Good News for Charging Convenience

[DucRider]

I’ve been looking for data on how speed effects the mileage/range for Rivian vehicles. How much range does one lose by driving 80 mph verses 70 mph.?

There is nothing specific to Rivian vehicles since none have been produced (a few prototypes but they will have some differences).
The power source has no effect of the physics that reduce mpg (or mi/kWh). Air resistance has an inverse square relationship to speed and at freeway speeds is the primary factor affecting the energy needed to maintain a given speed.

You can search for something like "F150 mpg 70 vs 80 mph" or something similar, or just "mpg vs speed" and gets lots of data to look at. Less aero efficient vehicles will suffer a greater penalty with speed.

My guess would be about a 10-15% reduction in range at 80 vs 70 mph.

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[skyote]

I’ve been looking for data on how speed effects the mileage/range for Rivian vehicles. How much range does one lose by driving 80 mph verses 70 mph.?

Great question, & I have a feeling we won't know until owners will be able to answer.

 

[Havec]

Good answer.
I wonder what the test conditions were for the 400 mile specified range. Consumer Reports gives their highway mileage by driving both directions on a highway at 65 mph. So elevation gain is cancelled out by elevation loss along with wind. Can’t wait until CR tests the RT1.

 

[ajdelange]

I’ve been looking for data on how speed effects the mileage/range for Rivian vehicles. How much range does one lose by driving 80 mph verses 70 mph.?

That's an impossible question to answer because range depends on things other than speed. We all know that drag force depends on the density of the air (which depends on temperature), the effective frontal area of the vehicle and the square of the airspeed. Note that I said airspeed - not ground speed. Work is the product (or integral of the product really) of force times distance and power the first derivative of that with respect to time and speed is the first derivative of distance with respect to time of the distance traveled. Thus the power required to overcome drag depends on the cube of the speed but the energy per unit distance depends on the square. Thus, at first look, the energy required to push the air out of the way per unit distance traveled at 80 mph will be (80/70)^2 = 1.306 times that required at 70. But I said airspeed. Suppose there is a 10 mph headwind. The ratio is now ((80 + 10)/(70 + 10))^2 = 1.266 and for a 10 mpH tail wind ((80-10)/(70-10))^2 = 1.361.

But drag isn't the only thing that consumes power. There is also wheel slip (no slip - no torque). This is a substantial energy sink which can exceed the drag load. For the moment lets assume that it is equal to the drag and that the drag is 1 unit at 70 mph and, as shown above, 1.306 units at 80 mph. Then the ratio of total battery load (70 to 80) is
(1 + 1.306)/(1 + 1) = 1.153 so that we see the total increase in consumption is more like 15% under this assumption that slip loss is the same as drag loss. There are other losses too such as rolling resistance and the really big ones: inertia and gravity. None of these other losses depend on speed and, as they tend to swamp the drag losses, the assumption that range is reduced as the square of the velocity ratio is generally not valid.

Now drag does get you another way. You must supply extra energy to overcome drag and that means that the wheels must supply more power to the road and that power is the product of torque and rotation rate. Rotation rate goes up linearly with driving speed but total energy requirement, even though the other components do not involve speed, is still a non linear function of speed and so extra torque is required. This means that slip must increase a bit and that means more slip loss. I'm sure this all sounds complicated. I'm not going to put the math here unless someone wants it. Just ask. Even if I did put the math we wouldn't know what numbers to put into the formulas. As has been noted were are just going to have to wait and see. BEV's have software that lets one keep pretty detailed records of how much energy is used in each drive and to monitor how it is being consumed during the drive. From this you will get a feel for how what you do influences your energy consumption. Gut feel is that going from 70 to 80 would cost you 5 - 10% in range but, as per the discussion, other factors (head/tail winds, grade, terrain, weather) may have larger effects.

 

[ajdelange]

I wonder what the test conditions were for the 400 mile specified range.

This would be their estimated EPA range. This is determined by following an EPA specified speed profile and measuring the consumed power on a dynamometer. The dynamometer is subject to rolling resistance, wheel slip and drive train losses but aerodynamic drag, inertial and gravitational values have to be programmed in. I really don't know much about this but evidently the manufacturer can program these loads in any way he wants as long as he can convince the EPA it is valid. In previous posts I've indicated that I think Rivian is highly motivated to do this in a way that will not only satisfy the EPA but will come reasonably close to matching what the consumer will experience.

Consumer Reports gives their highway mileage by driving both directions on a highway at 65 mph. So elevation gain is cancelled out by elevation loss

If going up the hill increases the potential energy of the vehicle by 100 units that means that something more than that had to be taken from the battery say 110 units because of losses. When coming down the hill the 100 units of potential energy are returned to the car but only perhaps 90 go back to the battery because of losses. 110 units expended; 90 recovered. That's a loss of 18%.

..along with wind.

Remember from the earlier post that to drive at 70 into a 10 mpH head wind would cost us (70 + 10)^2 = 6400 units of drag energy i.e. 6400 - 70^2 = 1500 more that if there were no headwind. Converesely, with a tail wind of 10 mpH we'd use (70 - 10)^2 = 3600 units which is 4900 - 3600 = 1300 less than we'd need with no tail wind. IOW you do not get the headwind loss back when driving with an equal strength tailwind.

Can’t wait until CR tests the RT1.

I learned long ago to take what Consumer Reports comes up with with a large grain of salt.

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