Anyone have an idea as to what the most fuel efficiant rpm is? Obviously low is good, but not too low. I try and cruise between 2000-2500 if I want to save gas and its possible (based on my speed). I'm wondering if it might be more fuel efficient closer to 3000 than to 2000.

 

I don't think it's the RPMs as much as the speed. By the time you reach 3K RPM in 6th (assuming MT) you are definitely going too fast for optimum gas milage. Up to a point you're right that the RPMs matter but after about 70 MPH the greatest influence on milage is further speed increases regardless of the fact that the engine might run better (more efficient) at a higher RPM (aerodynamics takes over).
Doesn't the Navi have instant milage? That could tell you right away.

 

OK lets forget about highway because on the highway I am going to be in 6th gear and thats that. But lets assume I am at a speed where I can either be in 5th gear at 2800 rpms, or 6th gear at 2200 rpms. Which should I choose?

 

In just about any car the best milage is accomoplished by being in the highest gear possible with the engine "lugging" (turning the fewest RPMs possible). Also remember faster means more wind resistence to overcome thus poorer milage.

 

See I'm not so sure this is true. If i'm lugging my engine am I really going to save gas? (especially if i have to accel at times). I might be wrong. I mean i know lower is better, but to a point.

 

Well, as you know varying conditions and different cars will all run differently.

That said, usually, the most efficient mode of operation is in the highest gear possible, and with the throttle as wide open as possible. If you are crusing down the freeway at a constant speed:

1) Slower engine RPM means a smaller number of piston strokes as you are travelling down the road. Less piston strokes = less movement of everything in the engine = less friction.

2) Wide open throttle = less pumping losses. If the throttle is closed during the intake stroke, this would give you a high amount of energy wasted in creating vaccum at the intake.

If you've ever looked at those fuel economy projects that university students do, you'll notice that they all have tall gearing, and that they don't use a throttle to control engine output. It is typically run at WOT until you get up to speed, then cruise with the engine off until they begin to slow down again.

YMMV... but as a general rule of thumb, you get the most efficient operation with the engine RPM as low as it can be without lugging. (Since running at WOT all the time is impractical).

Test_Drive

 

I love to rev the engine, of course. But I always think about wasting gas since it's so expensive. How much gas am I wasting when I rev it up to 6k or 7k?

 

7200

 

The highest gear part is correct, but the wide open throttle part is completely incorrect. To repeat, in the real world you want highest gear and the absolute minimum amount of throttle. Also, go as slow as you are willing to go since wind resistance is also hugh detramint to fuel economy. The example given by fuel economy experts is to drive as though you had an egg between your foot and the gas pedal. The wide open throttle thing may be true in the custom college test vehicles where you go wide open until you get to say 25 mph and then turn the engine off and coast until you get to say 2 mph and then turn the engine on go wide open to the 25 mph then turn the engine off and coast to 2 mph and keep repeating the process, but not relevant in the real world of every day driving.

 

Yes... you will save gas by always being in the lowest gear possible. You will get the lugging effect but you do save gas. I had a Honda CRX HF once and it had a dummy light that wanted me to get to 5th ASAP regardless that the car almost wanted to "stall" in some gears (too high of a gear for the speed)...

out of all the cars that i have ever owned and all the miles that I have driven wouldn't you know that I ran out of gas 3 times with the CRX HF (HF stands for High Fuel). There was no low level gas light and the tank read out was off if you went a long distance without shutting off the engine. Each time I ran out I thought I had a 1/4 - 1/8th of tank left.

 

Yes, you are correct of course. As I said,
"Drive around at the lowest possible RPM without lugging"

...with all that other stuff I was thinking specifically of engine efficiency... which of course is not the same as improved mileage.

Oh well, chalk it up to weak coffee.

Test_Drive

 

OK, so then I gather that the most fuel efficient rpm in any given gear would be around 1500? (I think the engine startes to lug below that. Is that fair?

 

7200

 

6th.

 

More like 2000.

 

As mentioned in this thread it's a trueism(or at least it was with older motor technology" that engines run most efficiently at low rpms(less friction) and wide open throttle(less pumping losses) . I'm not sure how all the I-vtec(timing advance ) plays into it but I assume it's still true.

 

If you can maintain a constant speed, then 2000 is a good number. If you have to vary the speed (like, in real world driving), then forget about 2000 rpm's. You want the revs high enough so that you have flexible acceleration.
This car will drive you nuts if you keep it at 2000 rpm. Every Civic on the road will pass you.

 

You're also going to be a nuisance on the road. 2000rpm in 6th on the 6MT is around 78kph.

 

you cant make an assumption of the correct RPM to be running at for fuel economey. its all in how hard you have to press on the gas verses how fast your going.
with the TSX its a fuel efficient car because of the gear selection and the fact it makes so much torque at low rpm.
an engine will make max efficiency when running slower revs and also when it is running in an RPM range that has the most torque.
when your in 6th gear your causing a significant load on the engine. depending on how fast your driving should determin what gear your in and what the road is like.
if you have to go up a hill or are pushing a head wind you sould run in 5th because the engine would require a lot more throttle in 6th to overcome the resistance of going up a hill or pushing a wind.
bottom line the faster you plan to drive in 5th or 6th your going to loose fuel economey.
if your trying to be a fuel conservationst then here is what i would tell you.
1. drive the speed limit with the cruise control set. this prevents you from varying your speed.
2. if you see a hill comming up rather than have the car try to push you up that hill in 6th gear downshift to 5th gear so the car can get over it with less throttle required to maintain the speed.

as for city driving i would say never push the gas more than just a little and change each gear very soon after you shift into it.
in other words allow the transmission to speed up the car, not the rpm's.

driving your car this way will probly significantly improve fuel economy.
im assuning you dont have the nav because with the nav it tells you your instant and average fuel economey, i seem to average 21-22 mpg in the city and about 30+ on the highway driving in a normal manner.

 

True, Bass Mechanic, although I don't think the TSX running in 6th at 2500rpm does need to downshift going up a hill witgh a gradient of less than 10% (11°).

I've been watching this ever since I got the car, and it seems the TSX is most efficient when at 2500rpm, you shift to the next gear, and if you go down below 2000rpm, you downshift one gear.

 

So sauceman...if you are driving in a range of between 60-70 km/h ( a range where you can basically pick your rpm) what rpm would you cruise at for max fuel effeciency? Lets assume you can drive at a steady speed and there are no hills.

 

Drive in 5th gear, you'll be around 2000 - 2250rpms. (6MT)