Slow Warm Up?

welcome!

are you tracking your fill ups manually?

 

Check the vent behind the drivers seat. I've seen this a lot where a bag or blanket is covering the vent hole.

 

imo, there are too many variables in short term mpg's on the dash. there's no substitute for tracking fill ups for finding mpg inconsistencies.
easier than a wild goose chase to find what is wrong, when nothing may be wrong.

 

Agreed, though... a 33% drop isn't something that normally happens because of weather or driving conditions... usually. If he's sticking to the same route and driving mostly the same, I could see a drop of 10 mpgs at most. Going from 50 mpgs to 35 mpgs is a lot. Especially if it's not cold.

 

My guess is too much ethanol in the fuel he's purchasing. We have an issue with "Circle K" here in Indiana blending more than the normal 10%. Not all the time, but at least 3 times a year. My son and brother will both see cars pulling into the dealership in droves complaining of poor mileage, hard starts etc.

I'd try a different brand of gasoline just as a test. My not be it, but it's possible.

 

A decent headwind can make that kind of difference, especially if it's compared against driving the same route in the opposite direction with an equivalent tailwind. And especially if it's driven at a high speed. Aerodynamic drag at high speeds makes a huge difference in fuel economy. Winds are often not at all obvious when you're driving except for their impact on economy. Whenever I see that "l/100km" display doing something I don't expect the first thing I do is look for flags or trees to see if there's a wind blowing.

 

50 mpg to 30 mpg because of wind speed and drag would be a terrible design... nobody would ever drive over 55 in the PC if that were the case.

 

Well, presumably the kind of person who would buy a Prius is the kind of person who places a value on fuel economy. If so, then they should be aware of how high speeds hurt that fuel economy. Air resistance increases as the square of the velocity, so it takes almost twice as much energy to plow through the air at 70mph as it does at 50mph. Which is exactly what you get if you drive at 50mph with a 20mph headwind. And it's very similar if you drive at 50mph with a 10mph headwind and compare it with the same drive in the opposite direction with a 10mph tailwind.

 

I've driven in some seriously adverse weather before and I've never seen 30mpgs. In the high 30s, sure... but not lower than 35.

 

Both wild guesses are totally without merit.

A small percentage change in the ethanol content will have no noticeable impact on engine temperature.
And the A/C settings have no impact on the engine temp either.......but cooling outside air this time of year DOES make the A/C work a LOT harder, which would make the engine warm up faster if any impact at all.

The combination of the A/C setting plus the blocked vent probably accounts for your fuel mileage change.......but not because of engine temperatures.