rtd@gypsy.UUCP (01/08/86)
The recent discussion about speed limits, seat belts, etc. has
prompted me to post this note containing some questions I've
been wondering about for a while. As I recall, one of the reasons
that the 55 MPH limit was proposed was to reduce gasoline consumption
during times of (apparent) oil scarcity. And I seem to recall having
read reports indicating that nationwide gas consumption had been
reduced.
Now, the owner's manual for my '86 Celica (5 spd. manual transmission)
states that to maximize fuel efficiency, you should get into as high
a gear as possible as soon as possible.
Three questions come to me from all this:
1. If the engine is running at 2500 RPM, what difference does it
make what gear I'm in or how fast the car is moving?
2. If Toyota says that going faster saves fuel, and the U.S. Gov't.
says that going slower saves fuel, who is right?
3. If rapid starts and quick acceleration wastes fuel (another
U.S. Gov't. contention) then how can up-shifting as quickly
as possible save fuel?
Bob Dillberger
Siemens Corporate Research & Support
Princeton, NJdcn@ihuxl.UUCP (Dave Newkirk) (01/09/86)
> 1. If the engine is running at 2500 RPM, what difference does it > make what gear I'm in or how fast the car is moving? If you had a boost gauge you could tell that at 70 mph, the engine is working harder that it was at 30, even if the engine RPM's were equal. After you pass ~40 mph, wind resistance becomes important, and at higher speeds it the major resistance to overcome. It's like going downhill in gear (high vacuum) and then going up a steep hill (low vacuum) at the same revs. You're going to burn more fuel to get up the hill. > 2. If Toyota says that going faster saves fuel, and the U.S. Gov't. > says that going slower saves fuel, who is right? No matter what kind of car it is, your fuel consumption increases with speed. Depending on the aerodynamic drag of the car, the rate of increase differs. Since an Audi 5000 has less drag than a Chevy van, it could go much faster with same amount of fuel consumption. > 3. If rapid starts and quick acceleration wastes fuel (another > U.S. Gov't. contention) then how can up-shifting as quickly > as possible save fuel? I think the Gov't had automatic transmissions in mind here, since early up-shifting does help fuel economy in stop & go traffic. But early shifting doesn't mean spinning your tires at stop lights to get to second gear faster. Some moderation is required. In my Saab Turbo, I try to hold acceleration at or below the beginning of the boost range while up-shifting just under 2000 rpm. This seems to give you an extra few percent in economy. If you tried this with an automatic, it would work just the opposite. The harder you accelerate, the longer the transmission holds you in the lower gear. When you decelerate, then it up-shifts! -- Dave Newkirk, ihnp4!ihuxl!dcn
ags@pucc-h (Dave Seaman) (01/09/86)
In article <25300019@gypsy.UUCP> rtd@gypsy.UUCP writes: >Now, the owner's manual for my '86 Celica (5 spd. manual transmission) >states that to maximize fuel efficiency, you should get into as high >a gear as possible as soon as possible. > >Three questions come to me from all this: > > 1. If the engine is running at 2500 RPM, what difference does it > make what gear I'm in or how fast the car is moving? Fuel efficiency is measured in miles per gallon, not revolutions per gallon. Upshifting sooner gets you more miles for a given number of revolutions. The higher gear is likely to give you fewer revolutions per gallon (because of wind resistance) but more miles per gallon. > 2. If Toyota says that going faster saves fuel, and the U.S. Gov't. > says that going slower saves fuel, who is right? If Toyota said anything about going faster, you did not include it in your quote. > 3. If rapid starts and quick acceleration wastes fuel (another > U.S. Gov't. contention) then how can up-shifting as quickly > as possible save fuel? Most people can upshift sooner without resorting to rapid starts and quick acceleration. Toyota is saying that you should upshift as soon as you reach the appropriate RPM, not that you should try to reach the appropriate RPM sooner. -- Dave Seaman pur-ee!pucc-h!ags
ark@alice.UucP (Andrew Koenig) (01/10/86)
> No matter what kind of car it is, your fuel consumption increases with > speed. Depending on the aerodynamic drag of the car, the rate of > increase differs. Since an Audi 5000 has less drag than a Chevy > van, it could go much faster with same amount of fuel consumption. I don't believe that fuel consumption per unit distance always increases with speed. Consider: a car standing still with the engine running gets 0 miles per gallon.
mat@mtx5a.UUCP (m.terribile) (01/14/86)
> > 1. If the engine is running at 2500 RPM, what difference does it > > make what gear I'm in or how fast the car is moving? > > If you had a boost gauge you could tell that at 70 mph, the engine is > working harder that it was at 30, even if the engine RPM's were equal. > After you pass ~40 mph, wind resistance becomes important, and at higher > speeds it the major resistance to overcome. It's like going downhill > in gear (high vacuum) and then going up a steep hill (low vacuum) at > the same revs. You're going to burn more fuel to get up the hill. > > > 2. If Toyota says that going faster saves fuel, and the U.S. Gov't. > > says that going slower saves fuel, who is right? > > No matter what kind of car it is, your fuel consumption increases with > speed. Depending on the aerodynamic drag of the car, the rate of > increase differs. Since an Audi 5000 has less drag than a Chevy > van, it could go much faster with same amount of fuel consumption. This assumes that in neither case is the engine being operated in an inefficient part of its speed/torque range. The problem is just NOT all that simple. After all, with a manual transmission, you CAN get better fuel economy in many cars by accelerating with about 2/3 of the throttle, and upshifting as soon as possible. BUT with all manufacturers building cars for the CAFE requirements and testing at 50-55 MPH, most cars will have their fuel economy ``knee'' pushed up into the 50-55 MPH range, and pushing that knee on the curve will probably make the drop-off steeper afterwards. Because the CAFE requirements average all cars sold by a manufacturer, less-widely sold lines can be built to be way off average. This includes 'vettes, of course. -- from Mole End Mark Terribile (scrape .. dig ) mtx5b!mat (Please mail to mtx5b!mat, NOT mtx5a! mat, or to mtx5a!mtx5b!mat) ,.. .,, ,,, ..,***_*.