A generic calculation (non tire model specific) shows an 11.3% difference between the 40 and 55 tires. Also, there is a 3.2 inch diameter difference. Meaning, your car has the potential of being 1.6 inches taller off the ground. Real world, the rolling diameter is less, so it's more likely to be maybe only an inch or so taller.
I agree on highway mileage going up, but all other mileage will go down since you have a larger mass with a larger moment (diameter) you are trying to accelerate. You'll also find that your stopping distance would increase.

He's found some good tires in the correct size anyway so technicalities no longer important, unless you want to discuss the speedometer and odometer accuracy with stock size tires?

I've never heard of whistling from a valve cover, ever. Unless the cover is cracked? I'd expect a check engine light for a vacuum leak large enough to hear. My guess is a pulley is bad. Or an exhaust leak.

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Olympic Peninsula, WA, USA
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2005 Mini Cooper 100k, British Racing Green/Silver-Black
04/2002 4.6is 140k, Silver/Black-Alcantara *FOR SALE*
1989 325iX 270k, Diamond Black/Black
1984 318i 370k *SOLD* (First Bimmer)
Honorable Mentions: '71 2002, '87 325, '90 325ic, '92 525it, '93 325i, '94 530i