Fuel efficiency: Unexpected differences (comparing each half of a round trip)

[7milesout]

Agreed - I've been driving the Blueberry real nice (not too low rpm, not too high rpm), and will continue until 5,000 miles. After that it's big ole smokie burnouts and banging the rev limiter at every shift.

[MetroMPG]

(This post will appear a bit out of context with the rest of the thread... but it's partially an answer to a PM that 7milesout sent me...)

In my experience, the factor most likely to cause different MPG results on opposite legs of a highway trip is wind conditions. That said, I also drive in fairly flat terrain. Big hill/mountain driving would probably change that (if there's a significant elevation change).

Wind resisitance is the single biggest factor affecting cruising MPG above about 25 mph (below that, mechanical & rolling resistance are the big factors).

The way to significantly increase highway fuel consumption is to cut wind resistance: either by, (A) slowing down, or, (B) reducing drag, or (C) both.

Mirage speed vs. MPG chart for inspiration:



From thread: Speed vs. mpg/mileage/fuel economy chart - Mirage 1.2L 5-spd and CVT

75 mph = 47 mpg (indicated)
62 mph = 59 mpg (indicated)

Almost all of my driving is on secondary rural/scenic highways with a max. posted limit of 50 mph / 80 km/h. Cruising at moderate speeds is one of a couple of reasons I'm able to post stupid high MPG numbers. The aero mods I've done probably boost my MPG another 5-6% more. Using the engine kill switch where appropriate probably boosts it another 5-10%.

[MirageRally]

Agreed - I've been driving the Blueberry real nice (not too low rpm, not too high rpm), and will continue until 5,000 miles. After that it's big ole smokie burnouts and banging the rev limiter at every shift.

I'll let you know if l encounter any problems from shifting at redline...

[gregsfc]

I'm a new owner and only recently re activated my visits to this site, and stumbled upon this recent thread. Good stuff!

My theory is terrain and normal prevaling winds. Sometimes these factors are counter to each other.

What OP's observed with one direction always out performing another direction at the same speed, driving style, routes, etc is always my observation on any routine trip that I've ever observed or measured.

Even though my trip to work starts with colder temps than my trip home and is partially driving into a normal head wind; my trip in always out performs my trip back home according to the onboard vehicle mpg calculator. My F150 has shown up to 29 mpg going in to work and as low as 25 on the way home on the same day. If this was a one off, one could chalk that up to wind/weather shifts, but this is a consistent result. The least difference I've observed on this trip on the same day is about 1.5 mpg. One caveat: I've observed with my F150's calculation that the higher mpg that is displayed, the higher that the optimistic error becomes. I manually measure only tank to tank; my highest manually measured mpg in history, after accounting for the pessimistic trip meter, has been 26.7. At that super-high mpg level, the displayed figure is about 2.0 higher than actual or about 28.7; but if my actual is more like 23; the display will be more like only 1.2 higher or 24.2. With an average tank, the error will be around 1.3-1.7.

My trip to work is northeast but then turns northwest; but I drive to a fish hatchery adjacent to a river, so I'm ending up at a lower altitude and so it obviously has more downhill than uphill overall even though it's hard to notice.

But it's not just the work and back trips. It's any trip that I've ever observed more than one time...If I drive west towards Nashville (usually a head wind), I'll always get better mpg driving from the Highland Rim into the Nashville Basin along interstate 40 than I will get coming home despite the wind. If I go the other direction and drive eastd to my parents home in Knoxville (tail wind), I'll always do better going than coming. So it's an opposite result driving to Knoxville and back than driving to Nashville and back. I live about half-way in between near Cookeville. On that trip, I start out going from the Highland Rim another 1,000 feet above sea level to the Cumberland Plateau (officially the Appalachian Plateau); then down off the Plateau into Ridge and Valley terrain (both up and down hills) to my parents' house on the northwest side of town. The hills probably cancel out more or less; but prevaling winds probably play a bigger role in this trip, but I can't be sure. This Knoxville trip direction difference is not as significant as the Nashville trip, but all my trips that I've ever observed always result in a similar result; at least with respect to one direction out performing the other.

