Curious about MPG in cooler weather

[Seagull6]

I live in NJ and have had my 06 Prius since mid-June. Up until last week my overall calculated MPG was around 52.6 and the MFD MPG was close at around 52.1 (my log book is in the car and I am operating from memory right now).

The problem is that since the weather has become cooler (between 45 - 55 degrees) my MPG has dropped to 48.5 MPG. My driving habits are identical to before and the only thing that appears to have changed is the temperature, however, a friend suggested that NJ may have switched to the winter blend of gasoline alreay. I recall that in my other vehicles my mileage took a hit when we switched to MTBE gas, but I don't think they use MTBE here anymore. Incidentally, I checked my tire pressure and they are at 42 front and 38 rear. I'm using the same gas stations as before as well.

Any ideas what's happening to my MPG?

[Jeannie]

I've had the same happen to me. I live about 30 miles from you, and I'd been averaging around 49 MPG since I bought my Prius in April, but my last tank was a little over 44. That fillup was a little over 3 weeks ago, but the first week I was getting my usual mileage. I dropped a lot once we started getting mornings in the high 20's or low 30's and I HAD to use my defroster. I'm assuming it's because of the cooler temps and the need for the defroster. I'm 100 miles into my current tank and the MFD says 48, but it's been slightly warmer in the morning - high 40's and low 50's. Since my trips are relatively short, I'm pretty sure I'm seeing this much of a drop because of the temps - it takes longer for the car to 'get warm'.

Like you, I check my tire inflation regularly, and most of my gas has been from the same station.

I know we don't use MTBE in New Jersey, but there may still be some sort of 'winter blend' that's also playing into this.

[daronspicher]

I dropped off too... In Chicagoland... Seemingly am in the realtime 46-48 range now rather than my 52-54 in the summer.

It sure seemed like a sudden drop off too.. I thought it would be gradual.. a mpg here, one there... it pretty much just dumped in one weekend.

[Marlin]

Two reasons...

1) The Prius gets less gas mileage, 25-35 MPG or so, until the engine is warmed up. This usually takes around 5 minutes in the summer. In the winter, the engine begins at a colder temperature, so it takes longer to warm up, and therefore the warmup mileage hit is a greater percentage of your commute.

2) You are undoubtedly using the heater these days. The heater uses the engine's warmth to heat the cabin. In order to work, the engine must be hot and the engine doesn't get hot if it's not running. Therefore, until the engine warms up enough to provide cabin heat, it stays on, even when you are stopped at a traffic light. This lowers your mileage.

[john1701a]

That's normal. You'll learn to adapt... and then understand why the veteran owners all gravitate toward LIFETIME data. It's the only thing that takes seasonal cycles into full account.

This is the 7th time I've experienced a Winter drop. It brings an entirely new meaning to the arrival Spring.

[stanleyjohn]

I got my prius in june and had my best MPG tankfull av. in early oct (55.5).Its starting to slip now! last tankfull was about 52.Im interested on how low it will go this fall and winter before bottoming out.Putting snow tires on in early dec.(michelin x-ice),not sure how much additional mph hit that will give me.I know that other pc members have enigine heaters for the winter.Just wondering on how much that helps in relieveing the mpg cold weather hit? <_<

[meezercat]

<div class='quotetop'>QUOTE(daronspicher @ Oct 26 2006, 05:46 PM) [snapback]338909[/snapback]</div>

I dropped off too... In Chicagoland... Seemingly am in the realtime 46-48 range now rather than my 52-54 in the summer.

It sure seemed like a sudden drop off too.. I thought it would be gradual.. a mpg here, one there... it pretty much just dumped in one weekend.
[/b]

I'm so glad you said this. I too went from 52-54 to 46-48. I'm attributing it to winter gas formulation (assuming they must have changed it) since it happened immediately after a fillup.

[jfschultz]

<div class='quotetop'>QUOTE(Marlin @ Oct 26 2006, 05:49 PM) [snapback]338910[/snapback]</div>

Two reasons...

1) The Prius gets less gas mileage, 25-35 MPG or so, until the engine is warmed up. This usually takes around 5 minutes in the summer. In the winter, the engine begins at a colder temperature, so it takes longer to warm up, and therefore the warmup mileage hit is a greater percentage of your commute.

2) You are undoubtedly using the heater these days. The heater uses the engine's warmth to heat the cabin. In order to work, the engine must be hot and the engine doesn't get hot if it's not running. Therefore, until the engine warms up enough to provide cabin heat, it stays on, even when you are stopped at a traffic light. This lowers your mileage.
[/b]

Add to this

3) The engine cools off faster when stopped or running electric so it simply runs more to stay in the proper temperature range. The last half mile of my 4 mile commute to work is driven on campus with a low speed limit. I can almost always run it electric in the summer, but the engine will start towards the end in the winter. (A side effect is that the battery will have a higher level of charge than in the summer.)

4) The winter gasoline blend has more additives and less energy. All cars get lower gas mileage in the winter, just not a big a hit as HSD cars.

[tomdeimos]

You lose a little mileage in winter from winter fuel, slush and snow on the roads or even sand. More rain can hurt too. The other reasons are a cold battery and faster cool downs.

If you lose more from the other factors like a cold engine it is because you have not winterized your car and equipped your car with proper instrumentation.

If you don't use a block heater you will lose lots of extra mpg for the first 5-10 minutes. So this item is most critical for those with short trips!

If you don't have gauges and a properly set winter front the car will never even get close to warmed up on any length trip unless you drive very fast.

When I got my car I started out at 55 or better mpg. As temps got down below 50 degrees my mpg dropped to 50. When temps dropped to 10 degrees F my mileage was down to like 35 mpg.

The second year I added gauges and instruments. Now I still get mostly around 55 mpg summers. And in winter at temps down to 0 degrees I stay over 48 mpg. with at least dry roads. All that is required is to keep the engine up to normal operating temperatures but not allow overheating.

I use canview for instruments. Convenient, easy and complete enough for even monitoring the motors as well as the engine. Before that was available I used the scan gauge that at least measured the most critical item: engine temperature.

All this is covered in other threads, but it may not be so clear how very much this stuff affects winter mpg.
With the gauges I could see before I had any winter front, my engine temps would be maxing out at 120 deg F instead of a normal 170-190 degrees. when I was doing city type driving. This not only gives bad mpg but keeps the heater and defroster from working very well at all.

[eagle33199]

<div class='quotetop'>QUOTE(Marlin @ Oct 26 2006, 05:49 PM) [snapback]338910[/snapback]</div>

2) You are undoubtedly using the heater these days. The heater uses the engine's warmth to heat the cabin. In order to work, the engine must be hot and the engine doesn't get hot if it's not running. Therefore, until the engine warms up enough to provide cabin heat, it stays on, even when you are stopped at a traffic light. This lowers your mileage.
[/b]

Now, you know what they say about assumptions.... LOL. Personally up here in MN, i have yet to use my heater once (just the defroster for a few seconds to clear the window when it fogs up) And yes, it has been rather chilly here lately...