased my diameter by 8.79% so one revolution of the wheel used to travel 1913.3 mm and now one revolution of the new wheel travels 2097.7 mm and increase of 184.4mm
So here's my question
If my odometer shows I have travelled 400km with the bigger tyres on how far have I really traveled ??
and vice versa
If I have traveled say 100km between 2 towns what would it now read on my odemeter ??
I did manage to work out my speedo differences
40km is now 36km on my dash
50km is now 45
60km is now 55
70km is now 64
80km is now 73
90km is now 82
100km is now 91
110km is now 100

I cheated from here http://www.1010tires.com/TireSizeCalcul ... ion=submit to get the above measurements.

multiply or divide by 1.0879 to get the difference.

ie 400 x 1.0879 = 435.16

or 110 / 1.0879 = 101.11
meaning to stick to a 110kmh speed limit you need to drive at 101kmh according to your speedo (assuming it was accurate to begin with)

for very easy (and slightly rough) maths try a bit less than 10%, ie:

60 - 5 (which is a bit less than 6) = 55, therefore drive at a displayed speed of 55kmh in to achieve 60kmh.
or
250 + 22 = 272km travelled for an odometer reading of 200

Hope that helps.

Thats great! thanks Jamesd just curious how did you work out to use 1.0879 ?

Because 8.79% larger is 108.79% of the original (100 + 8.79)

Percentage is just a way of expressing a multiple of something.

To get 100% of something times by 1.0
To get 50% times by 0.5
To get 108.79% times by 108.79

Whether to use time or divide just depends on which way you are trying to make the conversion.

Hmm, I would probably not make a good maths teacher.

Ah ha thanks

Best to get hold of a GPS and use that to work out.. but my thoughts are you are out by 10%

I down loaded some speedometer iphone apps and it's pretty much the same as the calc's above eg 60km on iphone app is 55km/h on my dash.