as far as the car goes cant say, but you dont get as good a mileage with winter grade gas, at least in the northern states, due to a higher amout of butane mixed into the gasoline to aid in cold weather starting. i would think also the engine has to work harder longer with cold fluids, and probably runs richer for a longer period of time. also the a/c on modern cars kicks on in the cold weather to help with defrosting. all this combined knocks the heck out of your winter mpg.

I've noted about a 1 1/2 mpg difference between summer and winter in my car too. I just assumed it was something to do with the fuel. There is virtually no drop in mpg due to ac (defrost) usage on my car with a 350 cid v-8. A smaller engine probably has a harder time dealing with it. As far as the car goes, I'd think all that magical computer stuff would keep the air/fuel mixture the same regardless of the temperature or air density.

Cold weather can reduce mileage due longer warmup time. Another factor is more rolling resistance from the tires. Around here, a factor is our winter blend of gasoline. Extra ethanol and oxygenators are added (by EPA mandate). Two weeks ago, our local newspaper reporter was able to run 4 tanks of fuel through his vehicle: two with our winter blend and two outside this location with conventional blend. All driving was under approximately the same road conditions and speed, mostly highway. He reported a 2 mile per gallon reduction for our local blend. I know this was not an exacting test, but it does show the same trend I've expereinced. (I keep a log of miles and fuel used in all my vehicles.)

In the winter, I drive a 4x4 Jeep Cherokee, 6cy, 4.0 L engine. In summer, it tops out at 20 MPG and falls to 16 in the winter. The temperature was 15 F below 0 this morning, and it took a trip of 7 miles to bring the engine up to temperature.

I didn't know they changed gasoline in the winter? Is this a good thing or bad? Do all gas stations do this? When do they change back to summer gas?

As far as I know, all gasoline suppliers adjust the fuel blend on a seasonal basis. They've been doing this for many years. Originally, the main change was to adjust the volitility index. Winter fuels were blended to be more volatile for cold air operation.

The SE corner of Wisconsin has additional seasonal changes. We are under an EPA mandate to use cleaner burning fuel during the 'bad air' hazy months. Extra oxygenators are added. In some cases, this is extra ethanol with an additive. It costs us 2 to 5 cents more per gallon for this special blend, since it is not used outside our localitity, and the refinery supplies many other customers outside our EPA zone.

Adding ethanol, or alcohol, to gasoline lowers its heat content per volume of fuel because the energy content of ethanol is less than gasoline. This reduces mileage (and horsepower) by a porportionate amount.

The winter blend may be a little of it.But warming a car up sucks up gas.Running the defroster means the compressor is working just like AC.Also the car is getting more gas until it warms up.Every thing is stiffer from the cold oil at start up mechanical parts do not turn as free.Then there is snow and ice causing slippage perhaps.Also tires loose air in the winter faster then warm weather and you probably do not check the pressure as often because its cold.Its normal winter knocks the heck out of gas mileage.

Also I forgot to mention.There is more drain on your battery and a increase on the charging system.The alternator has more pull load on it.

low rvp gasoline season starts in march in mich, ohio, in april in new york not sure for the rest the country.