[Dave M]

What's up guys. I bought my car with 50k miles last year . 08 535i sport

It came with stock run flats in the back and Bridgestone in the front.

Just the other day I got Michelin super sports. Man what a difference. Tires ride if I'm on air and improved handling. I would usually get around 17-18 mpg on my old tires. Now I'm getting 21-22mpg.

Kinda crazy, could tires make this much of a difference? Maybe because before I had two different sets on?

[jymmijamez]

Tire size for your 08 535, stock tires weigh in at 31 lbs front and 32 lbs rear.

Michelin Pilot Sport PS2 come in at 27 lbs front and 30 lbs rear, so you lost 12 lbs of un-sprung weight, which would make a noticeable difference.

However there were probably some other factors here such as low air pressure in your old tires that were causing you to only get 17-18 MPG

[Nanometer]

You are going to actually get worse mileage actually with the Michelin tires. Run flats are stiffer, and thus they have less compliance losses. This means that they don't soak up the bumps as well which is good maintaining energy of the car's momentum. This is also precisely why soap box cars use hard rubber tires and not soft, not the best comparison, but you get the idea. And the Michelin tires have a higher rolling resistance due to adhesive and cohesive forces, both of which reduce your mileage more. This is all in the interest of high grip and performance.

[Eleventeen]

Did you get an alignment at the same time?

[LessIsMore]

Originally Posted by Nanometer

You are going to actually get worse mileage actually with the Michelin tires. Run flats are stiffer, and thus they have less compliance losses. This means that they don't soak up the bumps as well which is good maintaining energy of the car's momentum. This is also precisely why soap box cars use hard rubber tires and not soft, not the best comparison, but you get the idea. And the Michelin tires have a higher rolling resistance due to adhesive and cohesive forces, both of which reduce your mileage more. This is all in the interest of high grip and performance.

Agreed on your points, but seems that the unpsrung weight decrease, which is huge, might more than offset any additional higher rolling resistance due to stickier tires.

[Nanometer]

The unsprung mass change isn't significant to mileage, at least for highway cruising. It becomes an issue when you accelerate as you have both mass and inertia fighting you always. The weight differences would be not measurable for mpg for distance driving. For stop and go traffic? maybe... it's a factor but I don't think it would be a big difference