Why my RPM so high when driving on HWY?

[mxmaz]

Look, you want proof? Buy a full tank of gas, and redline it EVERYWHERE until it burns away, then buy another one, and drive it as low an rpm as you possibly can, and you'll find that you are sorely mistaken.

What you are saying here makes complete sense, and it worked that way on any other car i have driven. But on my mx, no matter how hard i drive i get the same gas milage (if not, maybe even better when driving hard) then if i drive around like a granny. I have tested this many times, and it always works out the same way.

[Gro Harlem]

In your example of 2500 vs 3500 rpm this is what will happen- The throttle plate will be more open at 2500 than at 3500 rpm. Since the engine is spinning slower you will have to move more air into the cylinder to light off the required amount of fuel to make the exact amount of power you need to propel the car at what ever speed. At 3500 rpm the throttle plate will be closed more than at 2500rpm. Since your doing more combustion cycles per second it will take less fuel(in turn less air) per combustion cycle to make the same amount of power you were at 2500rpm. The Amount of fuel the ecu puts into the engine is not just dependent on the rpm. The throttle position and air meter have huge effects on this. T

Untrue. at highway speeds your car doesn't need much HP or torque to keep its speed maintained. You don't need to open the throttle MORE at 2500rpm to keep the speed up.

At 2500 compared to 3500 rpm ur car isn't making all that much less power or torque and you won't have to compensate it much at all if you are already going 70mph. The amount of compensation will not affect the fuel economy nearly as much as the fact you are revving the engine 1000 rpm lower.

[lakersfan1]

But on my mx, no matter how hard i drive i get the same gas milage (if not, maybe even better when driving hard) then if i drive around like a granny. I have tested this many times, and it always works out the same way.

Same here. Always get 22MPG no matter what I do it seems.

[Grants]

Idle it in the garage for 4 days - you'l get 0 mpg. And yes the fuel will get burnt, even quicker if you idle it at 3500 rpm instead of 700. Try it!

[matt_fulghum]

Hahahaha, he's gone quiet!

[TsiMiata]

Ha Ha he just got finished driving 17 hours from daytona, florida to austin, texas.

YOU still don't get it. Your driving to redline analogy has nothing to do with highway cruising. High rpm's, high throttle openings, high loads means shitty gas mileage. The only one of that criteria that highway cruising even comes close to meeting is high rpms and even then its more like half of redline.

I am to tired right now to deal with the ignorance on here right now.

[IanL]

This has been said higher up, but it's worth repeating:

• Higher rpm means higher fuel consumption per minute
• Higher rpm in the same gear means higher speed (or more miles per minute)
• Higher speed makes more drag and requires more fuel to overcome it

(Point 2 above eliminates the trivial example of idling in the garage)

The only way you can determine, for a particular model, which of the above effects predominates is to TEST. It's no use argueing about it - the result is not predictable.

[IanL]

Another thing:

The arguments about the throttle plate are flawed, too. The airflow depends not only on the plate position, but also on the engine vacuum, which is higher at higher rpm. So you can't say what position for what rpm without making a measurement.

[Grants]

I agree Ian. My trivial example was exactly to prove the point that a lot of the arguments are just plain silly. For example the argument that to move a car at a certain speed will use the same fuel no matter what gear it's in is idiotic. I guess it was more aimed at the inanity of getting the same fuel consumption no matter what. NO MATTER WHAT! Maybe the "type" of driving is consistent and makes a wrongful conclusion. Always same weather, traffic, roads surface, etc. Maybe because of these consistencies it can only vary 10% or so (hardly notciable especially over a couple of tanks).

I didn't want to get involved in this thread but at the end of the day, what rules it all is physics. Engineers try their best to find a suitable compromise. Not majic. Just try and squeeze some efficency in without too much loss in performance - hence engine management and gearing.

You don't get nothing for nothing. If you don'tlike the gear ratios or fuel consumption, just buy a different car. Bet you'll still whinge but I still won't give a rats arse. I'm happy with mine.

[IanL]

You don't get nothing for nothing. If you don'tlike the gear ratios or fuel consumption, just buy a different car. Bet you'll still whinge but I still won't give a rats arse. I'm happy with mine.