Just thought I'd post about this in here...
I've noticed that my MPG can vary, and while the most obvious culprit is slow speeds, I've had bad mileage at highway speeds as well. So I'm thinking it's not so much speed, but rather engine RPM's, that are to blame?
What is the ideal relationship between speed and RPM's to produce good MPG?
MPG/Speed/RPM=Efficency?
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Elheru Aran
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MPG/Speed/RPM=Efficency?
It's a strange world. Let's keep it that way.
Every car has an optimum cruising speed for best fuel economy, with very few exceptions it's somewhere between 45-65 mph. Every car also has an optimum acceleration curve.
As a general rule, you want to keep the engine RPM down as much as possible at any given speed, this reduces the frictional losses in the engine and allows you to use larger throttle openings which reduces pumping losses (power losses from sucking air into the cylinders and pushing the exhaust out). The perfect case would be to have the engine just a touch above idle with the gas pedal all the way to the floor while cruising at a given constant speed. But that doesn't happen, so we gotta get creative.
If you're driving on the highway, always use your highest gear to keep the RPM's down and resist the urge to speed at 80mph. Also keep a good distance between you and the car in front of you so that when speeds change you don't have to slam on the brakes and then step on the gas, that wastes fuel. If you have a good gap you can just back off the gas pedal and let the car coast down, then ease back onto the gas when the speeds pick back up.
In city driving, let off the gas if you see a light turn red in the distance and coast into the light instead of keeping your speed and then slamming the brakes at the light.
If you have a stick shift you can skip gears when accelerating from a stop. Instead of going 1st-2nd-3rd-4th-5th you can go 1st-3rd-5th, skip a gear or 2 so the RPM's stay lower. Several GM cars use a 1st to 4th skip shift to get better fuel mileage on the EPA city cycle which keeps them from getting dinged with a gas guzzler tax.
A real world example. Driving to my friend's cottage and back is about a 250km trip. If I don't use any of the above tips and follow the flow of traffic along at 110-130km/h, it takes me a bit over half a tank of gas to make the trip. If I take the back roads which is the same distance, I can cruise along at my best fuel economy speed without getting run over, combine that with the other tips and I can make the trip on under 3/8th of a tank. That I'd say is a pretty significant gain in fuel mileage.
As a general rule, you want to keep the engine RPM down as much as possible at any given speed, this reduces the frictional losses in the engine and allows you to use larger throttle openings which reduces pumping losses (power losses from sucking air into the cylinders and pushing the exhaust out). The perfect case would be to have the engine just a touch above idle with the gas pedal all the way to the floor while cruising at a given constant speed. But that doesn't happen, so we gotta get creative.
If you're driving on the highway, always use your highest gear to keep the RPM's down and resist the urge to speed at 80mph. Also keep a good distance between you and the car in front of you so that when speeds change you don't have to slam on the brakes and then step on the gas, that wastes fuel. If you have a good gap you can just back off the gas pedal and let the car coast down, then ease back onto the gas when the speeds pick back up.
In city driving, let off the gas if you see a light turn red in the distance and coast into the light instead of keeping your speed and then slamming the brakes at the light.
If you have a stick shift you can skip gears when accelerating from a stop. Instead of going 1st-2nd-3rd-4th-5th you can go 1st-3rd-5th, skip a gear or 2 so the RPM's stay lower. Several GM cars use a 1st to 4th skip shift to get better fuel mileage on the EPA city cycle which keeps them from getting dinged with a gas guzzler tax.
A real world example. Driving to my friend's cottage and back is about a 250km trip. If I don't use any of the above tips and follow the flow of traffic along at 110-130km/h, it takes me a bit over half a tank of gas to make the trip. If I take the back roads which is the same distance, I can cruise along at my best fuel economy speed without getting run over, combine that with the other tips and I can make the trip on under 3/8th of a tank. That I'd say is a pretty significant gain in fuel mileage.
Lusankya: Deal!
Say, do you want it to be a threesome with your wife? Or a foursome with your wife and sister-in-law? I'm up for either.
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Elheru Aran
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Cruise control usually helps, as long as the software is decently smart and there aren't too many hills. My cruise control unfortunately is pretty dumb, it's great at keeping a constant speed but when I press the accel/deccel buttons it revs the engine like mad to get to the new speed. On hills it'll downshift the gears and send the RPM's way up to keep a constant speed.
