Shifting for mpg efficiency...help solve dissagreement
01-08-2013, 10:13 AM
#1
Shifting for mpg efficiency...help solve dissagreement So got in a "dissagreement" with a buddy of mine about the proper way of shifting a manual transmission with only goal being increasing fuel economy efficiency. For this intance the only scenario is pulling away from a dead stop to 35mph in city driving:
His theory is to keep the throttle light and shift each gear before a certain RPM (say 2000), then when finally reaching cruising speed of 35 pop it in highest gear possible (say 4th) and go on from there.
My theory is to give it more throttle and accelerate to the desired cruise speed quickly (as quickly as possible without getting in trouble) and then pop it in the highest gear.
His idea is to keep RPMs as low as possible even if that means taking much longer to accelerate to cruise speed. My idea is to forget about RPMs to a certain degree but instead get to your desired cruise speed as quickly as possible to avoid long periods of slow acceleration which we all know is not efficient for fuel economy.
So what do you say, is there one process that's more efficient than the other...or are they both relatively the same and trying to find a difference between the two is like splitting hairs?
His theory is to keep the throttle light and shift each gear before a certain RPM (say 2000), then when finally reaching cruising speed of 35 pop it in highest gear possible (say 4th) and go on from there.
My theory is to give it more throttle and accelerate to the desired cruise speed quickly (as quickly as possible without getting in trouble) and then pop it in the highest gear.
His idea is to keep RPMs as low as possible even if that means taking much longer to accelerate to cruise speed. My idea is to forget about RPMs to a certain degree but instead get to your desired cruise speed as quickly as possible to avoid long periods of slow acceleration which we all know is not efficient for fuel economy.
So what do you say, is there one process that's more efficient than the other...or are they both relatively the same and trying to find a difference between the two is like splitting hairs?
01-08-2013, 10:43 AM
#3
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It's a mix of RPM and engine load. The higher the load, the more fuel delivered, no matter what gear. So it's really a combo of what you both said. Lugging a car at WOT in 6th gear at 1200RPM is not saving you any fuel over 2200 RPM, in 4th gear at 30% throttle.
Acceleration is by the far the biggest user of gas. I don't think taking 10 seconds to get up to 35mph saves much more (if any) gas than accelerating it in 6. However, in 2 seconds and I bet you will use more.
If you have a car with an instant gas gauge, you can test my theories out quite easily. Too bad 4th gens do not have them.
Again, throttle input & load on the engine also have to be factored into the equation.
Acceleration is by the far the biggest user of gas. I don't think taking 10 seconds to get up to 35mph saves much more (if any) gas than accelerating it in 6. However, in 2 seconds and I bet you will use more.
If you have a car with an instant gas gauge, you can test my theories out quite easily. Too bad 4th gens do not have them.
Again, throttle input & load on the engine also have to be factored into the equation.
01-08-2013, 12:48 PM
#4
From a lot I have read low rpm/high throttle opening scenarios minimize pumping losses. Hence the super tall 6th gear/skip shift in these cars. They are vastly more efficient at lower rpms. I would bet your friends method would produce a better result. Whether that amount is statistically significant or easily measurable is up for debate.
01-09-2013, 07:56 AM
#7
Car and Driver took a bunch of their engineers a while back and played with these techniques. They have written a few good summaries on coasting and lugging: http://www.caranddriver.com/columns/...-lug-and-coast
Even though your buddy is right about lugging, you can pop in your new found knowledge of coasting.
By coasting to stop signs or coasting while you wait for a light to change, you can save a ton of gas vs. driving the speed limit (or faster) up to a stop and then hitting the brakes hard. (This dissipates all of that energy you've spent on gas to get the car moving in to heat going out through the brakes.)
Even though your buddy is right about lugging, you can pop in your new found knowledge of coasting.
By coasting to stop signs or coasting while you wait for a light to change, you can save a ton of gas vs. driving the speed limit (or faster) up to a stop and then hitting the brakes hard. (This dissipates all of that energy you've spent on gas to get the car moving in to heat going out through the brakes.) Trending Topics
01-18-2013, 01:30 PM
#8
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i've discussed this with my dad before, and we concluded that filling the cylinder by operating at wide open throttle your engine is at its maximum efficiency. I always accelerate at wide open throttle (not redlining, just upshifting when I hit 2k rpm or so) and get up to 6th once i'm above 30mph and reach cruising speed. I typically get 24+ mpg in mixed city and highway and about 20 in city. I wish my 6th were 0.25:1 so i'd be at wide open throttle on the highway and at maximum efficiency 
anywho, I think you are more correct, accelerating hard up to your highest gear. Your friend saying be light on the accelerator means he could be gassing it more and upshifting much earlier. Maybe carburetor's venturi would send too much gas into the intake immediately, but I can't believe fuel injection would have that problem.
anywho, I think you are more correct, accelerating hard up to your highest gear. Your friend saying be light on the accelerator means he could be gassing it more and upshifting much earlier. Maybe carburetor's venturi would send too much gas into the intake immediately, but I can't believe fuel injection would have that problem.
01-18-2013, 02:09 PM
#9
i've discussed this with my dad before, and we concluded that filling the cylinder by operating at wide open throttle your engine is at its maximum efficiency. I always accelerate at wide open throttle (not redlining, just upshifting when I hit 2k rpm or so) and get up to 6th once i'm above 30mph and reach cruising speed. I typically get 24+ mpg in mixed city and highway and about 20 in city. I wish my 6th were 0.25:1 so i'd be at wide open throttle on the highway and at maximum efficiency
anywho, I think you are more correct, accelerating hard up to your highest gear. Your friend saying be light on the accelerator means he could be gassing it more and upshifting much earlier. Maybe carburetor's venturi would send too much gas into the intake immediately, but I can't believe fuel injection would have that problem.
anywho, I think you are more correct, accelerating hard up to your highest gear. Your friend saying be light on the accelerator means he could be gassing it more and upshifting much earlier. Maybe carburetor's venturi would send too much gas into the intake immediately, but I can't believe fuel injection would have that problem.
If you want to accelerate efficiently, look and see what a modern automatic transmission does. These are programmed for maximum efficiency by engineers and are thoroughly tested. Steady and slow at a high gear and RPM just above idle will be the most efficient.
01-18-2013, 04:34 PM
#10
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I always got best mileage in my LT1 m6 by letting skip-shift do it's thing and trying to go through the least number of gears possible. I hardly ever actually used 3rd gear unless i was racing. Either went 1-4 or 1-2-4. In other words trying to keep rpms low.