Disappointing hwy MPG

At 73mph our Prius never gets 47mpg. Add wind. Add incline. ~40mpg sounds about right.

 

Take into the dealer and see if that is the case.




Merged.

That is about what i get.

 

I was in fl for a few weeks, just got back, honestly, once you get near the state the mpg's drop, I honestly think it's humidity and heat, the hybrid battery just goes into stay alive mode, if... the state is using e85, and boy, what a state for hiding the truth (chits, reality, global warming) it wouldn't surprise me, but I'm still going with heat.

 

I don't think most people appreciate just how much high speeds hurt fuel economy. The force required to push your car through the air increases with the square of the speed. So doubling your speed doesn't just double the air resistance, it actually increases it by four times.

Going from 60MPH to 73MPH takes almost 50% more power to push your car through the air. (73 squared divided by 60 squared = 1.48, which means 48% more power). Therefore you should expect a significant fuel economy penalty at the higher speed.

 

I like reality more, Toyota made everyone aware of the mpg figures, what's the problem?

Aerodynamic Drag:
Aerodynamic drag is a force in the opposite direction of the vehicles forward movement. You need enough power and torque to overcome this force and move the air around the vehicle. At low speeds drag is not a major concern, but as the speed increases, the aerodynamic drag will increase with the square of the speed (speed2). This means, in order to double the speed you will increase the drag force by four times.
If your drag force is 70 lbs at 50 mph, then the force at 100mph is 280 lbs. At 200 mph the drag force would be 1120 lbs, and so on.

Horsepower is affected by the speed as well, but HP = Drag force x velocity. This means as the speed increases the Horsepower required is now the cube of the speed. Every time the speed doubles the HP required to overcome the drag forces is increased by 8 times!!
If it requires 15 HP to overcome the drag forces at 50 mph, then it would take 120 HP to overcome the drag force at 100 mph. To reach 200 mph you would need 960 HP. 300mph would require 2700HP!! This is why you can add 50, even 100HP to your engine and not see a big gain in top speed.

 

@75 mph ours tends to stay around the 45mpg mark

 

2014 c 1, eco off on highway, no cruise, 200 mile round trip to our lake cabin, a few times a month

 

First of all, there's actually over a third more drag at 63 than 73MPH. (73*73) / (63 *63) = more than 1.34, which is 34% more drag. It increases with the square of the speed, remember?

Given that increase in drag alone, without considering any other factors, if your fuel economy is 53MPG at 63MPH then at 73MPH it should drop to (53 / 1.34) = about 39.5 MPG. That's pretty close to what you're getting, so I really don't see any big mystery there. The 1.5MPG difference is, at most, a minor mystery that could so easily be explained by other factors.

High speeds are really, really bad for fuel economy. It's just a fact of life.

 

I'm a bit skeptical of the 25% figure because aerodynamic drag is so much larger an influence than the other factors such as rolling resistance. The calculator you linked to does say it only provides an "indication of aerodynamic and rolling resistance" - so it shouldn't be treated as gospel.

But - even at 25% you're still talking about dropping to around 42MPG at 73MPH from your original 53MPG at 63MPH. That's within about 10% of what you're reporting, which in my experience is within the margin of variation caused by all kinds of other factors - including whatever inaccuracies are inherent in that 25% calculation in the first place.

 

I have a 40 mile commute every day. 1/2 highway and 1/2 not. The highway portion is relatively flat. In my 2012 C with just my 185 lb self aboard, ECO on, I average 50 plus mpg, as long as I keep the speed at 71 or below. At 72 or above it dips into the 40s. But 38 mpg at 73 is way too low. Sounds like something is wrong. I don't use CC, as I find I can do better with my own driving techniques. On a flat road it should make for better mileage, but just for testing purposes, I would try the same trip without it and see if there is any difference.

On the off highway portion, where my speeds run from 35-50 mph, with a few hills, I get around 70 mpg. This is where the car really shines as it can get into electric only mode. You say only 60 at those speeds, so again, I think something is wrong. Cold weather, wind, number of people in the car, are all factors, but I would not expect 38 mpg at a steady 73 mph on flat roads.

 

Yes, a conspiracy, does it involve tiny droplets dropped from airplanes?

What do you weigh? Subtract that weight from the vehicle and figure mpg's, bet they are now spot on.

 

GM, so NOT third party... yes the c is size-ably less aerodynamic than gen3. Lower speeds or live with it. Plug in a obd reader and check engine Load. it will be slightly higher at 70 plus than below... I also find that NOT using cruise can help mpg. cruise overcompensates. p&g will also help.

 

My 2010 Prius 11 got 47 mpg on a 1300 mile trip to Erie,Pa from Dallas,Tx. Ran 75 to 85+ mph using cruise control as much as possible. most of the speed limits around here on the interstates and main highways are 70 t0 75 mph. most people are driving 5 t0 10 miles over the limits because they can get away with it. My Honda used to get 23 mpg on the same trip at the same speeds. No reason to drive under the speed limit for better mileage. Actual mileage was 46 mpg figured manually. Air pressure in tires was at Toyota recommended psi. Made the ride much smoother.