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Old 01-08-2015, 12:14 AM   #1
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Sweet spot crusing speed

Most cars have a "sweet spot" cruising speed, a balance of speed and economy. It was long thought that 56 MPH was the best speed to cruise at for optimum economy, but I'm not sure that still stands today. A slower speed and you will be using more throttle and take longer to reach your destination, a higher speed means higher revs but less throttle, there will be more wind resistance but it will take you less time to get there.

As I don't use highways very often, I'm just wondering what kind of cruising speeds people stick too for best economy? 60, 70, 80 MPH, or anywhere in between? Are there any mathematical calculations you can do to help work it out?
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Old 01-08-2015, 06:09 AM   #2
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When driving my car I tend to stay at about 70MPH which puts me right about 2k RPMs in 5th gear.
Granted, I work from home now, and don't really use my car that often anymore, but when I was commuting back and forth to school years ago, it was a good balance between speed limits, traffic flow, and economy.
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Old 01-08-2015, 07:14 AM   #3
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Thats the kind of speed ive been experimenting with, given the limit on the highway is 70 MPH, although 90% of people stick to 80. I got a steady 74 MPG at 70 MPH, but as soon as I came off the highway and slowed down, the economy dropped which I thought was weird!
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Old 01-10-2015, 02:42 PM   #4
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My '15 Fiat 500, 1.4 Multiair 5 Speed prefers around 50 mph for best MPG... :/... 60mph can still hit the target of EPA Rated Highway of 40mpg. 65mph is "acceptable", and I can pull 35-37mpg or so. 70mph and it drops to about 33mpg. 75mph and I'm down to about 31mpg.

This is sad, of course, because speed limits on my rather lengthy (80 mile each way) commute vary from 65mph to 75mph. I actually IMPROVE my mileage during city driving, if I keep my foot out of it. Freeway time at 75mph just kills it. I wonder if a 6th gear would have made a difference? Or is it simply that the 500 is too much of a brick at these speeds?
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Old 01-10-2015, 05:07 PM   #5
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I stick to 60mph, that's a solid 45mpg+, if I drop to 50-55mph, I can net 50mpg with a bit of work. But as the most frequent freeway trip I take is mostly 75mph speed limit, 55 is just too slow to handle.

There's no stock calculation you can make, it varies too much for one rule to cover all vehicles. The best you can do is find a reasonably long flat spot on which to do some A-B-A testing at various speeds. A ScanGauge or UltraGauge is critical for that, however.
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Old 01-11-2015, 01:23 AM   #6
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Originally Posted by aelfwyne View Post
My '15 Fiat 500, 1.4 Multiair 5 Speed prefers around 50 mph for best MPG... :/... 60mph can still hit the target of EPA Rated Highway of 40mpg. 65mph is "acceptable", and I can pull 35-37mpg or so. 70mph and it drops to about 33mpg. 75mph and I'm down to about 31mpg.

This is sad, of course, because speed limits on my rather lengthy (80 mile each way) commute vary from 65mph to 75mph. I actually IMPROVE my mileage during city driving, if I keep my foot out of it. Freeway time at 75mph just kills it. I wonder if a 6th gear would have made a difference? Or is it simply that the 500 is too much of a brick at these speeds?
I used to have an abarth 500, the 500 obviously started life as a city car, so it's gearing is more suited to urban driving. I did have a normal fiat 500 when mine was in the garage once, non multiair, and the economy was worse as it had no turbo, so you had to work it much harder to get up to speed. Not an issue in the multiair though, I think in time you will get better MPG. On the day I traded in my Abarth, I was getting close to 60 MPG (about 50 US MPG)
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Old 01-11-2015, 01:26 AM   #7
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Originally Posted by Jcp385 View Post
I stick to 60mph, that's a solid 45mpg+, if I drop to 50-55mph, I can net 50mpg with a bit of work. But as the most frequent freeway trip I take is mostly 75mph speed limit, 55 is just too slow to handle.

There's no stock calculation you can make, it varies too much for one rule to cover all vehicles. The best you can do is find a reasonably long flat spot on which to do some A-B-A testing at various speeds. A ScanGauge or UltraGauge is critical for that, however.
I guess if I keep the revs at around 2k, or just below if possible, then I should get good economy. When I learnt to drive, the intructor told me the best time to shift was 2500 RPM, but cruising, less revs will be more efficient.
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Old 01-11-2015, 01:37 AM   #8
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A slightly taller final gear might make most cars give sluggish acceleration but that can be overcome with smarter automatics transmissions and manual transmission drivers. That's why I'm disappointed Mazda didn't see fit to give my manual transmission at least as tall a final gear as the automatic. Maybe even taller since manual drivers should be given enough credit to assume they'll know enough to drop it down when they need some speed. I suppose they won't figure that out before the cars start driving themselves. At that point we should see a big jump in average fuel economy since the dumb drivers will only be texting instead of driving their fuel mileage down.

When cars start talking to each other a lot of MPG possibilities will open up. I just hope the network handling the communications don't get hacked.
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Old 01-11-2015, 07:31 AM   #9
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Maybe even taller since manual drivers should be given enough credit to assume they'll know enough to drop it down when they need some speed.
Part of the reason Mazda and others may not have put a taller final in the manual is because manual reviewers aren't smart enough, or they are but word it such that the need to downshift is a bad thing.

There are manuals with tall finals out there, even in the US. The six speed in the Sonic has taller 5th and 6th than the automatic.
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Old 01-14-2015, 10:34 AM   #10
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That's why Porsche has a 7-speed manual now in the 911... they had to get that 7th highway cruising ratio to get the economy reasonable vs all the other terrible choices in the 6-speed close-ratio racing gearbox.

My Subaru STi, while not an economy car by any stretch of the imagination, is really irritating on highway trips because bringing it up to 110 km/h or higher comes with a noticeable fuel economy penalty. The sixth speed is way too short for economy (fifth in the same year 5-speed WRX is taller than 6th in the STi).

My "sweet spot" is only like 90 km/h and is still 9 L/100 km (31 mi/UK gal), lol.
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