Does anyone run tyres apart from 245/40/18 XL on 18" wheels?

[MarkTM]

Reason I ask if have been offered a set of new Michelin Pilot Super Sports in 245/45/18 XL size at £450 that may be too good to pass up.

They are going on Audi Ronal wheels.

[DieTryin']

Reason I ask if have been offered a set of new Michelin Pilot Super Sports in 245/45/18 XL size at £450 that may be too good to pass up.

They are going on Audi Ronal wheels.

You'll probably find the speedo is more accurate and the ride quality will be smoother.

FWIW I would do it.

[MarkTM]

DT,

Soz do you mean more accurate with the 45's or with the original 40's?

I thought all cars are calibrated to read more on the speedo than your actually doing, so at 40 on my speedo my GPS tells me I'm doing 38. More pronounced at 72 when GPS tells me am doing 68. So I take it your saying that the extra 5 may do away with this anomoly with the rolling radius increased just a smidge?

[DieTryin']

I believe so. Remember the 40 or 45 sizing of the tyre is a percentage of the width, so 40% vs 45% (IIRC about 12mm in your case).
So 2⫪r means bigger circumference... so for a given engine rpm, for each revolution of the wheel you'll be travelling slightly further, therefore slightly faster.
So you'll probably find you're closer to your GPS speed.
I haven't done the calcs, so don't quote me :p

[MarkTM]

Will have to remember the increased accuracy if 'plod' stop me :biglaugh:

[DieTryin']

Will have to remember the increased accuracy if 'plod' stop me :biglaugh:

:biglaugh:

[MarkTM]

Great just bought them and collect Sat night

Not bad reviews on them here:

Michelin Pilot Super Sport | the Michelin Pilot Super Sport reviewed and rated | the online tyre guide (http://www.tyrereviews.co.uk/Tyre/Michelin/Pilot-Super-Sport.htm)

[DieTryin']

For £450 they're a bargain!!

[MarkTM]

Gup just gave me a link where you can compare current & upgraded tyres and it seems that 245/45/18's will make the speedo read 3.62% faster which should about address the deviation between speedo & actual ( also compared a smaller tyre)...if I'm reading it right Tire Size Calculator - Tire & Wheel Plus Sizing (http://www.1010tires.com/Tools/Tire-Size-Calculator)

Apologies for the messy cut n paste below:



Stock Tyre Size


245/40R18
Imperial
Metric


Section Width:
9.65 in
245.00 mm


Rim Diameter:
18 in
457.2 mm


Rim Width Range:
8 - 9.5 in


Overall Diameter:
25.72 in
653.20 mm


Sidewall Height:
3.86 in
98.00 mm


Radius:
12.86 in
326.60 mm


Circumference:
80.79 in
2,052.09 mm


Revs per Mile:
808.82/mi


Actual Speed:
62.14 mph
100.00 kph


Speed Variance:
-




-




Tyre Size 1


235/40R18
Imperial
Metric


Section Width:
9.25 in
235.00 mm


Rim Diameter:
18 in
457.2 mm


Rim Width Range:
8 - 9.5 in


Overall Diameter:
25.40 in
645.20 mm


Sidewall Height:
3.70 in
94.00 mm


Radius:
12.70 in
322.60 mm


Circumference:
79.80 in
2,026.96 mm


Revs per Mile:
818.85/mi


Actual Speed:
61.38 mph
98.78 kph


Speed Variance:
1.24% too slow.



-1.22%


Tire Size 2


245/45R18
Imperial
Metric


Section Width:
9.65 in
245.00 mm


Rim Diameter:
18 in
457.2 mm


Rim Width Range:
7.5 - 9 in


Overall Diameter:
26.68 in
677.70 mm


Sidewall Height:
4.34 in
110.25 mm


Radius:
13.34 in
338.85 mm


Circumference:
83.82 in
2,129.06 mm


Revs per Mile:
779.58/mi


Actual Speed:
64.47 mph
103.75 kph


Speed Variance:
3.62% too fast.


Diameter Variance:
3.75%

[techfreak]

At that price can I get some too please?

[joshA6]

The variance should be 2% or under, so you are a little out speedo wise.

check with your insurance company as well, you don;t want them to not pay out if you have a prang due to the wrong size tyres