MPG Cruising on Interstates at 75, 70, 65?

Around western Pa I'm getting between 62-65 mpg (ScanGauge) at 55 mph.

Sorry I couldn't help more but my Prius hasn't been over 60 mph.

 

Find the graph that plots MPG and MPH. It will give you an idea. Basically it's a straight downhill slide, the faster you go the less your mileage

 

I commute 45 miles each way to work and 95% of it is highway at 70+ miles an hour. It is relatively flat with a few minor hills, but on average I maintain 50-54 MPG. Now that it is a bit cooler, I am averaging closer to 50 MPG.

 

On my return trip from over the river and through the woods last night, I stopped to refuel and reset the Trip A. Over the ensuing 110 or so miles, according to the trip computer, I averaged 58 mph and 49.6 mpg. This was running brand new Continental Extreme WinterContact tires, an OAT ca. 32°F and with about 2100 miles on the engine. Overall, not a bad performance in my book. Not bad at all.

 

Flat, warm Florida - Real highway numbers
MPH MPG
65 -- 50
70 -- 48
75 -- 46
The more local driving per tank, the better the numbers get, but never better than 53MPG.

 

Drove from NC to Illinois yesterday, probably 70 - 75 mh most of the way. Grill totally blocked on bottom and half on top. Car had 3 occupants, luggage, Christmas gifts, 2 cases of ginger ale, new Michelin Xice i2 tires on 15" wheels (down from stock 17s). Temps started off in 50s and dropped down to high 30s along the way. Speedometer read 1.5 - 2 mph high. 1st tankful purchased in Tenn. had ethanol. I'm pretty sure 2nd tank, purchased in Indiana was 100% gasoline.

Calculated average of 2 tanks was 38.62 mpg.

I've never had such bad mileage. What in the world could have happened?

 

You've already answered the question:

• NC to Illinois - yesterday the winds were blowing to the east in excess of 15-20 mph. Your airspeed, the major drag at highway speeds, was probably in the range of 85-95 mph. (Yes, I look at the wind maps when I check the weather.)
• new tires - often have initially higher rolling drag and the larger diameter (rubber hasn't worn down) could lead to errors in indicated speed. New tires need to have the distance and speed calibration checked.
• cold air is denser - this increases the drag significantly
• ethanol - has an effect and then we don't know what you started with.

I would not worry about it. I've gotten 39 MPG at 80 mph with the defroster left on by accident.

Bob Wilson

 

Just did a trip to from LA to PHX and back. Speed definitely effects the mileage. Once I pushed up above 70 I'd see a pretty big drop. 50+mpg under 70, if I drove over 70 I'd find myself in the mid 40s.

 

About 48mpg (45 calculated) driving on left lane between 70-85 mph with rolling hills and trying to keep up with the traffic on Hwy 280 between SJC & SFO. If I take Hwy 101 with some traffic driving between some stop & go and some 70 mph area, I get about 51+mpg (about 48 calculated).

 

I think the wind resistance is proportional to the square of the speed. Or something like that. So there is probably significantly more wind resistance difference between driving 75 vs 65. The difference between 45 vs. 55 is probably minimal compared to the difference between 65 to 75.

 

The force of wind resistance is proportional to the square of the speed. The power required to overcome the resistance is proportional to the cube of the speed.





(source: Drag (physics) - Wikipedia, the free encyclopedia)

My reckoning is that on Gen 2, the power needed to equal wind resistance at 75mph is 15kW, while at 65 it's only 10kW and at 55, 6kW. Gen 3 is a little lower - maybe 14.5kW at 75 - because of improved aerodynamics but I don't have a figure for drag area (A in the above equations). Gen 3 is 20mm wider than Gen 2 but has an improved Cd of 0.25 vs 0.26 (though these figures may be rounded to nearest two decimal places, and it could be closer than it appears).

 

Your numbers look low. Power to over come drag is the cube of of velocity (adjusted for wind). I'm sure its more than 15kw at 75 mph, you also need to add power to overcome rolling resistance the square of velocity, and hvac and other power Your mpg is inversely proportional to the square of velocity - drag force - and rolling resitance which is propertional to velocity. IIRC drag is about half the power at 40 mph, which will make it over 80% of the power at 80mph. Does anyone know power at 60 mph, it should be around 13 kw?

One other trick, the prius spedometer reads a little low.

 

I think that is right. In perfect conditions I would estimate 13-18hp to cruise at 60mph. I know that is a large range but the values change quickly as terrain changes so it is hard to peg one specific number.

 

Last April, I drove my 2010 Toyota Prius in the rain all day from 70mph to 80 mph, eastbound along I-70 from Missouri to Maryland. My tire pressure was 38 psi front and 36 psi rear. I was getting about 44 mpg calculated ( the MFD was saying I was getting about 48 mpg) so your 45mpg at 75 mph sounds about right. When my tire pressures is a bit higher at 40 psi front/ 38 psi rear and with the cruise control set between 52 to 55 mph running along the Oh and PA turnpike in the spring time - I've been able to get about 60 mpg calculated ( MFD = 65 mpg). While Driving with load is suppose to get better fuel efficiency than the cruise control on the superhighway - I find it hard to do at high speeds.

 

I watched the HP on SGII today and it seemed to sit at 13-14HP when I thought I was on flat terrain but would jump to 15HP or even more with the CC on. This was at 60mpg. At 55mph it was pretty steady at 12-13HP.

 

Something to think about:
(numbers rounded obv)

100 mile trip, $4.00 per gallon

Person A drives it at 70 mph (50 mpg)
Person B drives it at 60 mph (60 mpg)

Takes A 1.43 hours and 2 gallons at a cost of $8.00
Takes B 1.67 hours and 1.67 gallons at a cost of $6.68

Person A gets there 14 minutes before Person B at a cost of $1.32.
Person B gets paid $5.50 per hour to sit and listen to music.

Some people will argue that most people have a cost of time threshold and can make far more doing something worthwhile in their job or other parts of their life, and that has some merit. However, I would argue that there is only a very tiny fraction of our society that never wastes any time throughout their day. I think if most people were realistic about their lives they would admit they waste many hours every day.

 

When I drive from Los Angeles to Sacramento, about 400 miles, I'll try to average about 80 MPH, which would take about 5 hours, and I get real bad MPG (about 35 MPG in real cold weather). If I averaged 60 MPH, it would take me about 6.66 hours. My biggest issue of saving the 1.66 hours of drive time is not about wasting time. It's that my mind isn't quite as wasted in those final hours, so I'm a more alert driver. However, with the 100 miles you state, the time isn't quite as different, so your point is well taken.

Mike