Guys and Gals,
Just a reminder on playing with tyre sizes.
For those contemplating going to wider tyres, just keep in mind that the tyre width has an impact on the tyre height by way of the second number in your tyre size is a percentage figure and it's a percentage of the tyre height.
So in short, 245/35 means that the tyre height is 35% of the tyre width of 245mm.
What's the concern you ask? Well it ultimately impacts the rolling circumference of your tyres and as a speedometer counts wheel revolutions, changes to the rolling circumference is going to impact the accuracy of your speedometer and trip computer.
I'm going to work in millimetres in the following, it's how my brain is wired and how you will find a lot of the calculators work. For those in imperial, 25.4mm is 1"
The factory sizes on the B9 A4 Australian delivery are as follows, check your placard inside probably the drivers or passengers doors to confirm for your country.
245/40R18 - 653.2mm circumference*
or
245/35R19 - 654.1mm circumference*
*TRA figures - Tire and Rim Association standards.
So the average of the two is 653.65mm
The rough rule of thumb when keeping the same size wheel diameter is if you want to maintain the same circumference, and go to a 10mm wider tyre, you need to reduce your aspect ratio (the percentage figure) by 5%.
There's a good definition of tire nomenclature here -
http://www.tirerack.com/tires/tirete....jsp?techid=46
So 255/30R19 is going to be pretty close in rolling circumference to 245/35R19.
The tyre size of 265/35R19 is mentioned above and it has a circumference of 668.1mm which is only 14.45mm larger, though as a percentage against the reference, it's an increase in circumference by 2.2%.
So with this example, at 60mph in the car your actual road speed is 60x1.022 = 61.3mph.
I'm not sure of tolerance of the speed cameras in the USA and Europe, though here is Australia we have about 7-10% of the speed limit to play with.
If you typically factor a tolerance in what you set your cruise control at, dont forget to factor your actual road speed if you've changed tyre size and impacted your rolling circumference.
And your MPG calculations.
The other stage is when you increase rim diameter, this is called a +1 fitment (1" bigger rim) or +2 fitment (2" increase) and there's a formula for these too.
This is a fantastic calculator for anyone after references.
http://www.inawise.com/tyre-calculat...alculator.html
Happy travels.
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