[7milesout]

Greg - You get better mpg driving away from home, versus going back home. Mine is the same. They're usually fairly close, but the pattern is I get about 0.5 mpg better going north (away from home) than I do going home. The point of my thread was not only was this trip different, but it was a HUGE difference. I got WAY better mpg coming back home (same speeds). And it didn't feel like I had a tailwind. I tried to look for flags but never saw any. In my case, I think it must have been wind direction, I just didn't feel it like I thought I would have. The Honda Rebel 250 I mentioned is SUPER SENSITIVE to wind direction. Perhaps due to the low power (~17 hp). Maybe the Mirage is also very sensitive to wind direction, due to the low power.

BTW - When I turned the key ON this morning, my odometer was at 3,000 miles exactly.

[gregsfc]

So Without reading back through the thread, I think I underdtand that you had one particular trip where you got an unbelievable result. If so, I've had those kind of runs. I once frove from my home to the VA medical facility in Murfreesboro and in Murfreesboro and loggged 30.1 mpg on my truck computer when I reached the destination. It just kept magically climbing the whole trip, and like I stated, the error may have been up to 2.2 or so at that high displayed level; but, by the time I got home, even though it was still an amazing run, I was down to about 28.3 for the entire trip; which means I was likely just over 26. So what ever caused the anomaly, it canceled itself out in my case.

I'm always blown away when I do have an anomaly, because I usually get a huge dose of consistency recording mpg; aka, I usually see the same sort of results over and over and when something does go up or down, I usually know why; but every now and then, I can't figure something out like that 97 mpg run at the Vetter Challenge on my CTX700.

[MetroMPG]

The Honda Rebel 250 I mentioned is SUPER SENSITIVE to wind direction. Perhaps due to the low power (~17 hp).

Motorcycles also have very high drag coefficients compared to cars. That's why fairings / tucked riding position make such a difference on top speed... and fuel economy ... and wind sensitivity.

[MetroMPG]

... like that 97 mpg run at the Vetter Challenge on my CTX700.

Ha! So you know all about the dramatic impact of aerodynamics on motorbikes. Craig Vetter has made some fantastically efficient streamlined motorbikes for those competitions.

[Mark]

I'll let you know if l encounter any problems from shifting at redline...

"Every Man Should Know How to Drive Stick" article -

https://www.artofmanliness.com/artic...stick-a-shift/

This is why I like affordable cars like the Mirage. If they are highly discounted, I can buy them brand new & not have to worry about the previous owner shifting @ redline.

I'm sure many rental/lease vehicles are slightly abused, too. Buying new doesn't offer any special guarantees (full warranty however). It's just nice knowing your vehicle hasn't been abuse when bought this way. If I know I am going to keep a vehicle for a long time, I am willing to pay slightly extra for a new one (especially when they are highly discounted). Keep in mind - I paid under 66% window sticker for a new 2017 Mirage & all closing costs were under $860. At that price, a new Mirage was cheaper than some of the low mileage used one. Buying a manual limits available choices, too.

I paid $5,300 for a brand 1990 Ford Festiva & I drove it until 2004. Selling that car was the biggest mistake I made with any of my vehicles. It still ran perfectly fine (didn't look great) & liability insurance was only costing me $99. I don't remember if that was $99/6-months or $99/year. Either way it was super cheap car to have on the road. I should have drove it until the wheels fell off or a deer took it out.

I plan to do the same with Mirage. Unless my Mirage is giving me problems, I am not making the same mistake of selling it to soon. I doubt my Mirage will ever experience redline!

When you start shifting at redline, please record that for us. I want to see what that looks like with a Mirage.

[MirageRally]

I do believe fuel cuts off before I even get to redline BUT when I get my little recording camera again I will do just that for you