I'm on along weekend trip out to west texas.
Most driving was 80~100 mph with a few sections 110~120. I averaged 21 mpg, I 'm satisfied but wonder if I had kept @ 75 what improvement in mpg I might have seen.
wj
2008 xk conv

 

The last thing I'm concerned about is my XKR's mpg.
MPG takes a back seat to SPG (Smiles Per Gallon).


FYI ...

See: https://www.mpgforspeed.com/

 

I had a 1988 Thunderbird 5.0 which would get 36 mpg at 80-83 mph. Anything above or below that and the mileage would fall. I'd not seen a sweet spot that high before, but with many, many trips back and forth to Winnemucca NV all on Interstate 80, I had a lot of time and info to figure this all out. Gearing, torque and drag all came into synchronicity at that speed for me. Course, keeping my foot out of it made it between 25-28 anyway.

 

Just got a bit under 27 mpg on a 1400+ mile trip, almost all highway, Morgantown WV to Birmingham AL, so a good bit of hillwork. Speed was 75-80 most of the time. NA 5.0L engine, 93 octane gas.

As mentioned in a different post, front lower air lip was broken off halfway through the trip. Not sure if that had any effect, but I guessed not.

 

Mid 20's if I use cruise control @ 75 mph
High 20's using CC on country roads, @ 65 mph

 

I’ve tried this experiment and I agree with others, the “sweet spot’s” in the 65-75 mph hour range on all the V8s I’ve had.

 

Occasionally having the opportunity to go to the German Autobahns, I've experienced going between 200 and 250 km/h for long stretches. That sucks your gas tank dry very very quickly. It might be an exponential effect, I don't have the numbers, but seems like it.

But I agree, with cars like the ones we're interested in, it's about Smiles Per Gallon. Worrying about mileage, is like finally getting that date with the hottest girl in school and worry all evening about how to make the evening as cheap as possible...

 

That’s true, at 100 to 150 mph fuel economy drops to about 9 mpg. But the subject of the article’s how driving at freeway speeds has worse mpg than highway speeds and debated for decades. In my experience this’s simply not true.

 

It is surprising what Serious Speed -- 85-105+ mph-- does to MPG. (Especially top-down in a convertible)
On x-country drives, I have my own gauge, which is time. How long will a full tank last?
In Speed Country (NV, UT, rural CO, TX, etc), could definitely suck full tank dry in 3 1/2 hours.

 

same here, with either the XKR or the XJR, both with the 5.0L

Seems that there might be about 1 MPG difference between Mid-Grade and Premium, so I use mid-grade most of the time.

 

I forgot of those times, 7 or 8, when I put it in S and was clicking off some curvy sections @ 100 mph. As I stated, satisfied with the mpg.
wj

 

Kinetic energy (in plain English: momentum) varies like the square of speed (E = 0.5mv², to be precise).
And given any kinetic energy acquired by a car in motion comes from its tank at the end of the day...there you go. So it's not exponential, but squared.

 

Since I'm not as smart as others, can you give examples of say, 65MPH vs 80MPH? After that, how do your numbers equate to MPG?

 

I wish I could, but there are so many other variables involved (friction inside the engine and drag, gear ratio, etc.) between the tank and the speed.
Just keep in mind that consumption grows faster than speed.

Oh and BTW, you guys: the world is metric, just FYI.

 

The THIRD world is metric, just FYI

 

Denial is not a river in Egypt...just FYI.

 

Read the article linked in my Post #2, above.