Fuel Economy - manual transmission

[KuroNekko]

I have accumulated meticulously 13 months of fuel economy on my car, since it's averaging so horridly.

I usually get <20 MPG per tank (calculated) but the associated average speed is usually <20 mph. The EPA city MPG is tested at a average speed of 21 mph (while the htihway EPA avg speed is 48). I had a few tanks of 27-30 MPG on roadtrips with average speed ranging in the 40s mph. I know some people don't believe it, but I feel most of the time the average speed gives a good indication of how much city/highway ratio one drives and correlates well to the MPG, so it'd be helpful if people reset their avg speed with each fill-up and report those figures as well.

Kuro's figures are no surprise in that the MPG exceeds the EPA highway rating, since the EPA is tested at 48 mph avg speed, meaning there's still some slowing down and accelerating in the tested cycle. If you can keep your speed at 60 constantly without much dec/acceleration it wouldn't be surprising you are exceeding the EPA rating. Over 65 there will likely be a decline in MPG?

In 13 months after 28 tanks I am averaging 21.3 MPG, with a overall average speed of 21.5 mph. So I am not sure whether to blame the car or the slow city driving around here.

I know you have been complaining about low MPGs for a while, but given your data, I think it's on par with what it's supposed to get. 21.3 MPG for a car that averages 21.5 MPH indicates to me that you do mostly city driving. You also have a SLS with AWD meaning you have two disadvantages to fuel economy: The SLS has the larger 18 inch wheels which results in lower efficiency and the AWD system also adds weight (always) and drag when in use.

Given your car's specs, the fuel economy isn't that bad. It's on par with rival cars with AWD, in my opinion.

Regarding the decline in fuel efficiency over 65 MPH: Yes. Going faster meaning the engine is at higher revs and burning more fuel. Wind resistance is also greater at higher speeds. Cars are generally aerodynamically tested at around 55 to 65 MPH so these are the speeds in which you get optimal fuel efficiency.

It's also why there is the famous 55 MPH speed limit on most of America's highways. The 55 MPH highway wasn't a creation out of the government trying to curb highway deaths. It was actually a policy to reduce national oil consumption during the oil crisis. However, after setting the speed limit to 55 MPH, it was observed that highway related deaths also declined. This is why it's still in place today although safety wasn't the original concern of the government.

[~tc~]

Regarding the decline in fuel efficiency over 65 MPH: Yes. Going faster meaning the engine is at higher revs and burning more fuel. Wind resistance is also greater at higher speeds. Cars are generally aerodynamically tested at around 55 to 65 MPH so these are the speeds in which you get optimal fuel efficiency.

Not really. Air resistance goes with the square of the speed, so 2x the speed has 4x the air resistance. The idea for optimal fuel efficiency is to go the slowest you can in the tallest gear.

[Woodie]

Regarding the decline in fuel efficiency over 65 MPH: Yes. Going faster meaning the engine is at higher revs and burning more fuel. Wind resistance is also greater at higher speeds. Cars are generally aerodynamically tested at around 55 to 65 MPH so these are the speeds in which you get optimal fuel efficiency.

Not really. Air resistance goes with the square of the speed, so 2x the speed has 4x the air resistance. The idea for optimal fuel efficiency is to go the slowest you can in the tallest gear.

That's the general rule, but I've found exceptions. I think gearing is more important than we previously thought.

My brother got a Chrysler Voyager and told me it got better mileage between 70 and 80. I told him he was looney, the car is shaped like a brick, aerodynamics (speed) should be huge. I've now got a Grand Caravan as a company car and he was 100% right, the mileage keeps going up the faster you go (up to a point I'm certain, I don't usually drive it over 70). It's got a six speed automatic and seems quite high geared.

[Woodie]

- The observed MPGs from owners here show that CVT fuel economy and manual transmission fuel economy are more similar than what the EPA figures show.

