Why Mileage Gets Worse in the Winter

I get better mileage in the winter. My ac uses gas. Can someone help? I bought new Toyo tires. My Mileage dropped from 48 to 37. Have 117 miles. Had 60m on michellin tires.

 

Thanks for the note on the bad mileage during winter!

 

clean your throttle body and maf sensor, i thought everyone knew this. do you realize how much air density changes in winter?

 

The TB has almost zero influence on performance or FE, but can contribute to an unstable idle condition. The MAF should only need to be cleaned once, maybe twice in a vehicle's life if at all. This is because no air filter catches 100% of the mist or grime entering the intake system and some of that crud lands on the MAF sensor element, causing it to read incorrectly. However, the most common cause for a noticeable loss of FE is a dramatic change in rolling resistance or a failed AF Sensor (Wide Band Oxygen Sensor). If the AF sensor goes rouge, you often do not get a DTC because the ECM just thinks the AF ratio is too lean and adds fuel. You can gauge this by looking at the Long term fuel trims, if you have tooling that allows those PIDs to be read.

 

I have a 2008 Prius with about 140K miles and have recently noticed the same drop in mileage. Mid 40s MPG to 37ish. Using the vertical bar graphics that defines mileage in 5-minute increments, I always get under 25 MPG in the first 5 minutes and then it goes up from there. Is that a clue as to what's wrong? Does that confirm its a Mass Air Flow sensor problem. Thanks!

 

it is a clue that mpg's suffer during the warm up stages of the engine.

 

I noticed above that one of the culprits in a decrease in FE is a change in rolling resistance. I noticed a drop of about 10%, high 40s to mid 40s instantaneously with a purchase of 4 new tires. Same size. Thought it was perhaps they were under inflated at first, but inflated back to same as before. Took back to tire dealer who said it couldn't be the tires. Is it possible a change in times could impact rolling resistance?

 

Sure can, if the new tires have a higher rolling resistance when compare to your previous tires.

Having said that, it doesn't make the new tires defective or returnable, unless you specifically asked and were told by your tire guy these new tires were low rolling resistance (LRR) tires. Even if they are marketed as LRR, in the US, the manufacturer determines relative RR between each of their tires and each manufacturer can have different criteria, so comparing brand to brand is very difficult.

The flip side is that LRR tire are more expensive than the others, and you can get tires that are not specifically marketed as LRR, but perform well.

Tire selection is a continuing challenge for the economy minded.

BTW, how did you determine the rolling resistance of your old and new tires? Just by the change in FE? If so, give it a few hundred miles to settle down. I wouldn't jump to conclusions on the basis of just one or two tanks. Plus, you really need to record your MPG in a spreadsheet or on a website like Fuelly.com to get meaningful statistics. Just using the MFD and your memory can play tricks on you.

 

Many thanks for the insight! I'm wondering in the future how to determine if tires recommended by the sales guy at the local NTB or Discount Tire have a LRR. They didn't seem to have any insight into tires making a difference in FE when I questioned them after the first two months on the new tires. It's been roughly 6-8 months now since the new tires with consistently lower mileage measured by total mileage on a tank divided by miles since last fill. The change was stark from the first nine years I drove my 2008 Prius. I think this is the 4th or so set of tires on my Prius with 145K miles now. Previous new tires did not have the same affect on FE.

If anyone has any insight into how to know what the rolling resistance is of a particular brand and model of tire is before you purchase that would be great for future. Cheers.

 

Any new set of tires, LRR or not, is going to negatively impact FE by a few mpg for the first 10k miles or so.

 

Determining if new tires rolling resistance is going to adversely effect your mpg requires some internet research; unfortunately there's no standardized testing in North America as yet. Also, manufacturers occasionally change the formulation of tire over time, effecting rolling resistance, one way or the other.

I have had new tires that had no apparent reduction in rolling resistance, specifically: going from Bridgestone Insignia SE200_02 to Bridgestone Ecopia EP20.

 

Here in Washington State I normally get about 43 to 45 mpg. However, I live in a costal town surrounded by 500ft hills. Any direction I go it up hill out of the area. When I'm in the Seattle area the Prius gets between 47 and 51mpg.

However, in the wintertime the MPG drops an average 3mpg. This winter we've been making fewer trips out of town so the Prius does not adquetly warm up. The MPG can drop to 36 to 35 MGP with all those cold low mileage trips. I'm looking forward to Spring when it warms up and the MPG climbs.

Also, here on the left coast the three Pacific States also change their fuel formulation during the winter. This adds up to a double whammy to lower fuel mileage.

I admit that when I see the MPG go that low I silently panic. I'll worry about it until I can take a longer trip out of town and make sure that on those trips the MPG is average for a warmed engine.

 

fuel economy also decreases because of the Fuel Blend for winter...Winter additives like kerosene, ethanol , and other substances will decrease fuel economy by 10% or more...winter blend usually starts usually around the end of October and summer blends kicks in around end of April into May...

 

Do they change the fuel blend for winter months even in south Texas (Houston) where I live? Still just trying to figure out how I've experienced such a significant drop in my 2008 Prius mileage after getting new tires and rolling into the winter months. Went from 45-47 MPG down to around 38 MPG. Pretty consistent in 3-4 tanks.

 

I've had one opposite experience with gas: filled up in a small town up the coast with Chevron regular, and NEVER had such good mpg, and easy. Maybe small towns are ethanol-free, I'm speculating: there was some mention of that when I researched a bit.

Now, back in the city, it's back to normal.

 

I always get 199.9, Winter, Summer, Day, night. How can the car do better?

 

200?

 

what has the temp been? have you checked the 12v, tire pressure, air filters, oil level and etc.?

 

Never check, everything works perfectly. Why annoy it?