For the first 64000 miles of its life, my tC rolled on its stock Potenzas, consistently getting between 28.50-30.5 MPG on each fill-up (I'm obsessive about calculating each time).

For the past 3000 miles, I've been on my new set of Cooper Zeon A/S, and I've only been getting between 25.5-27.0 MPG. May not seem like a lot, but it calculates to a possible worseining of fuel mileage by 15%. It's pretty substantial, then.

Can tires affect MPG this much? Assume everything else is the same, such as tire size, vehicle weight, driving style, PSI (I stick to Scion recommendations).

Someone told me it could be the additive the gas companies are required to put in during the cold weather months, but I never had such a decrease in the previous 2 winters.

 

how sticky are the tires. The softer the tires are the more traction you'll get. This in turns creates more friction and reduces your MPG.

 

The weight of the sidewall and the your tire pressure will also have a significant affect. Wheel/tire modifications have the most effect on MPG than just about any other mod. . . even FI (assuming you stay out of boost).

 

assuming ur still on stock rims, the tires weight and size themselves can have an effect.

 

but 25-27mpg is average/normal for the tC pretty good for the winter. as the gas companies change to a "winter gas" that leads to lower fuel economy for improved starting in cold weather.

i say you're doing fine.