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Old 05-03-2018, 02:02 PM   #11 (permalink)
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100 mph is not particularly common here, but speeds in the 90's are (rural freeway speed limit is 80). I think many drivers feel that crossing the three-digit threshold will incur greater wrath from the State. Not a bad assumption.

The Nissan Altima with a 4-cylinder and CVT gets in the upper 30's to low 40's at freeway speeds here (altitude around 4 to 5 thousand feet; usually very dry air, especially on rural freeway). The Camry is a little less efficient at speed (drove a rented Camry from SLC to Las Vegas and back. It averaged about 38 mpg at 80 to 85 mph most of the way.

Engine and drivetrain efficiency are big factors these days. 6 and 8 speed auto-shifters or CVTs can make former gas hogs sip fuel. The Subarus are a good example of that.

I have seen Prii going very fast on the freeway. I raced one, unintentionally, from Auburn to Reno on I-80 and the Prius was doing 85 comfortably up the grade. I was driving a rented Hyundai Elantra (a pig in crosswinds or headwinds with a drop in mpg to low 20's or even high teens). (A heads up to cross country travelers. Driving a rental car at the posted speed limit (I always slow down for cities and towns) while reclined, wearing shades and sporting a full beard will get you facially profiled in Nevada. I didn't get a ticket, but I had a very long chit-chat with the nice officer before he let me go.)

Cars that have a good Cd do not necessarily perform well cross-country or at high speeds. Cross-winds and head winds, as well as shifting and gusty winds are often not replicated in wind-tunnels, especially for cars (and when the Cd is to be used as a marketing tool), so what performs well in the lab does not always do well in the real world.

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Old 05-03-2018, 10:07 PM   #12 (permalink)
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THE Altima I had got about 30-32 on the freeway here in WV, going 80mph... it was the QR25DE/CVT model and at that speed it turned like 2400 RPM...
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Old 05-04-2018, 02:10 AM   #13 (permalink)
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Quote:
Originally Posted by 19bonestock88 View Post
THE Altima I had got about 30-32 on the freeway here in WV, going 80mph... it was the QR25DE/CVT model and at that speed it turned like 2400 RPM...
Thin air does make a difference, and long flat, straight stretches. Not much of that in WV, iirc (I had in-laws in Nitro). I can get around 30 mpg in my T&C doing 80 mph here, unless there is a strong headwind or crosswind, which is fairly common. A good tailwind pushes it into the low 30's, but the return trip more than cancels it out. Not rushing up grades, of which we have many long and steep ones, keeps the overall trip efficiency up.

A big car with a big engine can get pretty good gas mileage. I drove an old Impala (Sixth generation) to the Bay Area and back in the '80's. It wanted to go 80 and got significantly better gas mileage at that speed (I think around 18 mpg, to the chagrin of my cousin who insisted on driving his turn at the posted 55 mph and got crappy(er) mpg). I drove a Pontiac station wagon of similar vintage and model (with the towing package) back from the Bay Area and it cruised quite comfortably at 90 to 100 mph -- until I got a ticket 20 miles outside Austin, Nevada. The patrolman was somewhat apologetic, but said it was a safety issue, as there were free range cattle in the next valley. He was correct. I could see them from ten miles away.

Again, thin air makes a big difference and most of my driving has been at 4,000-5,000 feet in semi-arid to arid environments (daytime humidity levels commonly below 10 percent outside the Wasatch Front).
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Old 05-04-2018, 02:40 AM   #14 (permalink)
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It’s the hills that kill my highway(freeway) mileage... my Saturn averages about 36mpg on the freeway, running 75-80mph but for the brief moments the road becomes flat, my instant MPG swings up into the forties
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Old 05-04-2018, 07:17 AM   #15 (permalink)
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I am curious if an audi a5 or Tt or a posche would yield better FE in this application.
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Old 05-04-2018, 08:11 AM   #16 (permalink)
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Id personally go with a diesel A5, given those three choices...
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Old 05-04-2018, 10:08 AM   #17 (permalink)
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I was wondering if that type of car woult trump a 1.2 tdi VW polo.
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Old 05-31-2018, 11:49 AM   #18 (permalink)
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My understanding is that diesels (especially the bigger ones) dominate the Autobahn. They might not be quite the fastest, but if you have to use it a lot it becomes a lot cheaper with a big diesel.

In the US, GM LSn V-8s have been great at this, although other cars now seem to have the huge overdrives as well.

Good aero is key. Aero losses will double over 70mph. Unfortunately while coefficient of drag is typically published, frontal area is not. Car companies would rather sell a CUV or SUV, while you are obviously looking for a car.

To find a car that will operate in the BSHP "island" try to find a car that has an unlimited maximum speed of ~150mph. BSHP tends to operate at 30%-50% engine load, and 100mph should be ~40% of the load needed for 150. Unfortunately most cars that can do this have limiters here or lower. [Any guesses? I remember 80s muscle cars having top speeds in the 140s and hp between 200-240, but hopefully aero has gotten better. And don't count on those hp/speed numbers being quite the same era].

Ideally you should have a gear that is at near peak torque at 100mph, or slightly lower. That should be right about the center of the BSFC island, but you don't want to be so high in revs that you wear out your engine.

CVTs aren't quite as impressive as we might hope. Que Jason Fenske:

Of course, this implies that Toyota CVTs with said gearing shouldn't have trouble at tall gearings. Still, look for tall gears that put you in the BSFC island at 100mph.

Good luck!
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Old 05-31-2018, 12:04 PM   #19 (permalink)
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My understanding is that the island of efficiency is more like ~75% rather than 30-50%.


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Old 05-31-2018, 12:13 PM   #20 (permalink)
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Specific models are one way to make recommendations. Another way is to suggest design characteristics to seek out. Seek taller gearing and lower drag coefficients. I think you want a car with drag below 0.30 & preferably as low as the Prius. And you would want the tallest gearing you might get, as found in the Chevy Cruze Eco in the USA or any CVT equipped car stateside or in Europe. Basically, keep RPMs & drag down as much as is practical.

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