What mileage could you achieve on this commute?

43.8 for a touring going 72mph cruise control is respectable. Drop the speed by 5mph and your mpg will probably get to 45mpg.

370mi/week is 74mi/day assuming a 5 day workweek, and 37mi each way. At 72mph, that is 31 minutes. At 65mph, that is 34 minutes.

 

drop your speed by 7 and you'll get closer to that 45 as 2k1toaster said... now.. drop a little more than 7mph, and you could get higher... when i towed my trailer around the s.f. bay area doing 55 to 60, i was getting about 50mpg.. which is way above what i usually get... speed is the key.

you're getting good mpg for the speed you hold. i haven't been babying my car lately. a lot of short trips... keeping up with faster (not fastest) cars on the highway, and jackrabbit starts... i'm getting slightly below 40mpg... at times it's 40 to 45... if i baby it i can get more.. but.. shrug.. i like torque

 

You will start to see a slight increase as the weather warms. One contributor is that for short commutes, the engine warms up faster. You don't really apply with this. The factor you will see is that summer fuel blends have more energy per volume then winter blends, so you get more energy per gallon.

There is a graph around here someplace that shows the effect of speed versus miles per gallon. Basically 55mph is the sweet spot. You lose a couple average mpg for every 5mph faster you go.

This is for a standard 30mpg car:

Definitely help. You wont "save" money by buying two sets of tires, but you will increase your average mpg.

 

Conditions permit, you just have to slow down.

If you had a tank, then maybe. You have a flexible fuel bladder that expands and contracts with temperature, pressure, mood, whatever.

 

It will improve with the weather, but likely by a smaller proportion than commuters going shorter distances at lower speeds. This graph shows my seasonal variation, but I average half your commute distance at half your average speed, in a newer generation car.

I see a quadruple whammy in those tires. Non-LRR costs fuel. 205 width costs fuel (see F8L's frequent posts on this as he swaps between wide and narrow, depending on his mood.)

And since both sizes are the same 55% aspect ratio, the 205s will have a larger rolling diameter, requiring adjustment of both the speedometer and odometer measurements. Without adjustment, the 205s will cause you to drive faster than you think, costing extra fuel, and produce a lower odometer reading, producing an artificially low mpg calculation.

But remember that Prius is not a standard car. Bob Wilson's mpg vs speed graphs show a much different shape. Prius has no 'sweet spot' or 30-55 mph plateau.

That chart shape isn't even right for the manual transmissions in my household.

Lower speeds, other different conditions, plenty of hypermiling skills, and numerous Gen3s in the mix.

It does hold more than that -- usually. But the Gen2 bladder is infamous for inconsistencies, and you probably don't want to get stranded by the problems and short tanks it is known to produce. I'd have no qualms about going a whole week on a no-bladder Gen3.

 

I see two things that haven't been mentioned (or maybe I missed it):
74'F is a nice cozy temperature, fine if you're padding around the house in your slippers and bathrobe, but in a car (assuming you're wearing a jacket and good shoes for the weather), you could probably keep it at 68-70'F and still be comfortable. Heating up the car when it's cold outside takes a hit on your first several miles (or until you get on the freeway, whichever comes first), since the car can't turn off at lights or when coasting. I have a short commute so the effect is magnified for me, I'm not sure how much of a difference you'll see, but it will be something.

You use cruise control in rolling hills. If you're serious about fuel economy, you can do better by driving it yourself. Slow down a bit on the last bit of the incline, and recover your speed on the drop. That can give you at least another 1 mpg (more or less, depending on what your hills really are like, I know Washington state has a lot of variation in terrain).

But in the end, you're not far from your goal. 43.8 for the weather and conditions is reasonable, you'll easily get 45 when it's 60'F outside.

 

One thing about the Prius, it has a humidity sensor in the cabin. I'm not sure exactly how this affects things, but my guess is that it runs the A/C less if the air is already dryer if it's not needed to keep the interior cool. I know when I run the climate control in auto with A/C on in winter, it barely runs at all (but it does run a tiny bit here and there). As the weather warms up, it runs more. The Prius A/C is electric, and it's also a special type of compressor based on a variable speed industrial compressor design. This allows the car to run the pump at only the intervals and speeds that are actually required to meet your temperature demands, which is completely unlike most cars. I've tried running with A/C on and off this winter in my '08 touring, and I've never seen any difference in mileage from it. In the summer there is a small to significant difference.

When I've had similar driving conditions here in CO to what you describe, my mileage is right in line with what you're getting in yours. It looks perfectly normal to me-- I've had my car since August and have put about 16,000 miles on it so far. I deliver pizza in it 5 days a week, so I have tons of experience with it now.

When I drove it from CO to AZ shortly after buying it and the temperatures were still 50+, and I was driving at interstate speeds with cruise set at 69, 74, or 79 MPH, I averaged between about 44 and 48 MPG on every tank (about 2,000 miles), so you can probably expect to hit around 45-48 MPG without altering your driving style at all come warmer weather. Warmer weather also gave me more consistent fill-ups because the bladder seemed to have less variation than in colder weather. You may yet be able to reach your 1 tank per week goal.

That said, if you can adjust your commute and driving habits just a bit, learn how to maximize pulse & glide over your commute, and drop your top speed down, you should be able to get into the 50s pretty easily. If I stay below 60 MPH in warm weather, I average 52-58 MPG doing those things alone. But only if the temp is warmer and I avoid things like cruise control on hills, too much highway merging, too much passing, not keeping an eye on my consumption screen etc.

One thing that I've had to adapt myself to, I'm used to driving by ear but you can't do that with a Prius and get your best mileage. I've noticed that under many circumstances, I can let my foot off the gas pedal quite a bit (under light to light-moderate acceleration) without negatively affecting my acceleration noticeably. The engine revs lower, but my rate of acceleration changes very little in some cases-- that helps increase my mileage without a sacrifice. It takes time to develop that "butt feel" of when letting off the gas helps vs. when it hurts mileage. It's taken me around 15,000 miles in 6 months to really start to get the feel. And it doesn't work that way all the time, only in specific circumstances.

You can start practicing that by watching your instant MPG on the consumption screen-- try to aim the instant MPG to between 1/2 to 1 MPG per MPH (so MPG between 25 and 50 MPG while accelerating at 50 MPH, 20-40 MPG accelerating at 40 MPH etc.). Keeping your MPG within that range while accelerating will give you conditions where you can develop that butt feel for when you can let off the gas a bit without lowering your acceleration rate and improve your mileage. For me, I can add an extra 4-7 MPG by doing that-- at least until I lose most of it again by getting ticked and passing someone or trying to merge into heavy highway traffic.