Optimum speed - GreenHybrid - Hybrid Cars


Fuel Economy & Emissions Talk about the mileage database, EPA, hypermiling, gas and driving strategy.

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Old 08-24-2006, 10:49 AM
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Smile Optimum speed

Hi all im new to this forum and new to driving economically in general, but i have been thinking of ways to get my MPG higher over the past few days and ive been trying to calculate what speed would be best for my car on the highway.
I know that the government says that on average about 55mph is the most efficent speed to drive at.
Does anyone know of a way to specifically calculate the optimum speed for a specific type of vehicle by using weight and power output etc.
Obviously its going to depend on a variety of factors, ive been conducting my own tests by driving at steady speeds and seeing what my cars mpg calculator is saying but it only shows averages and i get a slightly different reading every time i repeat it at the same speed.
Anyway thanks for reading this far and for any replies sorry for long post.
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Old 08-24-2006, 12:33 PM
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Default Re: Optimum speed

For conventional cars the wisdom is that you'll get your best mileage at the lowest speed possible in the highest gear. For instance if you can drive in fifth gear (assuming a 5 speed) at any speed above 50 mph then 50 mph will give you your best mileage.

However, with my TCH going faster does not necessarily cause the engine to spin quicker due to the unusual configuration of the car's ECVT. Instead of a transmission it uses a combination of the ICE and motor/generators through the power split device instead of a transmission. I find that there's not much of a difference in mileage for any speed between about 55 and 65 mph. The speed limit's 55 here and the cops enjoy enforcing it, so I generally don't get above 65.
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Old 08-24-2006, 06:47 PM
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Default Re: Optimum speed

It's really pretty much as slow as you can go. What I mean by that, is the slowest speed you can safely drive on the freeway. On surface streets where you can drive slower, the ideal speed might be closer to 40 MPH or so.

At higher speeds what starts to impact your gas mileage is wind resistance. But even with an Insight, the most aerodynamic of any hybrid, 55 gets you better gas mileage than 65. Don't get me wrong, 65 or even 75 MPH will still get you decent gas mileage in an Insight, but 55 will give you 10% or so more than 65.
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Old 08-24-2006, 08:07 PM
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Default Re: Optimum speed

If you do some searches on drag or wind resistance, you'll find some interesting info (both here and on the internet in general). What you'll find is that up about 40 MPH, more energy is expended overcoming friction (from the engine, road, etc.). After 40 MPH, wind resistance becomes the biggest factor. The amount of energy needed to overcome wind resistance is significantly greater as you speed up. So the optimum speed for FE is probably right around 40 MPH. I've found in my Civic that between 40 and 50 it does very well.
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Old 08-24-2006, 08:54 PM
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Default Re: Optimum speed

I agree with Tim, around 40 is best. The Prius has a weird bump in its graph that you want to avoid. I think mid thirties is best in a Prius.
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Old 08-25-2006, 12:24 PM
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Default Re: Optimum speed

I asked this in the Ford Hybrid forum here and some members with scanguages were able to determine the ideal speed for fuel economy by running tests at different set speeds. For them, in a FWD Ford Escape Hybrid, the ideal speed was 48mph at which they achieved 48mpg. Of course, for the mile or so you can drive in the city on battery power under 35mpg gets you 'unlimited' fuel economy!
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Old 08-25-2006, 01:00 PM
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Wink Re: Optimum speed

Quote:
Originally Posted by lakedude
I agree with Tim, around 40 is best. The Prius has a weird bump in its graph that you want to avoid. I think mid thirties is best in a Prius.
I recommend these speed ranges:

0-38 mph -> should see 55-70 MPG, more at slower speeds.
51-65 mph (for at least 2 miles) -> should see 50-55 MPG



At 42 mph, the vehicle control laws have to protect MG1 from over speeding. So above this speed, the ICE has to run. Below this speed, the ICE is optional. Avoid passing through 42 mph since it can trigger engine strart and stop, a fuel expending process that initially takes power and provides no motive force on its own. This chart from an ODB scanner shows what happens:



THANKS to Jeff Muller who posted this data in August 29, 2001. "ICE+" means ICE rpm with a positive MG1 rotation applying power to wheels. "ICE-" means ICE rpm with a negative MG1 rotation (aka., idle for power but ICE is turning.) Learning how the Prius transaxle works is not easy.

Bob Wilson

Last edited by bwilson4web; 08-25-2006 at 01:10 PM.
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Old 08-25-2006, 04:05 PM
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Default Re: Optimum speed

Mutley wrote:
Quote:
new to driving economically in general
Hands down the best write-up for driving for efficency can be found here:
http://www.cleanmpg.com/forums/showthread.php?t=1510

Those are written for what ever you drive being hybrid or not.
Some tips are simple- easy to adopt and advances all the way up to the latest extreme hypermiler techniques.
These brought my HCH from the 40's to what I get now.
Also can bring our Grand Caravan from 16-19 up to about 30.
If you choose to follow any of those tips only begin with what you're comfortable and safe with.
-Steve
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Old 08-26-2006, 02:38 PM
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Default Re: Optimum speed

Quote:
If you do some searches on drag or wind resistance, you'll find some interesting info (both here and on the internet in general). What you'll find is that up about 40 MPH, more energy is expended overcoming friction (from the engine, road, etc.). After 40 MPH, wind resistance becomes the biggest factor. The amount of energy needed to overcome wind resistance is significantly greater as you speed up. So the optimum speed for FE is probably right around 40 MPH. I've found in my Civic that between 40 and 50 it does very well.
From a physics perspective, because of air resistance, the most efficient speed (least work done overcoming resistance) is as slow as possible. Gasoline engines however tend to do poorly at very low load, and very high load, so gear ratio changes are necessary. In lower gears, friction in the engine is effectively "multiplied" so this will offset the reduced air resistance. On hybrid vehicles, where engines tend to be downsized, and coupled to the wheels with CVTs, the optimum MPG point will be much much slower than on a standard car. On a pure elecrtic car, which develops its peak torque at 0rpm, and usually has enough torque launch in top gear, the optimum speed will be a crawl -- so driving at the absolute most efficient speed possible doesn't always make sense .

Optimum speed will also tend to be slightly faster if you decide to run air conditioning, as the amount of energy used will be a function of the time you're on the road, driving faster will require less cooling. For the elecrtic car that hits it top efficiency at 10mph, this will be more noticable.

I personally would suggest driving slower on regular commutes, but on long trips, it pays to drive fast. The difference between driving 60 and 80mph in an aerodynamic Civic or Prius, though it's fairly large swing in drag, only works out to about $3.50/hour-saved, the cost will be much greater on an SUV or less aerodynamic car. If you have 2 people in the car, that's $1.75 each to save a full hour.
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