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Since I started driving a plug-in hybrid with a 50 mile electric range I've become sensitive to things that affect my 50 miles. For instance, when it's below about 40 degrees out I lose up to 25% of my range due to heating the car's systems (even if I don't use the heater in the cabin.)

Recently I noticed that if I take flat surface streets at a constant 45mph (no stops) to work I end up using significantly less electricity than if I take the freeway--when I take the freeway I end up with a remaining range in the mid-20's miles but when I took surface streets my range remaining was mid-30's. The change only adds about 3 minutes to my 30 minute commute to save 1/3 of my "fuel"!

I was wondering if gas cars were this sensitive to hills/wind and I never noticed it because the tank size is so much larger and therefore I never paid enough attention. Could I have been averaging 30+mpg instead of 25 just by taking this flat, slightly slower alternative route?

My feeling is there could be something in the nature of the electric vehicle that makes it more sensitive to hills/wind but I can't put my finger on just what would cause it to be different.

Also note--the "Hills" on the free way are very subtle, I never even noticed them until I started paying attention to the energy gauge in the car. I guess you'd describe them as rolling hills, nothing you'd even consider unless you were on a bike.

  • I don't have a source, but I have read that the optimal aerodynamic speed for most Internal Combustion Engine (ICE) powered cars is around 50-55 mph, since wind resistance increases exponentially with speed. – MooseLucifer Apr 28 '17 at 17:23
  • Are you asking for a driving technique to save battery? – cory Apr 28 '17 at 18:55
  • More asking if a technique I found that saves battery in an electric car would work as well on a gas car. – Bill K Apr 28 '17 at 22:58
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My answer is - yes, gas engines are affected in the same ways by hills and wind... but that doesn't mean surface streets are the better option.

When you look at the question as a raw physics question you find the answer that all resistive forces do not discriminate between drive type. Doesn't matter if you all electric, hybrid, gasoline, diesel, car, truck , motorcycle or bicycle - a hill will apply the same amount of resistance in ratio to mass while the conditions are the same.

The big difference comes in "How does each system apply the energy it generates?"

You'll notice is most, if not all, gas vehicles that the "City MPG" is lower than highway. That I due largely to the fact that you're always burning gas, even when you're not moving. All types of EVs (to my knowledge) use a very small amount of power when they are stopped - all of which is required in some way - so there's very little waste comparatively.

Now, with EVs, you're driving an electric motor using electricity from a battery or generator type source. Electric motors always draw more amperage when they come against large resistance, and since EVs don't have a transmission, there's no way to change the forces being fed back to the motor. This mean, hill = resistance = increase in wattage used. However, when you cruise downhill, many vehicles recharge!! So there is a benefit to hills as well.

Aside from these things you have other factors like aerodynamics of the vehicle, friction ratios, tire pressure, power-loss ratios, driving style, ambient temperature and on and on. They all play a part in the complex mechanical system that happens when we drive our vehicles and they all factor into the cost of driving.


In Summary

Yes, all vehicles benefit from the least resistive path which means flattest or steepest downhill, but also no, most gas vehicles won't save gas by using streets on the same commute.


Update to comment

It can happen. As I noted and was mentioned in a comment, aerodynamics falls into play. You vehicle may just be more efficient at 45 mph.

Consider for example the angle of the hood. Depending on overall geometry, the hood can produce a good amount of downforce at higher speeds. When you add downforce to a hill (rolling or otherwise) you start having force vectors that are stronger.

Downforce is created by peeling energy off of one vector (forward) and redirecting it (down). You can show this on paper quite easily. The wind force (backward) is pushed against an angled surface (spoiler) and is redirected up. Using Newton's third law we know that forcing air up causes force to be applied down. However, also, forcing a redirection of a vector requires and external force, bringing Newton's second law into play as well. This means a force going forward, which is then in conflict with the other forces.

I'm sure I could ramble more on that, but I can't seem to find a good image illustrating my point... I could draw it, but we'll see.

The point is that different vehicles will have different aerodynamic properties. As an example, my wife's Honda Civic seems to get the best mileage in the 40-60 mph range. I did a cross country trip in a Chrysler T&C van and found peak 29 mpg at 85 mph and 21 at 65! So, it varies a lot, and 30% I can easily see.

  • Note that I probably shouldn't have said "Surface Streets" since in both cases we're tailing a constant speed without stopping, one is 45mph and flat, the other is 65mph with rolling hills. Stopping/starting in city traffic is a different story and I realize that will effect both (And will have less of an effect on the electric range). I would have expected to get better mileage on this slower rout, but wouldn't have ever thought it would be 1/3 more! – Bill K Apr 28 '17 at 18:09
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Bad terrain uses more fuel in a gasoline powered car .Bad terrain reduces range on an EV because it uses more battery power .This effect is much more apparent on an EV because the internal friction of all types of electric motors is far less than the Piston engine.When the terrain is flat and smooth the gasoline engine wastes more power in its internal friction .Say if driving not too fast on flat terrain needs 10HP to push the car along .Say the gasoline engine has a max power of 200HP at say 5000rpm.At this 5000rpm the friction could be 50HP .At say 2500rpm the friction power could be 12.5 HP .Clearly more than 10HP .Now the electric motor would have a friction power of less than 1 HP under the same driving conditions .The EV also benefits from lower drivetrain losses because there generally is no gearbox and no clutch .

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    So your feeling is that the amount you save in a gas car may be less significant because regardless of terrain a larger percentage is going to waste? – Bill K Apr 28 '17 at 22:58
  • @Bill K .Yes it is .You have summed it up . – Autistic Apr 28 '17 at 23:01
  • That doesn't make sense though because the "max HP" of a gas motor will be measured at the crank which is at the end of the motor's mechanical line. All the "friction" losses of the motor have already been subtracted at that point. From this point all loss is between the clutch or torque converter and the ground. Moreover, if the friction inside your motor is fluctuating as much as you've said then something is seriously wrong... that a 800% difference between the friction from 2500-5000 rpm! – kyle_engineer Apr 28 '17 at 23:39
  • @ kyle engineer .Yes the HP is at the crank .The mech efficiency of piston engines is around 70 to 80 % at full load and max HP rpm .At lower speeds the mech losses go way down but in % terms they are very significant at this relatively light loading of 10 HP . – Autistic Apr 28 '17 at 23:44

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