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On a short road trip, I created a chart of fuel economy at different speeds to determine the ideal 'cost at speed' to operate the car over long distance.
I'm a bit of a data nerd, so this is going to be overkill.

Car: 2019 Buick Envision Essence: 2.4L, FWD. Speeds from 32mph to 92mph in increments of 4mph.

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Immediate observation: There seems to be an 'upper limit' of 44.5 mpg, achievable at all speeds bellow 52mph.

Examples of where this information can be applied.
My Example, a 280 mile drive to Chicago from Detroit. I want to know how much it will cost me to arrive 'sooner', or how much money I can save if I just drive slower:
-- If I normally drive 72mph Avg: I'll arrive at 3hrs 53 minutes. (this will be my baseline)
(baseline):
At 72mph, I'll use 280/34.9= 8.02 gallons, or $32.10
-- If I drive 80mph Avg: I'll arrive at 3hrs 30 minutes (saving 23 minutes!). But how much gasoline will it cost me?At 80mph, I'll use 280/29.1= 9.62 gallons, or $38.50
Meaning: I'll save 23 minutes, but spend $6.40 more in fuel. This is a valuable proposition, if I was running late for a flight.
-- What if i'm really late?
travelling at 92 mph:cool:? well, i'll spare the math. I'd save 47 minutes over baseline! but would use $16.38 additional in fuel over baseline. (not to mention the speeding ticket!)
-- Conversely, what if I wasn't in a hurry? If I drive 60mph, i'll use 280/41.1= 6.81 gallons, or $27.25 (but, it will take 4 hrs 40 minutes - Wasting 47 minutes!!!!)
Meaning: It'll take 47 minutes longer than baseline, and i'll only save $4.85 in gasoline from baseline. For me, sitting in the car 47 more minutes to save $4.85 isn't worth it.


Detailed Info:
Fuel: Speedway Gas, Regular 87. No% ethanol, 'Summer Blend'.
Environment: 1200' ASL, Temp 50F, No substantial elevation changes during test. All areas tested are 'basically the same'.Illegal parts of this test conducted 'somewhere in mexico'. All other parts conducted somewhere between Chicago and Detroit, in Michigan.
Car/Settings: 2019 Buick Envision Essence 2.4L fwd. Climate Auto (Heat+AC for defogger) @72. Cruse Control On.
Route Factors: Speeds 32mph-58mph are done on rural highways.Speeds 62mph-92mph are done on interstate freeways. This is what I'd consider acceptable, as fuel economy should be recorded on the route which is suitable for the speed traveled (you'd never go 45mph on the freeway, so the test must be on a rural highway)
Testing Parameters: Each speed is set to cruise control, then once at speed the 'trip ODO is reset' after 5miles* the economy is recorded.If any disruption was recorded (stop sign, traffic interruption, anything that required disabling Cruise Control) the test is stopped, and restarted. *= speeds 88mph an 92mph were only recorded for 1.5 miles as traffic obstruction was an issue going at such speeds.
 
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