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Old 07-22-2007, 07:52 AM   #1
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My Driving Technique

I've continued to learn small details about how to get the most economy from our Mazda5 5 speed, so I thought for the benefit of other 5 owners I would summarize it here (though much of it can be generalized to any manual transmission vehicle).

1st off, when the motor and weather is cold, there just isn't much to be done that will improve economy significantly until the motor warms up. I've tried P&G, lowest rpms possible, and just plain not worrying about it, but on my 2.5 mile commute to and from work, fuel economy barely fluctuates outside of 25-28 mpg. It's pretty clear why after watching the Scanguage, as the GPH display holds a low rpm idle consumption at nearly twice what it consumes once warmed up. No doubt some of that carries over into higher rpm operation.

As for general maximum fuel economy without special techniques, the lower the shift point you can use, the better, so long as the next gear doesn't drop the rpms below roughly 800 or so. Cruising in 5th gear at idle speed (about 19-20 mph) on flat ground produces over 50 mpg. Even at 25 mph, economy still hovers around 50 mpg. The same speed in 4th drops economy down into low 40s. Shift at the lowest rpms possible as soon as possible.

Now as far as pulse and glide technique is concerned, I've found that attempting to utilize it at anything under 30 mph average speed is simply not productive. Because the rpms in 5th gear are so low already, it's difficult to make up any ground there unless one takes the extra measure of engine-off coasting. Even so, under 25 mph or so, even EOC'ing doesn't return too much to economy, as the bump start at the low end triggers a rich-start condition that negates much of the benefit. For the wear and tear on the car, I don't consider it worth the effort.

Where P&G seems to produce the most benefit is at average road speeds of 40 mph and above. A steady cruise in 5th at 40 mph nets 42-44mpg. P&G bumps that up by about 2-3 mpg. EOC P&G adds another 2-3 over that. Unfortunately it's a bit more difficult to assess the benefit at highway speeds due to traffic on the local two lane roads, but using a combination of 2+ second drafting and limited P&G I have been able to attain high 30s-40 mpg on a local 15 mile trip we do regularly with 75% highway driving. Sustained cruise control at 55 mph nets about 32 mpg without any other aids. Fortunately this car is so slick aerodynamically, economy still stays around 30 mpg at 65-70 mph. Not bad for a 6 passenger wagon!

Regardless, a bad day of traffic and light sequencing can bring my best effort in town down into the mid-20's. Short of just runnin' those reds, there's only so much you can do.
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Old 07-22-2007, 12:03 PM   #2
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1st off, when the motor and weather is cold, there just isn't much to be done that will improve economy significantly until the motor warms up.

there's only so much you can do.
Is the use of an engine block heater viable ? Maybe use an engine blanket as an insulator to retain "some" warmth when a block heater isn't available .

Yes there is only*so*much you can do,but it is very important that you do
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Old 07-22-2007, 12:30 PM   #3
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MnFocus -

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Is the use of an engine block heater viable ? Maybe use an engine blanket as an insulator to retain "some" warmth when a block heater isn't available .

Yes there is only*so*much you can do,but it is very important that you do
I was thinking of a block heater too (myself included).

I have one commute per week into "the heart of MPG darkness", aka a trip to the Westside near UCLA. With everything that I try, it's neary impossible to break 30 MPG because of the stop and go on the surface streets, and I refuse to take the 405 because it such a total disaster between 5 and 7 PM.

I like this post because every car has it's own nuances that need to be understood.

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Old 07-22-2007, 12:35 PM   #4
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A block heater is viable, but the reality is that it only takes 2-3 miles to get fully warmed up anyway, so the real issue is whether we combine multiple short trips or waste gas by making random trips here and there all day long. Being that my wife drives the car 95% of the time, I doubt I'm going to see regular use of a block heater as a means to make any difference.
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Old 08-10-2007, 05:45 PM   #5
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thanks for the write-up snax
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Old 08-10-2007, 11:00 PM   #6
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No prob.

We just took a road trip to Mt. Hood today which was about a 350 mile round trip. Even chugging up the pass to the tune of 12-15 mpg in many places, we managed over 34 mpg for the trip according to the Scanguage.

Of course it helps that to recapture the energy spent on the 6-7 mile climb I EOC'd down to the flatter terrain. We chalked up over 600 mpg for that brief run only to watch it quickly tick back down to the double digit range once I had to start it up again.

Brake response was pretty horrible once the vaccum assist ran out during the coast though. I would not recommend this to anyone without strong quadriceps to engage the brakes fully if needed. They truly take allot of effort to bite very hard.
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Old 08-11-2007, 06:57 AM   #7
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If you guys are going to insist on engine off coasting, I'd suggest investing in an electric vacuum pump for the brake booster. The brakes don't depend on variable vacuum from the engine, all they need is to see X amount of vacuum, and you can measure that by putting a vacuum gauge between the one way valve and the booster, in the line that goes from the engine to the booster. An electric pump wold keep vacuum on the booster, then there is no concern at all with the brakes working properly when the engine is off. If you don't want to do this, you can get a vacuum can from Summit Racing for about 100 bucks, which is designed to build a vacuum reservoir for brake operation on cars with low vacuum (high performance, lopey cam) engines. One of these will allow you more assisted brake instances per coast interval, you could probably get 4-5 more hits on the pedal this way. An electric pump with vacuum can would be the very best, a vacuum switch connected to the can to control the pump operation would mean unlimited EOC braking with minimal pump operation. Install all this after the 1 way valve going to the engine, and the pump would only run when vacuum dropped with the engine off, not when the engine was running.

Just a thought.
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Old 08-11-2007, 08:08 AM   #8
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It's an excellent thought.
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Old 08-11-2007, 10:01 AM   #9
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Quote:
Originally Posted by Snax View Post
A block heater is viable, but the reality is that it only takes 2-3 miles to get fully warmed up anyway, so the real issue is whether we combine multiple short trips or waste gas by making random trips here and there all day long. Being that my wife drives the car 95% of the time, I doubt I'm going to see regular use of a block heater as a means to make any difference.
It takes 5-10 minutes to get my engine temp to 190f depending on EOC's.
It takes 10-15 minutes to get my HAI temps over 100f
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