There is much more to gas mileage than just cruising speed. The weight the car is carrying for example. The way you get to cruising speed. Flat out is not good.
IIRC automatics are better for mileage because the gears shift at a pre determined time and you can’t wind them out.
This is generally not true, due to the higher internal friction and losses in the transmission and torque converter, as well as (typically) not having as high a top-end gear. Looking at many comparisons between automatic and manual transmission mileage on the same vehicle, one typically sees a 1-3 mpg difference in favour of the manual.
But again…doesn’t that rely on the driver not winding out the gears? And don’t most, not all, drivers wind the lower gears farther than the should?
scr4, using the fuel economy measure I suggest, if you are parked in your driveway, then your engine is off. Fuel economy is quite good under such circumstances. But I may be mistaken…
To be technical, a vehicle’s theoretical optimum gas mileage would be acheived with the smallest velocity greater than zero. But since we are talking about a 1.5l base engine in a production Civic…
The goal is to determine at what point does the “gas-in, horsepower-out” ratio of a given engine become so HIGH (gas increases and horsepower decreases) that the gas mileage would decrease the slower the vehicle goes (keeping in mind optimal gearing and torque transmission efficiency, like the OP said). This very point is the optimal gas mileage.
So in simple terms the space on the graph (RPM’s, gas consumption, HP Output, velocity which accompanies HP output are the variables) between where the ratio is increasing and decreasing is the exact horsepower (which also directly corresponds to a certain velocity) that optimal gas mileage occurs. There could be multiple points, but obviously you choose the least mph. The details of the math and graphs are a little beyond me, but this basic idea seems pretty obvious to me.
In the US, laws encourage carmakers to get good fuel mileage. A car’s official mileage comes from a carefully controlled test on an indoor dynomometer. The test regimen is performed at legal speeds (when the national speed limit was 55mph, that was the test speed; now it’s 65.) Because the US buys more cars than anybody else, carmakers design their cars to the standard rather than to real life. In real life, most people drive about 10mph faster than legal speeds. Your car’s drive train is geared so the engine’s peak efficiency happens at about 65mph. If it were designed for real life instead for the standard, you’d get best mileage at about 75mph.
The law has good intentions. However, most people will get less than ideal mileage, because the law forces carmakers into a box.
I don’t know about that. If you’re comparing the vehicles in terms of driving habits of their drivers, then it’s a whole other question. I’m talking about what the vehicle is capable of.
I think a lot of automatic transmission drivers tend to treat their gas pedal like an on/off switch. Routinely minivans and small economy cars pull away from me at stoplights, quickly getting to the speed limit + 10 in their automatics - an effect I can only reproduce when I rent automatic cars by stomping on the gas at every light. I notice this because my car (normally) is much faster than anything I pull up next to, and in routine driving I’m always left behind in a cloud of dust.
Most automatics have locking torque converters and at highway speeds, for fuel consumption purposes, get the same as a manual geared the same.
Also remember that an engine does work and uses fuel just moving itself (pistons, crank/cam shaft, belts, water pump, alternator) so you have a base drain of fuel before you even start actually moving the car. This drain means that the greatest MPG is NOT at just over 0 mph. Think of it this way:
To move at 0.1 mph for 10 hours you need 1 gallon of gas just for the engine, and another 0.1 gal to move the car.
You are going 1 mile in this 10 hrs, using 1.1 gal and getting apx 0.1 mpg
If you increase your speed durign this test to 1 mph, you still need that 1 gallon of gas during that 10 hrs just to power the engine, but now consume lets say a 2nd gallon in actually moving the car.
you are going 10 miles in 10 hrs, using 2 gal and getting 5mpg
Lets say we throw caution to the wind and take her up to 10 mph. Assuming that humans can survive that speed, lets say we use 1 gal for the engine and 10 gallons for motion.
you are going 100 miles in 10 hrs, using 11 gal and getting apx 10 mpg
Now this is a over linerar simplification but hope it shows that 0+ is not the most efficent way to travel.
Also for the suggestion start, accelerate, shut off engine, coast, repeat. some problems w/ that too. 1st is that if you use the starter to turn the engine on you will be draining the battery, putting a load on the altenator which does have to be paid for. If you pop the clutch to restart the engine then you may have a better shot, but again not for free. Also acceleration costs you, this is a basic principal of newtonian physics. Add to this engines have a most efficent rpm range which you would rarely be in due to accelerating (rpm’s would be too high).
I also have heard that the most efficent speed is usually about 50-60 mph.
Winding out the gears is not a real difference.
Automatic drivers tend to do it as much as if not more than the driver of a comperable manual driver, because of the loss of engine power during acceleration.
Think of two propellers inside of a sealed tank. There’s one that drives a thick fluid, and one that is driven by the motion of the fluid. That is how an automatic works, at it’s simplest form. When stopped (but still in drive) the rotor connected to the engine is spinning, but not with enough force to turn the one attached to the gears and driveshaft. This slippage is intentionally engineered into the car. As gas is applied, the rotor spins, moving the fluid, and INDIRECTLY transferring power to the driveshaft. As speeds increase, the loss becomes less apparent.
After reaching cruising speed, and shifting into overdrive (called so because the wheels are spinning faster than the engine in this gear) a device called a “lockup-torque converter” engages. This is essentially the same thing as a clutch in a manual shift car. It makes a direct, mechanical connection between the engine and wheels, eliminating slippage.
Older automatics didn’t have the lockup torque-converter, and thus had horrid highway mileage. On most modern automatics, the city mileage is far less than a manual, but highway mileage (assuming they’re geared in similar ratios) is very close.
The city mileage suffers though. This is also why automatics have that characteristic lag time between pressing the gas and feeling the forward motion. This is also why you don’t have to push in a clutch pedal when at a full stop or risk stalling. The slippage is there and necesary.
BTW, a common test to see if the clutch in a manual shift car is slipping is to place a pillow between the front bumper and wall. With the car pushed firmly against the wall, shift into 5th gear and slowly let out the clutch. If you can let out the clutch all the way without stalling, it’s time to go pay a mechanic for costly repairs.
Long ago in a galaxy far, far away my 1978 Honda Accord used to regularly get between 50-60 mpg. Fifth gear was overdive.
Highway speeds did not reduce the mileage, either. However, I consistently achieved this by purchasing regular (leaded) gas from a particular off-brand service station just off the main drag in Jackson, Wyoming.
Unfortunately, I was only in Jackson during the summer. The rest of the year the mileage dropped to 40-50 mpg every place else I bought gas anywhere in America.
I miss that car.
