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Old 09-29-2005, 12:46 AM
Uglyhat Uglyhat is offline
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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.
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Old 09-29-2005, 10:16 AM
jimandmandy jimandmandy is offline
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What engine do you have? If it is a diesel, then yes, 1900 rpm is closer to the torque peak and you may get better mileage. If you have a gas engine, I wouldnt do it, the mileage might be worse. 70mph is pretty fast for towing anyway, the limit is 55 here in CA.

Jim
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Old 09-29-2005, 12:09 PM
rusty70f100 rusty70f100 is offline
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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...
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Old 09-29-2005, 01:24 PM
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6000# tow load at 70mph? Dude.
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Old 09-29-2005, 02:54 PM
Uglyhat Uglyhat is offline
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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.

Last edited by Uglyhat; 09-29-2005 at 02:57 PM. Reason: correct an error
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Old 09-29-2005, 02:58 PM
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Yup. Whatever the torque peak is would be your best bet. Can you get the truck dyno tested?

Happy trails
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Old 09-29-2005, 09:05 PM
Uglyhat Uglyhat is offline
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The only dyno I know of in town is in a riceburner performance shop, but I've (ahem) never been in there Since the motor is stock, I was going to go by the factory spec. 'never actually had a vehicle dyno'd ... would it be worth it on a stock motor? How $$ should a dyno test be?
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Old 10-17-2005, 11:17 PM
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i have never priced one before but i have been under the under standing that they are very expensive, but if you do go that way, make sure its in its best possible condition first
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Old 11-01-2005, 12:05 AM
jcp123 jcp123 is offline
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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.
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Last edited by jcp123; 11-01-2005 at 12:07 AM.
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Old 11-20-2005, 08:46 AM
The SnoMan The SnoMan is offline
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Quote:
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.
What earlier posters said about engine uses least amount of fuel at or near its torque peaks is quite true when the engine is under a moderate to heavy load. You cannot equate MPG at 42 mph is a slick light car to that of the needs of a heavy drag prone SUV. As speed increase, so does drag and the amoutn of power to overcome that drag as well. As to what geaing is best for use? That depends on user needs, towing or not, lift or not, terrain and so on. There is no one solution to fits all conditions.
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Old 11-20-2005, 08:46 AM
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