Is 50 mpg at 65mph normal?

I drive 79mph on highway all the time. I have 46MPG with Costco gas. The dashboard reading is ~50, 51.

 

See #207 of "Prius Prime Plus in my hands" by Mr. Bob Willson. Temp, altitude, terrain, tire pressure, cargo weight, etc may influence MPG.

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This is my model and validation data points:

- The curve is generated from the EPA roll-down coefficients and 40% engine efficiency
- The Prius Prime Plus had fully inflated tires (max sidewall), engine oil, and transaxle oil change
- Temperatures were 59-62F, low to no wind
- Two runs, same route, opposite directions, 10 miles each way, summed fuel and distance for both, excludes acceleration
- Speed cruise control managed, ~1.6% error compared to GPS metrics
- Outlier, 35 mph: did not have a full, 20 minutes of warm-up; coolest temperatures and; was a loop
- On-curve, 35 mph: bi-directional in afternoon after doing hill climb and descent run

Bob Wilson

 

The EPA does not run the test at 65 mph

 

That looks about right for that speed. To get an idea of how strong air is, hop on a motorcycle & ride along at 50 mph. Get a feel for the force of the air. Then crank it up to 70 and feel the difference.

 

The EPA highway test is based on rural roads. The top speed is 60mph, but the average is 48.3mph. Then acceleration on it and the city cycle is very gentle.

With speeds faster on a freeway, you will get lower than the window sticker on them.

Detailed Test Information

 

Bob's graph is very telling: in the middle, pretty much a 1:1 inverse relation, between mph and mpg.

 

Clever, I hadn't notice (missed the forest because of these fascinating trees.) But it sets up a 'rule of thumb':

• 120 = MPG + mph :: 35-70 mph, +/- 5 mph or 5 MPG

So within the 35-70 mph range, we can say:

• MPG = 120 - mph
  
• mph = 120 - MPG

So who is going to win the first bet with this bit of math nonsense?

Bob Wilson

 

Is there a specific tire pressure on the 15" tires that a person should keep to get the best MPG? I know the manual but has anybody tinkered with tire pressure to find optimal performance?

 

The equation goes well in the middle, not so good at the extremes where the graph flattens, and even reverses. But really hits home: going faster costs.

 

I get 48 mpg at 75 mph.

winter tires gave me 44 mpg at 75 mph

From Portland to Seattle and back.

 

I wonder about the Tire Pressure as well. Just got my first oil change today, and they recommended 36 for FRONT and 35 for REAR. What do you guys think? The interesting thing is that my tires were at 48/50 and the max is 44. I have no idea why it would be that high. I have not touch them since I got the car on 11/11/16.

 

If you checked them hot, they will read high. Quite a few of us run the tires higher than recommended -- some even at max sidewall pressure. My max is 49 or 50, iirc. I run mine at 47/45. But dealers will often take it upon themselves to set your tires at the pressure on the door panel regardless of your wishes. Recommended pressure will be a little softer and quieter than where I run mine.

 

They leave the factory at high pressure, to avoid flat-spotting before sale. Dealership is SUPPOSED to lower to spec during Pre Delivery Inspection.

 

there are no tyre pressure tests that i know of, any info here seems to be guesswork at best.

 

is this hv mpg we're talking about? phev mpg? or gas only mpg?

 

I've heard of the tires being pumped up before loading on the ship to allow for pressure loss during transit. If true, perhaps your dealer hadn't adjusted them for delivery.

For fuel efficiency, higher tire pressure is better. The recommended levels for the manufacturer err on ride comfort, and they also apply to the tires that came with the car.

 

Remember that cold air in the winter is denser that warmer air in the summer, which increase aerodynamic drag at highway speeds, lowering your MPG a little bit.

 

I got over 52 mpg driving between 70 and 80mph from San Diego to Placerville, California. 0 percent EV charge to begin. From 8:30pm to 4:20am, mid 60s to low 50s temperature. I stopped to eat once and later to get gas between Stockton and Sacramento. Roughly a 550 mile drive. Gassed up at approximately 480 miles with 9.2 gallons. This was on the day I bought the car mid December.

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