Ok, this is crazy or I'm insane
 

Just put my snows on the MC (wife had to drive in the mountains of PA)

Here's the kick...

Summer tires 215/40/18
Winter tires 205/50/16

Suddenly my MPG went from 28ish MPG to 34 MPG, I'm just wondering if the tires are that far off and what in reality my mileage is for either set of tires. Also wondering how far the speedo/odo is off...50k service next week...possibly was over due?

Is it possible your mileage went up because you wife was driving the car instead of you? I suspect mine does (on the rare occasoins I let her drive it, that is).

:lol::lol::lol::lol::lol::lol:

From http://www.1010tires.com/TireSizeCalculator.asp

215/40/18 Revs/mile = 839.7
205/50/16 Revs/mile = 864.1
Speedo difference=2.905% too fast with the 16's compared to the 18's.

So theoretically, the change in tire diameter will give you a small apparent increase in mpg (but not 28 to 34 mpg...that's likely due to conservative highway driving!!!) :smile:

that and better mileage in cold weather.

my car shows 320+ miles range on the computer when I put my 205/45 runflats. When I switch them to 215/40 azenis, range drops to ~280

At first I was puzzled; lighter wheels and tires ought to over-compensate for more friction drag from stickier tires and yield better gas mileage.

Gas mileage is determined by many things and load is one of these. A lightened flywheel requires more load from the throttle to keep the engine spinning - the loss of inertia at low speeds hurts acceleration. I wonder if the same phenomenon is at work with lighter wheels and tires; are we indeed increasing load to keep things moving?

My wheels weigh in at 16.2lbs each and my tires at 20lbs each...about 20lbs less spinning weight per wheel when compared with stock 17" runflats.

Minicoop78 - mpg typically suffers in cold temps. The warm-up cycle is longer.

That's crazy talk...stop that you might hurt someone with your logic

difference of oversized circumference and weight vs proper circumference and weight? :smile: