RPMs for best mileage?
#1
RPMs for best mileage?
Generally speaking, at what RPM does one get the best mileage? I know this question has no simple answer, but I'm thinking RPMs in relation to a motor's torque curve.
I'm trying to determine if a ring and pinion change would result in significantly greater mileage while I'm towing, specifically 6000# boat (w/trailer underneath) behind a 350 2wd drw 4.10 diff. This truck-trailer combo isn't real heavy for this rig, and the boat is aerodynamic compared to, say, a monster 5th wheel rv trailer. At about 70 mph, I'm at or above 2500 RPM. 'seems like if I could cruise at 1900 RPMs or so my mileage might increase.
I'm trying to determine if a ring and pinion change would result in significantly greater mileage while I'm towing, specifically 6000# boat (w/trailer underneath) behind a 350 2wd drw 4.10 diff. This truck-trailer combo isn't real heavy for this rig, and the boat is aerodynamic compared to, say, a monster 5th wheel rv trailer. At about 70 mph, I'm at or above 2500 RPM. 'seems like if I could cruise at 1900 RPMs or so my mileage might increase.
#2
#3
It all goes back to the camshaft.
As far as the engine is concerned, the efficiency is best at the torque peak. This is part of the reason smaller cams cause better gas mileage. The torque peak is lower in the RPM range. The smaller cams also close the intake valve sooner, which builds more compression and therefore more efficiency, where more BTU's are extracted from the fuel to move the vehicle. Also, having the torque peak lower in the RPM range results in less frictional losses from spinning the engine slower.
In the real world, it all comes back to where the torque peak is, and how your gearing is set up. If your torque peak is up around, say, 2500rpm, then the frictional losses from the engine, and the wind resistance, may offset the advantage of running at the torque peak. That's a fancy way of saying switch to a lower gear and drive slower.
There's a lot more to it than that...
In the real world, it all comes back to where the torque peak is, and how your gearing is set up. If your torque peak is up around, say, 2500rpm, then the frictional losses from the engine, and the wind resistance, may offset the advantage of running at the torque peak. That's a fancy way of saying switch to a lower gear and drive slower.
There's a lot more to it than that...
#5
Er ... yeah, 70 mph, maybe 65ish ... this isn't in the mountains on a two lane highway, or with any appreciable traffic. This would be, for example, I10 west of Phoenix - straight, flat, big shoulders, multiple lanes, little traffic. It's a dual axle trailer with electric over hydraulic brakes on all four wheels, and new st185's all around. The truck is at a little over half it's posted towing limit (like 11.6K# or so). This certainly doesn't feel reckless to me, and I've towed said boat and trailer about 15K miles this way with a similar psd, and never felt I was even approaching the limit of what was prudent. (forgot to mention this truck is a 460).
Believe what you will from my question, I'm certain I'm one of the safer drivers on the road, and were you to see me on the road or ride with me, I think you'd agree.
So, shoot for a cruise speed near peak torque RPM is what I'm hearing.
Believe what you will from my question, I'm certain I'm one of the safer drivers on the road, and were you to see me on the road or ride with me, I think you'd agree.
So, shoot for a cruise speed near peak torque RPM is what I'm hearing.
Last edited by Uglyhat; 09-29-2005 at 02:57 PM. Reason: correct an error
#7
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#8
#9
My truck ('92 Bronco 302 4-speed auto OD) seems to like 45mph best, that puts it right at 1500rpm. Torque peak is 270 @ 2400. My understanding is that efficiency also depends a lot on the amount of vacuum you pull; the lowest RPM at which you pull peak vacuum is the best engine speed for mileage. Hence why some of the early gas mileage meters went off of engine vacuum.
Best proof I have of this is my Dad's Passat he used to have. It was EPA rated ~25mpg freeway, but at exactly 42mph (I believe it was about 1800rpm? Peak torque was 206@3200), it got 48mpg according to the onboard mileage computer thingy. Optimistic maybe, but even if it were reading 20% high there, it still says to me that engine vacuum plays a huge role.
Best proof I have of this is my Dad's Passat he used to have. It was EPA rated ~25mpg freeway, but at exactly 42mph (I believe it was about 1800rpm? Peak torque was 206@3200), it got 48mpg according to the onboard mileage computer thingy. Optimistic maybe, but even if it were reading 20% high there, it still says to me that engine vacuum plays a huge role.
Last edited by jcp123; 11-01-2005 at 12:07 AM.
#10
Originally Posted by jcp123
My truck ('92 Bronco 302 4-speed auto OD) seems to like 45mph best, that puts it right at 1500rpm. Torque peak is 270 @ 2400. My understanding is that efficiency also depends a lot on the amount of vacuum you pull; the lowest RPM at which you pull peak vacuum is the best engine speed for mileage. Hence why some of the early gas mileage meters went off of engine vacuum.
Best proof I have of this is my Dad's Passat he used to have. It was EPA rated ~25mpg freeway, but at exactly 42mph (I believe it was about 1800rpm? Peak torque was 206@3200), it got 48mpg according to the onboard mileage computer thingy. Optimistic maybe, but even if it were reading 20% high there, it still says to me that engine vacuum plays a huge role.
Best proof I have of this is my Dad's Passat he used to have. It was EPA rated ~25mpg freeway, but at exactly 42mph (I believe it was about 1800rpm? Peak torque was 206@3200), it got 48mpg according to the onboard mileage computer thingy. Optimistic maybe, but even if it were reading 20% high there, it still says to me that engine vacuum plays a huge role.