I drive a 1984 Honda Accord 5-speed. I can maintain 55 mph at 2500-2700 rpm in 4th dear. In 5th gear I can maintain 55 at 2000-2200 rpm. I assume, never having driven a 6 speed, that the rpm drop would be about the same. Is there a limit to how high you can gear a car? Would it be possible to gear it high enough to maintain 55 at an idle speed?
let me put it this way. stadium racing trucks put out 400+ HP but are only geared to do 60-66 mph.
Yes, but the problem would be the power available at idle. An old big block would be OK just off idle, but the smaller engines of today need to buzz a bit before getting in to their operating range.
If you’re driving on a perfectly flat surface I can’t see why you couldn’t run a car at 65 MPH and maintain it at idle RPMs. The moment you hit a hill, however, you’re going to have to gear down.
Basically, that’s what overdrive does, but they leave a little oomph in reserve so you’re not downshifting to climb every little rise in elevation you encounter.
Here is an explanation of overdrive.
I had a Ford F-150 with the 300 cubic inch 6 cylinder and the 5 speed manual transmission with OD.
At highway speeds in overdrive it ran at about 1500 RPM which is pretty low. Once on a highway down in Louisiana I decided to see how fast it would go so I floored it, in OD, at about 65 miles per hour. 7 miles later I had not reached 80 MPH but it was still accelerating.
Hope that helps.
Like B&I said, I doubt it puts out enough power at idle to overcome rolling and wind resistance, friction in the drivetrain, etc.
I drive a Ford Fairmont (Australian Ford) with a 4spd auto box behind a 4l/6 cyl engine. I know in Yank terms this isn’t a very big engine, but it more than makes up in the torque department. Max torque (don’t have the figure at hand now) is delivered at a very low 2200 rpm. The gear ratios are worked out such that 2200 rpm in 4th equates to about 120km/h, 3000rpm approx. 160km/h (3000rpm in 3rd gives about 120km/h). That said it still has enough grunt in top to pass most highway traffic easily. Sometimes I downshift to 3rd if I’m in a real hurry. I think this engine would do great things in a 4x4!
On a flat surface, if I just let go of the brake, the car slowly accelerates (at idle) through the gears up to about 65km/h. Idle speed is about 800rpm.
But it all depends on the type of engine. Mine is a slow-revving, high torque at low rpm engine. Smaller engines usually delivers maximum torque at very high rpm, which means that the torque available at idle speed is (usually) simply too little to accelerate the car and maintain any speed at idle. I don’t know if the V-Tec engines (Honda et al) can overcome the small-engine-no-torque syndrome at low rpm with their variable valve timing setups.
Not necessarily. It would depend on the gear ratios they used for the two transmissions. The Mazda Miata is available with both a 5 speed and a 6 speed. The rpm in the top gear works out to be about the same, however. They each cover about the same range, the ratios in the 6 are all just a bit closer together than in the 5.
Eric
55mph… you need approx. 20-50 HP to go that fast, depending on the size of vehicle. So at 1000 RPM’s, you would need between 100-260 lbs of torque to do that, with WOT of course. The everyday cars that are made right now do not offer that kind of low end torque to cruise at 55mph @1,000 RPM. (Except the new F-150).
But doesn’t momentum figure into the equation?
Yes, it does. If the overall resistance load the vehicle is fighting overpowers the torque at the wheels, the car will slow down, and vice versa.
Just out of curiosity Reeder, what advantage do you see in doing this?
RPM is not the only variable in how much fuel you burn. It’s going to take roughly the same amount of fuel to make the needed power, no matter what the RPM*.
If you are looking to reduce wear, consider that lots of power at low RPM is going to result in higher cylinder pressures. Higher pressures inside the cylinders may mean more wear on the rods, bearings, etc. You will also have reduced your detonation margins.
The Honda guys have a pretty good compromise going.
*I know friction will be less and BSFC tends to be better at lower RPM.
It is fuel mileage and I agree there are other factors. But the less times the spark plugs fire the less gasoline is burned. Is that not correct?
No, it’s not correct. Fuel burn depends on how much power you produce, not the RPM.
You need to produce X amount of power to move your car down the road at 55mph. You, as the driver, can do this one of two ways:
- Use high RPM with low cylinder pressure
- Use low RPM with high cylinder pressure
Using a higher RPM will mean you need less cylinder pressure, so there won’t be as much of the fuel/air mixture burned in the cylinder on each power stroke. BUT, there will be MORE power strokes per minute.
Using a lower RPM with a higher cylinder pressure means that you will have FEWER power strokes per minute, but each one is going to be burning MORE fuel/air.
Now, this is terribly over-simplified and leaves a few things out, but the basic concept remains. I hope I don’t get jumped too badly. 
Aerodynamics is probably the biggest issue with weight being a close number 2 with regard to fuel mileage at highway speeds.
(IMO) - no cite or research
county:
I’ve seen charts.
Aero drag is the biggest factor, followed by weight, which is actually represented by friction at wheel bearings. The transmission itself follows in the least, interestingly enough, the transmission’s power losses are highly proportional to RPM… so running your car at 60 MPH at 4,000 RPM is more taxing than running it at 60 MPH at 3,000 RPM, before the engine itself even enters the equation.
By the way, I’ve caught my car, a '96 Chevy sedan with a 350-cubic-inch V8, shift into overdrive at 25 MPH. I think that would work out to under 650 RPM… which is of course below stall speed, but that’s another thread.
I drive a 6 speed (Matrix XRS). And I’ve driven my friend’s 5 speed WRX. The revs in OD differ quite a bit.
At 65 mph I’m sitting rather high at about 2800 rpm and at 75 mph I’m at 3200 rpm. My friend’s 5 speed is about 500 rpm lower at both speeds.
To be fair of course all my power is at very high RPMs (about 6300) while the WRX makes power everywhere.
My vehicle’s top speed of 136 mph is reached in 5th gear only (an almost 1:1 ratio gear) because 6th is far too tall a gear (and the engine, while great, does not make enough power to fight wind resistance at that speed).
I have an antidote. My car drops by 5 mpg just for the extra drag of putting the top down.
BSFC (Brake Specific Fuel Consumption) is the term used to describe the amount of fuel you need to produce 1hp. BSFC is lowest at the rpm where the engine makes the most torque. However, friction losses increase with higher rpm, so the best cruise rpm is almost always lower than torque peak.
If you have a small 4-cylinder you just can’t go down the freeway at 1200 rpm. The car would slow down on its own… It needs between 2000 and 3000 rpm to make enough power.
On the other hand, large V8’s can run at a much lower rpm. I used to have a 6-speed Camaro with a 5.7 liter V8. In 6th gear it would rev 2000 rpm @80mph, or 1500 rpm @60mph. It would also get 27mpg driving 75-80 mph.
I installed a 5-speed OD tranny in my current car which has a rather large 7.4 liter V8 and I can drive it at 1200 rpm in 5th with no problems.