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I've found out that my car is capable of doing about 2× of the city mpg if driven on highways at 50—60mph in place of 70—80mph (going 80mph appears to retain the city-spec'ed mpg in my 2.5L Jetta).

Travelling between some areas of the US is rather straightforward in that there is only a single Interstate to take, which usually provides the best (and sometimes only) route; however, what's the best approach if I'm travelling between something like Phoenix and Austin, through Albuquerque, where neither Phoenix nor Austin are directly connected with any West-East Interstates?

I've noticed that Google Maps has an option of avoiding highways, however, it's not very clear whether or not that's something that I want. My car is still a normal passenger car, so, I'd still prefer going through roads with quality pavement, just not at 80+mph, so, the best route may not be the most straightforward one that's designed for passing at 80mph.

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    Interstates have the advantage that they're always at least two lanes in each direction, so even if you drive slow, faster traffic can pass you. Other routes, e.g. US 60, are often just one lane in each direction. So even if the speed limit is a bit lower, you'll actually be obstructing traffic that wants to drive that fast - which may invite unsafe passing or road rage. Or you'll have to pull over frequently, which will surely not help your mileage. – Nate Eldredge Mar 5 '18 at 3:38
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    Safety trumps economy in my opinion - you'll be best to take Interstates and drive a few mph below the speed limit if you like (lower, of course, in poor conditions). 70 mph in an 80 zone wouldn't be too horrid. Whatever you do, ensure you stay in the driving lane (right), not passing lane, and let faster traffic pass you. And be sure to adjust your speed as needed to accommodate merging traffic. If you really insist on much slower speeds, drive in poorer weather where that's a safe and prudent speed. – Jim MacKenzie Mar 5 '18 at 3:52
  • @JimMacKenzie, yes, of course; coming from someone who used to drive 80+, I'm obviously going to take reasonable care to ensure not to intentionally (or by omission) obstruct my prior selves. :-) Plus, even though the roads are designed for 80+, they're still usually posted at just 70mph or 75, which I'll likely cruise at 15 under, in the left lane. – cnst Mar 5 '18 at 3:58
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    At that speed I would prefer the back highways rather than the interstates. – Michael Hampton Mar 5 '18 at 4:57
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    The posted speed limit is not what traffic flows at on either I40 or I10 - that is more like 80 out there. Now, the drive Phoenix to Holbrook, or Phoenix to Socorro on back roads is quite beautiful and doesn’t have folks going 80 trying to tear-end you while they watch the scenery go by... – Jon Custer Mar 5 '18 at 23:00
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There is no single easy way to do this. You will have to construct routes yourself, although websites like Google Maps are certainly going to be useful. (You can use Street View to glean speed limits and pavement condition, for example, although not all areas are updated very often. Some highways in nearby parts of Montana to me haven't been updated in Street View since 2008.)

However, I'd caution you to some degree against this approach as a blanket approach. Certainly, sometimes, the secondary highways are perfectly pleasant to drive and quite safe. However, it's always more dangerous to drive on undivided highways than on grade-separated divided highways (dual carriageways).

Another consideration is that if you drive significantly slower than the general flow of traffic, you'll actually be a hazard, creating bottlenecks. This runs the risk of creating road rage, but even with other drivers being considerate, you're slowing their journey for your own benefit.

On the plus side, the secondary highways often have nice scenery in some parts of countries, and you have more ability to stop if you see something that's of interest for you (e.g. to take photographs).

You can go under the speed limits on controlled-access freeways and motorways, like the US Interstate highways, but unless traffic is light or road or weather conditions poor, I'd counsel against it. On quiet divided highways, people can pass you easily, but on congested ones, again, you're causing a bottleneck.

You might be better to choose your moments when to drive in a relaxed fashion and when not to.

If fuel economy is your primary goal, bear in mind that there are other ways to improve fuel economy that will have fewer harmful effects on traffic flow (such as removing unnecessary weight from your vehicle, keeping the engine in prime operating order, maintaining the inflation of your tires, avoiding add-ons to your vehicle that change its aerodynamic profile, and accelerating and braking smoothly and carefully instead of abruptly or repeatedly).

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