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Discussion Starter #1
I have now had my spider (MT) for a few months and at 13,000 I feel it is fully broken in. I have noticed my best mpg is in 5th gear until 100km/h (62mph). Most non freeway roads only go up to 80km/h (50mph).

I have done a few tests comparing 5th and 6th gear and if my fuel economy (car reading not my own calculation) is 6.0L/100km going 85km/h in 6th gear. Dropping down to 5th would read 5.4L/100km. At 100km/h they would both be 6.0L/100km and at 120km/h I would get 6.2 in 6th and 6.8 in 5th. The difference between the gears is the same with and without cruise control.

I normally drive in 6th at speeds above 80km/h, out of habit more then anything as I have always assumed the lowest gear with enough power (which this car has in 6th by 80km/h in my opinion) is best for longevity of the engine.

All that said, given how I drive, I am very happy with the efficiency of this car and am more perplexed then anything. Has anyone else experienced this? At what cruising speed do other people make that last shift to 6th?
 

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I tend to select between 5th and 6th based more on how much the engine has to pull at what speed and what rpm. If the speed has to go back and forth due to traffic volumes and flow, I'll stick to 5th. If its smooth running on a freeway or highway, I move to 6th. I think of 6th more as an overdrive that provides better fuel economy, but doesn't deliver as much in the performance aspect. In the west, we have a lot of long, straight roads, and even going through the mountains, I've been surprised at the excellent fuel economy I've gotten. On a run from Calgary to Jasper, I got well over 700km out of a tank. Great little car that loves twisty, turny roads... so the Rockies are great for that.
 

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Discussion Starter #3
Yes I understand a vehicle needs a lower gear for "more" power. Obviously anyone behind the wheel should know that.

Let my try again. On those drives where you go an uninterrupted constant speed on a flat road my best fuel economy is closer to 3000rpm then 2000rpm.

I am fine with this but it is contradictory to other things I have read on this site.
 

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Hmm. Very interesting findings. Conventional wisdom would say that the 1,000 extra RPM should burn more fuel since the crankshaft is literally rotating 1,000 RPM more per minute. The only thing I can think of is that the A/F curve is closer to 14.7/1 (stoichiometric) at 3,000 RPM and richer at 2,000 RPM, hence the better fuel economy at a higher RPM. 6th gear is an overdrive ratio technically (0.844 ratio). 5th gear is a true 1 to 1 ratio. Maybe a tuner could chime in here with his or her expertise?
 

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There should be slightly better mechanical efficiency in 5th gear since the power transmission doesn't pass through the two gear meshes as in all other gears, but passes straight through the tranny. Not sure if this is measurable though.
 
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