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Old 10-23-2023, 10:40 PM   #1 (permalink)
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Question Question about lower speeds

It's a common saying that reducing speed raises MPG, but I typically see it about going from 70+ to 50-60 MPH. Does this still apply to sub-40 MPH (not top gear, it refuses to go up until 45+), aka city driving, or do I have to rely more on PnG to get numbers up?

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Old 10-23-2023, 10:50 PM   #2 (permalink)
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Typically, down to 25MPH or so, depending on air density, & etc. Of course, returns diminish and get lost in the noise.

Other techniques are drive without brakes, and don't slow down for corners.
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Old 10-23-2023, 11:46 PM   #3 (permalink)
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I usually increase speed to get into top gear. The point of traveling is to arrive at a destination. In my Acura, I can be in 6th gear at 30 MPH. In the Mazda, it's 44 MPH.
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Old 10-24-2023, 12:12 AM   #4 (permalink)
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Quote:
Originally Posted by redpoint5 View Post
I usually increase speed to get into top gear. The point of traveling is to arrive at a destination. In my Acura, I can be in 6th gear at 30 MPH. In the Mazda, it's 44 MPH.
I'd like to do that too, unfortunately the route I take has many sections of relatively close together lights, and I don't want to risk accelerating to 45 just to have to slow down all over again. The longer stretches I am able to get up to 45, engage 5th, then slowly back off down to 40 and chug along at like 1.2K RPM. Otherwise I hover around 37 and do some b*stardized variant of DWL by choking the throttle at just barely above idle.

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Old 10-24-2023, 02:09 AM   #5 (permalink)
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Pulse and glide between lights is probably most efficient. Avoiding brakes is more important than overall speed in stop and go scenarios. In town, you need to go with the flow. You're not going to do 10 under in a 35 MPH speed zone, neither are you going to do 55 in a 30.
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Old 10-24-2023, 05:20 PM   #6 (permalink)
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I have observed that "it depends".

In my Insight, with the original drivetrain, fuel economy dropped off predictably (in relation to speed) above ~50mph. The faster I went, the more economy I'd lose. Below 50, the economy of steady-state driving would improve, but the car seemed more "sensitive" to outside factors, such as hills, weather, downshifts, needing to accelerate and decelerate, etc. Probably if there were a straight and level road where I could have traveled at 30-35mph in top gear for dozens of miles without any kind of interruption or grade change, economy would have peaked there, but I never really found those roads. In practice, rural roads with a 45-50mph speed limit were peak. In those conditions, the car would reliably deliver 90-100mpg on a warm summer day.

I have observed a similar pattern in my MX-5. Above ~80kph/50mph, economy drops even more sharply than it did in the Insight, likely because the car is less aerodynamic. The fuel economy display is far less granular, but it appears it would get phenomenal economy at lower speeds (it goes into 6th around 50kph/30mph). However, anything less than 80kph roads in-practice result in lower economy, because they tend to have stops, turns, yields, hills, etc. which hurt far more than the lower speed helps, even using good driving technique. The car will get (in freedom units) ~40mpg at 65mph, ~48mpg at 50mph, and might get 50mpg+ even crawling in traffic in 2nd at ~1200rpm, but the moment I need to accelerate and decelerate, that economy tanks, and I'm averaging ~35mpg city.
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Old 10-24-2023, 11:59 PM   #7 (permalink)
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Steady state isn't how you maximize fuel economy in an ICE powered vehicle. Pulse and glide.

You want to avoid minimal throttle positions.

I still don't like the question, because participating in traffic dictates the operating parameters.
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Old 10-25-2023, 12:26 AM   #8 (permalink)
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Quote:
Originally Posted by redpoint5 View Post
Steady state isn't how you maximize fuel economy in an ICE powered vehicle. Pulse and glide.

You want to avoid minimal throttle positions.

I still don't like the question, because participating in traffic dictates the operating parameters.
I agree with both points, though I'd add the caveat that everyone will have essentially incomparable driving conditions. We can only set up boundary value problems - most of which we can likely agree are possible edge scenarios which are pertinent to the discussion.

For instance:
Steady state driving at various speeds
Pulse and glide, engine on or off
Idling fuel consumption (and minimizing idle)
Stop and go traffic vs vehicle weight

And then there's a discussion of driving technique, and where it places your vehicle (chosen) and route (also chosen) within these conditions.


The Insight was basically at ~75% load in 5th on level ground, and did not benefit measurably from engine-on pulse and glide. Proper EOC requires a kill switch, which places it as a more advanced driving technique. Neither really applies to automatics.

The MX-5 has a very high compression ratio and runs deep Atkinson-cycle, which pushes the middle of the peak BSFC island much lower in the load range - closer to 55%. It's very easy to drop into fuel enrichment or deep timing retard.

I'm of the opinion that steady-state driving is still pertinent to discuss, even if it's rarely the most efficient technique. My driving isn't always to burn the least fuel, or spend the least money, or make the least pollution, though these are interests of mine.
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Old 10-25-2023, 07:48 PM   #9 (permalink)
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I got 71 mpg steady state driving here in Utah today at around 74 mph on the highway. Just flexing
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Old 10-26-2023, 12:45 AM   #10 (permalink)
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My company CX-5 is getting mid 20's lately. I'm sure something is wrong, but since it's not my car and I don't pay for the gas, the motivation just isn't there. No CEL. I think it smells rich when it starts up, which is kinda normal, but it seems more rich than it needs to be.

The CX-5 gets great reviews, but I just don't see the appeal. It doesn't handle as good as a Prius or any sedan. It doesn't have any more space. It doesn't get good fuel economy. It's a very basic car that sits a bit higher than a sedan. I don't hate it, but there's nothing I love about it.

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