Advantage...Prius

good observation, thanks! it would be very revealing to see high speed mpg graphs of hybrids vs gassers.

 

Bob should be along shortly, with a graph to shatter your thesis, lol. Just for warm up: there's no free lunch; mileage drops dramatically as speeds go up. You'll do a lot better on secondary highways, say around 80 kmh (50 mph).

 

but it's a comparison with gassers, not against itself over varying speeds.

unfortunately, without a prius gasser, there isn't a perfect comparison. maybe camry vs hycam.

 

The difference in mpg, for us, between 60mph and 80mph is minimal.

The same goes for having one passenger, vs four and all their luggage. Even holding it at 80mph there is not a huge difference in mpg.

 

Bob's thesis-shattering graph is right here:
Updated MPG vs MPH chart | PriusChat

Uncle Wayne over at CleanMPG.com has very similar graphs. Here is one for the 17" wheel version:
2014 Toyota Prius Package 5 Review | CleanMPG

 

I have numbers very similar to Wayne's post. I know I get around 50-55mpg going 60mph with many ups and downs. Then somewhere in the low to mid-40's mpg when I go a long road trip going around 70mph.

 

What is harder to quantify is the engine 'operating line' and Brake Specific Fuel Consumption (BSFC). On a constant-speed, flat, no-wind, 65-75F day, the Gen-3 engine will easily generate the power needed to cruise at high speeds. We have measured that as the engine rpm increases for more power, the EGR valve setting increases. The exhaust gas cools the combustion temperature allowing a leaner, more efficient engine. But the EGR valve opening peaks at 3,200 rpm.

In our Gen-3 Prius above 3,200 rpm the car moves towards a different strategy to avoid excessive exhaust gas heat . . . fuel enrichment. One of the Gen-4 features is an increased EGR capacity. It is very likely that this provides a significant part of the 40%, record breaking Gen-4 engine efficiency. It would also slightly reduce the peak HP which is found in the Gen-4 specs.

As for the OP's original statement, you are absolutely correct that the limited gearing of ordinary cars means the engine is always mismatched to the load. Worse, even the automatic transmissions have been poor at shifting to gears to maximize BSFC at peak efficiency. So they spin the engine faster and partially close the throttle plate so the engine is working against itself. They just can not compete.

Bob Wilson

 

The ramifications of EGR go completely over my head, but I do recall in the initial Toyota press releases they kept mentioning it, implying it mattered.

 

• dilutes fuel-air - with less oxygen, the flame burns cooler
• cooler flame - reduces NOx and protects the catalytic converter from heat damage
  
• allows leaner fuel-air mixture - less fuel burned, more efficient engine
• throttle can be opened more - the engine breathes easier without 'pumping losses' (i.e., think cocktail straw effect)
• slightly reduces peak HP - so the engine has lost a few but not so many affect performance

Sometimes I'm not very clear about why it is important. Perhaps Google might help find some better descriptions.

Bob Wilson

 

yes, unless you consider CVT.

why would that be? in my case mpg dramatically dives over 75 mph.

yes, this is exactly what toyota said (without hedging).

 

The difference in mpg, for us, between 60mph and 80mph is minimal.
The same goes for having one passenger, vs four and all their luggage. Even holding it at 80mph there is not a huge difference in mpg.

That is not the case for my 2010 - nor was it that way for my 2005. A difference of 20 mph always made at least a 5 mpg difference.

 

One trip does not make a trend. For example, the OP may have traveled in favorable weather conditions on both legs; especially the northern outbound leg could have had a tailwind the whole way from the traditional gulf breezes.

 

80 mph, roughly 130 kmh? Oy veh: I get antsy if the posted limit is 100, tend to stay right and around 90, 80 if no one's looking.

 

I've found that the length of the trip confounds MPG measurements, as MPG gradually increases over time as the transmission, tires, etc, all warm up which reduces pumping losses, rolling resistance, etc. It can take an hour of driving to reach peak efficiency depending on initial conditions and the speed you drive.

For example, I was experimenting with my ScanGauge on a recent 3 hour trip in our F-150. At the start, we were getting 18-ish MPG doing about 60mph (more traffic, lower speed limits), then 17-18 doing 70-75 (less traffic, higher speed limits) during part of the second hour, and finally 20-21 with the cruise control set at 63 in the last hour.

I've found the Prius to have an even more dramatic MPG drop with speed (relatively speaking). On the other hand, it can probably do 80+ and still get double the mileage of the F-150.

 

All true, except the part about pumping losses. They should remain very nearly constant with CVT, and increase with fixed gearing---although obviously not enough to overcome the other effects of warmup.

 

Oops, confusing choice of terms. I was referring to the losses pumping fluids, particularly those outside the engine where they don't warm up as fast (transmission, etc). Not to be confused with the loss due to pumping air through the engine, which is the canonical pumping loss. Way back when I had a manual transmission that actually used gear oil there was on obvious difference in shifting between starting out on a cold morning and after a couple hours on the highway.

Which reminds me of the time I tried topping off that car's engine with 10w-40 on a cold winter morning on vacation in NH. Turned the bottle upside down and nothing came out until I gave it a squeeze and that extruded a nice cylinder shaped piece of near-solid oil, sort of like dispensing toothpaste.

 

That's funny! The engine must have had metal grinding on metal through the warm up period.