[enkeivette]

Depends on the OD ratio/ final drive ratio and the power band of the motor. You get the best gas mileage in the highest gear at the lowest speed (rpm) that creates the most vacuum. Correct me if I'm wrong here, but I'm not. Vacuum level is inverse to the load demand on the engine, this takes into account wind resistance, etc.

My Vette gets best gas mileage at about 90 mph, my Neon... probably 55 mph. I can cruise at a slower speed in 6th gear in the Vette, but there is more load (lug) on the engine, consequently the vacuum isn't quite as high.

Low geared V8 trucks out there proabably won't even shift into the OD gear at 55 mph. My Dad's Avalanche is around 1800 rpm at 80 mph if I remember correctly.

[SeanPlunk]

Originally Posted by enkeivette

Depends on the OD ratio/ final drive ratio and the power band of the motor. You get the best gas mileage in the highest gear at the lowest speed (rpm) that creates the most vacuum. Correct me if I'm wrong here, but I'm not. Vacuum level is inverse to the load demand on the engine, this takes into account wind resistance, etc.

My Vette gets best gas mileage at about 90 mph, my Neon... probably 55 mph. I can cruise at a slower speed in 6th gear in the Vette, but there is more load (lug) on the engine, consequently the vacuum isn't quite as high.

Low geared V8 trucks out there proabably won't even shift into the OD gear at 55 mph. My Dad's Avalanche is around 1800 rpm at 80 mph if I remember correctly.

Not sure, but I HIGHLY doubt your Vette gets it's best mileage at 90mph. Remember wind resistance plays a hugh roll as speeds increase.

[Vettezuki]

Calculating the effects of Frontal Area and COD is not trivial. However, I could say that the frontal area, COD and lighter overall weight of the Vette means it could travel at higher speed with less of an Aero penalty than a more blocky car. I'd also say that most people burn up far more fuel accelerating unnecessarily than they do cruising at too high a speed.

[enkeivette]

Originally Posted by SeanPlunk

Not sure, but I HIGHLY doubt your Vette gets it's best mileage at 90mph. Remember wind resistance plays a hugh roll as speeds increase.

Wind resistance factors in to engine load/ lower vacuum. The increased wind resistance will cause your mileage to get worse not directly, but as a result of making your engine work harder. This can be seen by reading your vacuum/ boost guage. Because my car is geared so low and has such a big OD ratio, there will be more lug on the engine (at a lower rpm in 6th gear at 75mph) than at 90 mph. Once wind resistance becomes a factor again (at speeds higher than 90 mph when the vacuum guage reads less vacuum) then the engine will be working harder, and therefore be less efficient.

It's really simple, your vacuum guage takes into account EVERYTHING (excluding air to fuel ratio, but this SHOULD be constant) including wind resistance. It is simply telling you how hard your engine is working. Therefore, the more vacuum you see on the guage, the better gas mileage you will get (at the lowest rpm possible). Have you seen some new cars that display a read out of the gas mileage on the dash? Those use formulas to determine the mpg, using a reading from a vacuum guage as the variable.

With the smaller diameter tires it might be able to pull highest vacuum at 85 or so mph. I haven't been paying attention. But it's still above 80 for sure. If I were to throw Ben's tires on my car, it would go back up to 90. But, Ben's car will get the best mpg at a slightly lower speed, since he has a higher diff ratio.

[BADDASSC6]

Sorry dude, but your wrong. The lug that you feel on the engine isn't because there is more load on the engine. It's because the RPMs are way way down and the motor isn't making as much torque at 1100 rpm (approximately 55 in sixth for my car) versus 2000 rpm (80 mph my car). Also take into consisderation that as speeds increase the force of wind resistance increases exponentialy. My car gets ~23 mpg average going from SD to Phoenix when I keep it at or above 90 mph. It averages about 28 if I keep it around 75 mph.

[enkeivette]

Originally Posted by BADDASSC6

Sorry dude, but your wrong. The lug that you feel on the engine isn't because there is more load on the engine. It's because the RPMs are way way down and the motor isn't making as much torque at 1100 rpm (approximately 55 in sixth for my car) versus 2000 rpm (80 mph my car). Also take into consisderation that as speeds increase the force of wind resistance increases exponentialy. My car gets ~23 mpg average going from SD to Phoenix when I keep it at or above 90 mph. It averages about 28 if I keep it around 75 mph.

Very true, there isn't more load on the engine, my bad. But the engine is less efficient if you lug it (as you said, it's making less power) this is why the vacuum guage will read less vacuum. Semantics aside, you will not get the best gas mileage lugging your engine.

Your C6 does better as a lower speed as you have a higher diff ratio than I do. Just like Ben's C3.

Once again, mpg are calculated by using vacuum as the variable. And I'm sure there's no doubt that lugging your engine will cause the engine to make less vacuum.

BTW I've been able to get well over 20mpg at 90 mph coming back from SD, but always less than 20 at 70mph.

[enkeivette]

BTW, I could agree that your car might do better at 75 mph than at 90mph but I'm willing to bet that your C6 will get better gas mileage at 75 mph than it would at 55mph. I don't think your LS2 would be very happy with a 3.73 diff ratio and a .5 OD.

[Vettezuki]

Originally Posted by enkeivette

. . . (excluding air to fuel ratio, but this SHOULD be constant) including wind resistance. . . .

Que? My A/F is dialed in pretty good. As I recall it's 14.x:1 < 2,000 rpm and like 12:1 at >4,000. That's not a trivial difference. However, is relatively constant and lean at lower rpms if that's your point.


[QUTOE]It is simply telling you how hard your engine is working. Therefore, the more vacuum you see on the guage, the better gas mileage you will get (at the lowest rpm possible). Have you seen some new cars that display a read out of the gas mileage on the dash? Those use formulas to determine the mpg, using a reading from a vacuum guage as the variable.[/quote]

I would imagine it's also using duty-cycle readings from the Fuel Injectors. After all, that's giving a pretty accurate reading of the actual amount of fuel that's flowing.

[SeanPlunk]

Originally Posted by enkeivette

BTW, I could agree that your car might do better at 75 mph than at 90mph but I'm willing to bet that your C6 will get better gas mileage at 75 mph than it would at 55mph. I don't think your LS2 would be very happy with a 3.73 diff ratio and a .5 OD.

I bet you're wrong. I bet his car would get better mileage at 55 mph than 75 mph.

[enkeivette]

Good argument Sean.

You guys don't seem to be getting the point, different cars are geared differently. Different cars, have different motors. It's foolish to believe that 55 mph is a magical number for all cars, whether the C6 can do it or not. My motor, in my car CANNOT cruise at 55mph in 6th gear. IT CANNOT. My big duration cam motor lugs so bad at 55 mph in 6th gear that it requires so much throttle to maintain that speed that it will literally lean spit, case in point. Are you going to try to make the argument that I'd get better gas mileage in 5th gear at 55mph, (which would also lug BTW) Than in 6th gear at a higher speed?

55mph, is NOT a magical number, it is NOT where ALL cars get best gas mileage. Don't know what else to say.

If anyone disagrees with my understanding of the conditions that yield the best mpg, please explain why and provide your own set of conditions. Not going to play "No it's not!" "Yes it is!" Thaaat gets old.

And yes Ben, that's what I meant about the AF ratio. Obviously if the car were richer down low and leaner up top, the lowest rpm at the highest vacuum might not be the best spot to cruise. But all cars should have a consistent AF ratio at part throttle speeds.