engine just that posted speeds own unusual to the on as it at a
I believe people who claim to experience significantly better fuel economy at 78 than at 65 are deluding themselves (and I am including you in this group). Unless your car has an engine that is completely screwed up, there is very little chance that it is so much more efficient when running at 78 mph that it can overcome the increased aerodynamic drag (relative to 65 mph) to achieve higher fuel economy. And your claim that you get 7 mpg better fuel economy at 78 than at 65 borders on insanity. I unfortunately don't know what sort of car you are driving, so I suppose there is just the tiniest possibility that it is true, but it seems much more likely that an asteroid will strike the earth this afternoon.
Any decent production car will have an engine that has a relatively flat bsfc over the normal operating range. While it won't be perfectly flat, for a given set of parameters (throttle opening and rpm) it is not going to vary so wildly that it will be able to compensate for the approximately 30% more power required to go 75 versus 65.
I am so confident that I am right, I will propose the following bet:
1 - Your car must be a conventional car less than 10 years old, unmodified, and in good condition 2 - You will meet me at the intersection of US 258 and US 64 in Tarboro, NC early on a Sunday morning. 3 - We will fill your car up at the Exxon station near the intersection. We will fill it up until the tank actually runs over from the filler. 4 - We will head east on US 64 and maintain a steady 78 mph (the speed limit is 70, but it is unlikely the police will bother us if we only do 78). 5 - When we reach the next stop light in Williamston, NC (approximately 35 miles,at the end of the 70 mph speed limit segment), we will do a U-turn and drive back to Tarboro at the 78 mph (except as necessary to slow down for the U-turn and exit back to the gas station). 6 - We will return to exactly the same pump at the Exxon station and refill the car's tank as in step 3 7 - We will repeat steps 4 and 5, except the steady state speed will now be 65 mph 8- We will return to the Exxon station and fill-up exactly as in steps 3 and 6
If the car get as much as 7 mpg better when driving 78 (compared to driving 65) after comparing the results for the two runs, I will give you $500 on the spot.
When can I expect you to come?
BTW, your barbell analogy is lame. If your car goes zero miles per hour, it won't use any fuel (well, none if it is turned off, very little if it is idling). If you try to hold the barbell out in front of you without moving it- your arm will fall of. The human body generates power differentially than a car.
Ed