To get around this I speed up & slow down manually then set the cruise, and on hills I also disable the cruise and make careful use of my right foot. Instead of keeping a constant speed I'll bleed off a few km/h as I go up the hill to keep the transmission from downshifting.
Hills aren't nearly a big a problem with a stick shift, but stupid software can still be a problem when using the buttons to change speeds. Sudden changes in throttle position aren't good for fuel economy, you want to be gradual instead of going hard on to hard off or vice-versa.
To get around this I speed up & slow down manually then set the cruise, and on hills I also disable the cruise and make careful use of my right foot. Instead of keeping a constant speed I'll bleed off a few km/h as I go up the hill to keep the transmission from downshifting.
Hills aren't nearly a big a problem with a stick shift, but stupid software can still be a problem when using the buttons to change speeds. Sudden changes in throttle position aren't good for fuel economy, you want to be gradual instead of going hard on to hard off or vice-versa.
Lusankya: Deal!
Say, do you want it to be a threesome with your wife? Or a foursome with your wife and sister-in-law? I'm up for either.
I just checked my fuel & mileage logs and punched the numbers into my calculator. Driving the normal way gets me about 25-26mpg. Driving in full fuel saver mode rewards me with an econocar-like 36-37mpg. If I had the A/C off I probably could've picked up another couple mpg. This of course applies only to my car and my particular cottage trip. My car for reference is a 2000 Chevy Malibu LS with a 3.1L V6 engine. The EPA mileage rating for my car is 22mpg city & 30mpg highway.aerius wrote:A real world example. Driving to my friend's cottage and back is about a 250km trip. If I don't use any of the above tips and follow the flow of traffic along at 110-130km/h, it takes me a bit over half a tank of gas to make the trip. If I take the back roads which is the same distance, I can cruise along at my best fuel economy speed without getting run over, combine that with the other tips and I can make the trip on under 3/8th of a tank.
Lusankya: Deal!
Say, do you want it to be a threesome with your wife? Or a foursome with your wife and sister-in-law? I'm up for either.
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Elheru Aran
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...Interesting information to know. I drive a '93 Ford Probe SE, which's suffering a bit from a worn fan belt. When I bought it, it was doing approximately 37 MPG, down to around 30 now but that can vary considerably-- trying to keep it consistent these days...aerius wrote:I just checked my fuel & mileage logs and punched the numbers into my calculator. Driving the normal way gets me about 25-26mpg. Driving in full fuel saver mode rewards me with an econocar-like 36-37mpg. If I had the A/C off I probably could've picked up another couple mpg. This of course applies only to my car and my particular cottage trip. My car for reference is a 2000 Chevy Malibu LS with a 3.1L V6 engine. The EPA mileage rating for my car is 22mpg city & 30mpg highway.aerius wrote:A real world example. Driving to my friend's cottage and back is about a 250km trip. If I don't use any of the above tips and follow the flow of traffic along at 110-130km/h, it takes me a bit over half a tank of gas to make the trip. If I take the back roads which is the same distance, I can cruise along at my best fuel economy speed without getting run over, combine that with the other tips and I can make the trip on under 3/8th of a tank.
It's a strange world. Let's keep it that way.
Don't forget maintenance! You can pick up a few MPG by switchting to synthetics. My MPG went up to 18 from 16 when I switched and I picked up another half MPG when I put my transfer case on synthetics (although the fact that my car was overdue for a change may skew those numbers abit...)
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My '91 Celica gets 27 MPG doing 80MPH on highway- I normally get much worse on a normal basis at 17-23 or so doing my daily 3.6 mile commute (23 if accelerating like a grandma). I don't try to coast since I fear driving slow enough to irritate other drivers behind me.
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Alferd Packer
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A stickshift will also grant you the flexibility to stretch your fuel economy to the limit not only by skipping gears, but also by short-shifting. I manage to squeeze out an extra 10% (2 mpg) out of my city mileage by keeping the motor below 2900 RPM. Unfortunately, my heavy, slow truck needs me to use all five gears (First gear is 4:1, third is 1.4:1...do the math 
).
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