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Dammit. So you decided to stretch a little? Not much I guess.

How about going for a 35 profile? Not ideal for Winter I know, but the back of the car does seem to sit up higher than the front, smaller profile might fix this illusion no?

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I stuck with stock size and got michelins all around. So far no problems. 

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255/35R18 is the fitment on the 3rd Gen IS, and a far more common size but the rolling radius would be a full inch smaller than the stock 255/40R18 size on our cars.

The speedo would be quite inaccurate and you would rack up miles on your odometer at an accelerated rate.

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Ok but 245 40 18 would be a much smaller drop in rolling radius. I guess 40% of 245 is greater than 35% of 255 therefore closer to standard?

J Henderson. Any other issues apart from dazzling oncoming drivers? I have self levelling HID headlights so thinking this wouldn't even be an issue for me?

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Is there not a degree of calibration in the ecu? There is definitely an option in the menu concerning tire changes?

 

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15 hours ago, Billnick said:

Ok but 245 40 18 would be a much smaller drop in rolling radius. I guess 40% of 245 is greater than 35% of 255 therefore closer to standard?

J Henderson. Any other issues apart from dazzling oncoming drivers? I have self levelling HID headlights so thinking this wouldn't even be an issue for me?

Yes, 245/40 isn't that much smaller than standard. Its only about 6 or 7mm, something like that.

Regarding the self-levelling HIDs, I stand to be corrected but I believe they still work off a set base level and can only do so much. In the past I had the 245s on the rear, along with 235s on the front, so I had the scenario of the front sitting higher than normal, as well as the rear tyre being undersized, and with that setup I did occasionally get high-beamed by other drivers. Fitting the correct 225/40R18 size back on the front stopped this from happening though.

 

I reckon you would probably have to manually adjust the base level aim on the headlamps if you go with a 255/35.

16 hours ago, bobmc said:

Is there not a degree of calibration in the ecu? There is definitely an option in the menu concerning tire changes?

 

I think that Tyre Change option is for re-calibrating the satnav. 

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