I think this is rather important to note because the auto industry is trumpeting CVTs as the most efficient transmission for internal combustion engines. While they are known to be more efficient than tradition autos, the evidence that they are better than manuals is not very convincing. I consistently see EPA figures that show CVTs getting better fuel economy than their manual versions, but owner data shows otherwise in various makes and models.
My point here is that manuals have yet to be trumped by CVTs in terms of real world efficiency. Traditionally, it was known that manuals offered better fuel economy than geared automatics. This was verified both statistically and empirically. Now, CVTs have higher numbers on paper, but many owners are observing efficiency nearly equivalent to manuals.

I never believed that for a second. Any transmission with built in slip, and pumping around that much fluid is not a transmission, it's a mistake.

They're just computer tuned to do well on the EPA test, any other scenario whatsoever, and they're not going to perform as well.

[KuroNekko]

Regarding the decline in fuel efficiency over 65 MPH: Yes. Going faster meaning the engine is at higher revs and burning more fuel. Wind resistance is also greater at higher speeds. Cars are generally aerodynamically tested at around 55 to 65 MPH so these are the speeds in which you get optimal fuel efficiency.

Not really. Air resistance goes with the square of the speed, so 2x the speed has 4x the air resistance. The idea for optimal fuel efficiency is to go the slowest you can in the tallest gear.

While that may be true in terms of fuel consumption alone, keep in mind that fuel efficiency is a measure of distance traveled vs. fuel consumed. Driving too slowly will not allow you to cover that much distance so it's really not that efficient.

[Ehibb034]

MPG is all about how you use the throttle. By that I don't meant how fast you accelerate. The car is a momentum machine. The better you are at keeping it rolling, the better you are with MPG's. I honestly think it is how you are using the gas pedal. Most people say that they are driving like a granny and still getting bad MPG. The simple truth is that after learning how to modulate the throttle correctly, you can be going the same speed in the same car with one driver who knows how to use the throttle better than another driver in the same car, and the first driver will get better MPG's every time. I see this in my own driving. I can either get the instant readout to stay around 40 MPG while maintaining 60 MPH or I can get it to stay around 20 MPG while maintaining the same speed. It's all about when and how much pressure I put on the pedal. I see this in my wifes car and my parents cars. They all have their AVG MPG readouts and by the time I am done driving their cars, I have simply outperformed their avg's by using the throttle better. I am able to do about 5-8 MPG better on my wifes Honda CRV than she is every time I drive it. I am about to calculate my second fill up in a week and by the way I have been going in my Kiz, I should be right at 30-31 MPG, no doubt.

[~tc~]

Certainly throttle position has more effect than RPM - it's better to run in a lower gear at higher RPM with light throttle pressure than lug a higher gear with the pedal on the floor

[murcod]

Certainly throttle position has more effect than RPM

Try experimenting while climbing a hill with a constant speed. First use low revs/ a high gear with more throttle then compare higher revs with lower gear - do this while looking at your instantaneous fuel consumption. Keep the speed the same and use a constant gradient hill.

(I happen to drive a long steep hill near my home everyday and have done the experiment myself quite a few times. The results are always the same.)

[Woodie]

Certainly throttle position has more effect than RPM - it's better to run in a lower gear at higher RPM with light throttle pressure than lug a higher gear with the pedal on the floor

That is EXACTLY backwards. Assuming that you're not lugging the engine (car bucking and shaking), low RPM and wide throttle opening is more efficient. That's why economy cars have shift lights, to get you to upshift earlier.

[murcod]

Certainly throttle position has more effect than RPM - it's better to run in a lower gear at higher RPM with light throttle pressure than lug a higher gear with the pedal on the floor

That is EXACTLY backwards. Assuming that you're not lugging the engine (car bucking and shaking), low RPM and wide throttle opening is more efficient. That's why economy cars have shift lights, to get you to upshift earlier.

Yes, but it's always good to see it for yourself - as per my recommended experiment. I've got a really long steep hill to climb in a 50km/h zone and have played around to see the most economical way. Using the low RPM torque of the Kizashi's engine is a winner every time. On 98RON fuel it's a really tractable engine and loves lugging higher gears.