Here's the full chart from Bob's link above:

[TABLE="|"]Make & model|55 mph|65 mph|75 mph
Acura TSX 2.4l 4-cyl|39.9 mpg|35.5 mpg|30.7 mpg
Honda Insight 1.3l 4-cyl|51.9|44.8|36.5
Lexus RX350 3.5l V6 |30.9|27.4|23.0
Mercury Mountaineer 4.6l V8 |23.8|21.2|17.8
Toyota Camry 2.5l 4-cyl|40.3|34.9|29.8
Toyota RAV4 2.5l 4-cyl|34.6|29.3|25.9
Toyota Yaris 1.5l 4-cyl|42.5|37.9|34.0[/TABLE]

What I find interesting is that the difference between 55-65mph is just about exactly 10% in all cases except for the Yaris which is a bit higher.

And the difference between 65mph and 75mph is about 15-20%.

This type of data should be published for all vehicles, along with cost per mile.

Nice find, Bob.

That is interesting

Dividing mph by mpg gives gallons per hour, the actual rate of fuel consumption.

[table="|"]Make & model|55 mph|65 mph|75 mph
Acura TSX 2.4l 4-cyl|1.378 gph|1.831 gph (+32.9%)|2.443 gph (+33.4%)
Honda Insight 1.3l 4-cyl|1.060|1.451 (+36.8%)|2.055 (+41.6%)
Lexus RX350 3.5l V6 |1.780|2.372 (+33.3%)|3.261 (+37.5%)
Mercury Mountaineer 4.6l V8|2.311|3.066 (+32.7%)|4.213 (+37.4%)
Toyota Camry 2.5l 4-cyl|1.365|1.862 (+36.5%)|2.517 (+35.1%)
Toyota RAV4 2.5l 4-cyl|1.590|2.218 (+39.6%)|2.896 (+30.5%)
Toyota Yaris 1.5l 4-cyl|1.294|1.715 (+32.5%)|2.206 (+28.6%)[/table]

At these speeds the car will be in the highest gear it has - for the Insight CVT, at the highest ratio that the band can achieve - so the change in fuel consumption is only dependent on the power demand to overcome additional resistance.

The drag equation, for power to overcome drag, shows that the power needed is proportional to the cube of the velocity. The other terms in the equation are the density of the air, the cross-sectional area of the car, and the drag coefficient. The mass of the car doesn't come into it.

65mph is an 18% increase in speed over 55mph, 75mph is a further 15% increase. That would mean 65% more power required from 55 to 65, and 53% more power from 65 to 75. While drag dominates, you also have to account for rolling resistance, drivetrain and ancillary losses. You would expect the change in consumption to be lower from 65 to 75 than from 55 to 65, though, which it is on the Camry, RAV4 and Yaris, but not on the Acura, Insight, RX and Mountaineer.

Tesla posted an interesting power versus speed and range chart for their Roadster, showing the other losses for that car. There's a link to download the data at the end of the page.

Hi,

I added the Consumer Reports mph vs MPG data to the collected Prius model data:

Notice that vehicles with poor MPG in the 70-75 mph range also get poor EPA ratings, especially in the highway leg.

Now the following head-to-head mileage tests make sense:



The one thing I take is vehicles with higher BSFC at higher power settings do well in the highway tests, especially if that shows up in the 70-75 mph region and that include the diesels. It is in the city performance that diesels fall down.

The other thing is that knowing these mph vs MPG characteristics, individual drivers can then tailor their driving to achieve excellent mileage:


There is a reason why I'm running just over 52 MPG in my 2003 Prius is I drive per the mph vs MPG curve. I typically do this by route planning and on the highway I follow, at a safe distance, interstate truckers who are also trying to minimize their fuel burn, an expense. From my experiments, getting 90% of a vehicle's performance without adding to the driver burden is a good thing. For non-hybrid vehicles, they have no alternative.

Bob Wilson

Great info as usual Bob. So why not slow the nation down to 55 mph again and ruin a few middle eastern pipe dreams? The "Buck" no longer stops in the oval office, sad.

Its interesting in the MPG vs. MPH graph that the Gen I/II Prius don't change MPG much between 55 MPH and 65 MPH (only 1 MPG). That doesn't seem right to me.

Great Post Bob! Thanks so much.

Now when you see people complaining they are not getting expected mileage numbers but are driving at 75 and 80, well I think the graphical presentations speak for themselves.

One thing is that the Prius does fair a little better on the highway I think with overall MPG numbers if your taking it at 55-60.

Thoughts?

My observation is that when I dropped my freeway speed from 65 to 60 mph, I achieved an between 5 and 10 % increase in the MPG, or between 2 and 5 mpg.

Background...

The Gen 1 and Gen 2 Prius' were designed around the 'typical' driver... a JDM driver in a JDM situation.

When the 2004/05 Gen 2 burst onto the international scene most of the sales were to NA drivers. US. Our driving characteristics were significantly different than the JDM 'typical' driver. We pushed the hell out of the 1.5L ICE's at speeds above 70 mph on our Interstates. The 1.5L's do well but they are full out above 70 mph.

The new 1.8L ICE is more muscular and works easier at the higher speeds of our Interstates. Thus there is not so much of a drop off in FE above 70 mph.

Excellent post. Nice!

One thing to keep in mind, last I heard, Japanese cars have a warning buzzer in them that goes off above 100 km/hr and is very annoying. (confirm or correct please Ken). So while we North American drivers can blast along at -whatever- speed, in Japan they won't, not because of the possibility of tickets (though that is there), because it's annoying.

great info all. we really need to toss a few V-8's into the mix. so far this chart completely blows the myth that stronger engines dont suffer mileage hits at higher speeds.

i cant tell you how many people to talk to at work who claim their mileage GOES UP when doing 70 verses 60 mph. they all claim its because that is what their engine is tuned to drive at

P.S... most of the people referenced, drive 4 by 4's and get 14 mpg and are the first to complain about the price of gas!!

60MPH = 60MPG in a Prius.

You are correct our cars had to have 100km/h warning buzzer until 1986.
Foreign car manufacturers claimed it's a stupid law, then the warning buzzer was gone since then.
It's old story.

Ken@Japan

We had a 1981 Chevette, perhaps one of the worst cars ever made, the whole thing buzzed above 112 km/hr, which would wake me up and I would tell my wife to slow down!

Bob Wilson

I take issue with Consumer Reports figures for the Yaris. Or, at least, something is going on here. I just took my 2008 Yaris automatic sedan on a 393.5 mile round trip at 65mph, over slightly hilly ground with negligible wind, on 8.97 gal measured very carefully (with a lot of bouncing and retopping both times). Air off. Windows up. That works out to 43.9mpg@65mph. From experience, that's about typical for this car at that speed. (Note that the EPA numbers of 29/35 for this car are regarded as a joke by most owners I know. Yaris is a surprising little contender.)

I'm wondering how CS got 37.9mpg? The 5 speed manual generally does slightly better than the automatic. And the only engine available is the 1.5L vvt-i. Cd is 0.29 for both sedan and hatchback.

bring back 55 mph!

I can't drive 55!

But if we can't get the automakers building efficient cars, that's where we'll have to go during the next energy crisis.