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Old August 29th, 2010, 20:23
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Arrow General Manual/Gear/Mileage Question...

I have a question, maybe someone here has more know how to be able to answer this question and settle a slight argument...hahaha

The question...
Is a 6 Speed Manual car, at cruising speeds most fuel efficient at the 6th gear ?

My friends argument is at that speed (~75mph) with that gear your at the lowest RPM range and thus using the least power from your engine.

My argument is every engine has an RPM range that's most efficient and that isn't necessarily the highest gear at cruising speeds ?

Anyone care to comment, Maybe some numbers too ?

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Old August 29th, 2010, 20:50
joe_peters joe_peters is offline
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Re: General Manual/Gear/Mileage Question...

Think of it as throttle position. The less throttle, the more fuel efficient.

Cruising along in 6th gear, 1/4 throttle, is pretty fuel efficient. Trying to maintain the same speed into a headwind at 1/2, 2/3, or full throttle while in 6th, less efficient. Downshift to 5th, use less throttle to maintain speed, more fuel efficient than using 6th.

You can buy vacuum gauges marked with fuel economy ranges.
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Old August 30th, 2010, 08:44
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Re: General Manual/Gear/Mileage Question...

Are you saying using 5th a cruising speeds would be more efficient ?

Do you happen to have any mathematical reasoning behind it out of curiosity ?

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Old August 30th, 2010, 08:48
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Re: General Manual/Gear/Mileage Question...

no, he is saying that using less throttle is more efficient, not lower RPMs. If you are "lugging" the engine (running in a lower gear than you should, with a lot of throttle) you are (a) not doing the engine any favors and (b) wrecking your gas mileage.
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Old August 30th, 2010, 09:19
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Re: General Manual/Gear/Mileage Question...

also meaning you might be more fuel efficent in 3rd gear crusing at 25mph.
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Old August 30th, 2010, 13:45
srmitchell srmitchell is offline
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Re: General Manual/Gear/Mileage Question...

My xj with 4.56's now spins at about 2450 rpm at 65. Im probably getting 18 mpg on flat ground. I would call this 5th gear.

I used to get 20 mpg highway with 3.55's, but add any hills at all, and I would use so much throttle, that I would average 12-14 mpg. Think of that as trying to go up a huge hill in 6th, you will lug the snot out of the motor.

More gearing= less stress on the motor, which probably translates into less throttle input.
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Old August 31st, 2010, 13:02
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Darky Darky is offline
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Re: General Manual/Gear/Mileage Question...

I think it was Car and Driver who posed this experiment: If driving a stick, accelerate at 3/4 or greater throttle, while short shifting (2000 rpm or so). Their theory was that acceleration is the least fuel efficient thing you can do (aside from revving it in neutral for long periods of time), so the faster you get to cruising speed and can cease accelerating, the better your mpg. However, conversely, if you're redlining it, you're burning excess fuel there too, hence the short shifting. They tried it and saw small improvements. I tried it in my old Focus, and saw about 1 mpg better than driving normally. It was also more fun.
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