any advice?

This is a normal observation. MPG is highest during the warmest weather you can stand to keep the AC off. Some have devised a mods that keep your car warmer by blocking off part of the radiator. You must be careful not to go overboard or you will overheat. If you can stand it drive without using heat. Also recheck your tire pressure PV=nRT and all that rot.....

Install a block engine heater. Less fuel is needed to get her warmed-up

We don't have that problem in the deep south. When it initially gets cooler our mpg improves. This will be our first fall/winter with the Prius. It got 42mpg and change in late April in heavy city/short trip driving.In the full heat of summer it dropped to 40- 41 and change with one 39mpg tank.The most recent tank-today-was 241.8 4.85 gals-very close to 50 mpg.I think I "shorted" the fillup by stopping just after the 1st "stop" instead of pushing about another .3-.4 gals in like I usually do.Even factoring in another .4 gal, the tank would still be 46mpg. I know it isn't a great idea to overfill, but in hurricane I tend to want as much range as possible.
I'll keep good records and see if our peak cool season actually gives worse mpg than our peak heat. I'm betting it won't, but we will see.
Thanks,
Charlie

This is only true if you use AC.....If you can live without AC the hotter the better....

Winter formula gas has less energy per gallon than summer formula gas. You will notice up to a few MPG drop just due to winter gas. All gas brands change over to winter gas in the cold climates in the US around the first of October.



Cold startup is a major cause of lower MPG. Even in the heat of summer, short trips will reduce your mileage since the engine spends the first 5-10 minutes reaching operating temperature and thus will not shut off as readily when the vehicle is stopped. In cold weather, the engine takes even longer to warm up further increasing fuel consumption.



Your engine will run more when stopped in cold weather even when the engine is fully warmed up to keep the passenger compartment heater adequately supplied. This burns more fuel.



I installed an engine block heater and will be recording its effect on mileage this winter. The block heater will warm up the engine for my morning commute to the office. A timer will turn it on for 3 hours prior to leaving in the morning. The block heater will also make driving more comfortable as the interior will warm up much faster. The only pain is that I have to plug the car in every day.

LakeDude,
Yeah, if you can get by with no AC use it will improve your mpg, but I would wind up in divorce court, or maybe involuntarily committed.
Our Prius is black with a black dashboard, and lots of black interior trim. Now black is a nice color, but it is never my 1st choice. If I have a choice, I always choose white because of the bright sun/heat here(suburb of NO).I wanted a Prius with airbag pk; the price was right, so black was it. It looks sharp, but it can be very, very hot when we just get in. I measured the interior temp on a 85 degree day-111 degrees inside the car(the thermometer wasn't in direct sunlight). I'm not sure how much the black color has to do with it; the Prius has a lot of window area for such a small car.
The AC certainly hurts the MPG. After the Prius is warmed up I have done short stretches with and without AC-it is at least 2 mpg at "normal" hy speeds(60-70 mph). If I sit in a line-bank-I can see the trip mpg drop from 45 to 39 over just a few minutes if I have the AC on.
I'm guessing that it takes~at least 1 hp-750 wattHr to run a car AC for an hour(same as a 7500 btu window unit-10 EER).I think that is equivalent to 5-6 EV miles with the Prius at 30 mph.
My wife and son use the AC 11 months a year.
Luck,
Charlie

I hear you Charlie, I only mentioned it so folks would know it was AC not the hotter temps directly that caused the drop in mpg.

I hear you on the black car as well, my Vette is black and non-AC hypermileing is awful in hot temps. OTHO the HCH is silver and my hair is usually wet on the way to work so hypermileing in the HCH in the heat is not so bad.

A black interior with a large windshield makes for a perfect oven. You could benefit from the use of one of those sun reflectors behind the windshield when parked.

I have taken the hit also when it comes to A/C use. You can defray some of the hit, if you can wait to turn the A/C on until you are cruising at a speed where the ICE is continually on (i.e. getting on the highway). The HSD does a good job of converting excess ICE power into battery power.

LakeDude,
You have me wondering about how tricky it would be to attempt to transfer the heat from the cars surface and interior to the engine bay?
Like I said, this is our 1st black car; I'm not sure just how much difference a shiny black surface and a dull partially black interior make.We have about 60 or more days a year when the temp is in the mid 50's or less in the am-when the car is first started for the day.From past experience, I know that the interior of cars with lots of glass gets much warmer than the outside air on bright clear days.It will easily be 20 degrees above the outside temp after 2 hours of morning sun(say 10am).
If some of that interior heat could be siphoned to the engine bay then that cold chunk of aluminum would start with a 20 degree advantage. It might earn 10-20 seconds earlier engine shutdown, and a leaner mix until it shuts down.
I wonder if there is any easy(relatively easy) way to expose the heater core to the interior of the car?Sort of run the heater backwards, and use the warmed coolant to heat the block. Set it on a timer to kick on about 30 minutes before you plan to start the vehicle.
The core isn't ideally placed of course-too low-the hottest air will be high.You also don't want to waste a lot of energy pumping coolant around, or using a fan to draw warm air to the heater core.
Too bad there isn't an easy way to weld a radiator to the underside of the black hood.You could run the coolant thru it 30 minutes prior to starting up(better use rubber tubing with lots of slack).You would have to hook it up so it didn't operate at night-it would cool the coolant on a windy night.
Oh well.
Thanks,
Charlie