ghillie said:
1. Anyone know anything about the use of silica compounds in tyres - is Michelin just talking a lot of BS?
2. My preference is for the 175/60 because they are closest circumference to the OEM tyres and do not raise the ride height (keeping in mind Guillermo's comment above) - but would the 185/60's be better handling?
1. Most if not all quality tires include silica compounds some more then others. I've read marketing material that states it "improves grip on wet roads". But marketing loves to grab a key cool sounding tire ingredient that's not top secret and use it in marketing. The art and science of manufacturing of tires is one of the most highly guarded secrets in the automobile industry. Try to get a tour of tire plant... good luck. I would not take the marketing literature about tires too seriously because they reveal very little info that could be useful for comparing different brands of tires of the same category.
The michelin Energy series of tires are designed and marketed as low rolling resistant tires. They are OEM on many VW diesel cars.
2. Comparing the exact same brand and model tire in these 2 sizes I can say with confidence the 175/60R14 would result in better mileage then the 185/60R14 tire because it's 10 mm narrower so it's more aerodynamic. (and a less significant reason in this case would be because it's slightly lowers the car). Realistically the difference will be small. Mesureable difference? maybe. Tires are responsible for causing up to 30% of the total air drag in a typical car.
Using a GPS i've found that the OEM Insight tires cause the speedometer to display 2% faster then reality. The slightly larger diameter of the 185/60R14 tires actually cause the speedometer, odometer and mileage in the dashboard display to be more correct, compared to using the OEM tires. Therefore the Insight is about 2% optimistic in the mileage display with OEM tires. since the 175/60R14 tires are 0.79% smaller then the OEM tires the Insight would theoretically display a mileage that is almost 3% optimistic compared to reality.
Comparing the exact same tire brand and model on the same 5.5" wide rims:
The 185/60R14 tire would provide slightly better lateral grip and handling compared to the 175/60R14 tire simply because it's wider.
Michelin Energy XM1 tyres are all season tires just like the Insight OEM Bridgestone RE92 tires. So they both provide huge compromise in many tire characteristics for a long tread life and mediocre/adequate grip in most road conditions. The advantage of the Bridgestone OEM Insight tire is that it provides the best mileage for the car.
What advantage are you hoping to get by switching to Michelin Energy XM1 tyres?