I've asked this question within other threads, but never really received a response, so I thought I'd try it here .
Question: Optimum gas mileage- am I correct in my thinking that while towing at the rpm in which the horsepower and torque curves intersect is where one would get their best gas mileage?
Mine intersects @ 2500rpm, which translates to 55mph in 3 gear. At this speed, rpm & gear-I'm getting 13mpg.
So if any of you engine gruru's could set me straight, I'd be greatful-John
What engine/drivetrain/rearend combo do you have? To some extent, you are correct regarding the relationship between torque and HP. Remember though, its torque that actually moves you. Its the gear multiplication through the transmission and rear end that allow freeway speeds. Different engines respond differently to different loads and so on.
I tow a 25' airstream with a 1/2ton suburban with a 350 and 3.73 rear end. I usually get 12-13mpg regardless of speed. I ususally don't tow in OD unless its REALLY flat ground. Its much harder on the engine to lug it rather than rev it.
FYI, The rule of thumb that I've heard, as it applies to cars, is that for best mileage you should shift at your torque peak, and for best performance, shift at your horsepower peak.
Maximum gas mileage doesn't necessarily occur at stoichometric 14.7:1. It depends on the individual engine and my actually occur at 15:1 or greater. i.e. the power fall-off from being a bit leaner is not as great as the reduced fuel usage.
The ratio at which maximum hp occurs also depends on the engine. It may be as rich as 12.0, or as lean as 13.8, but generally occurs somewhere between 12.8-13.2:1
There's a school of thought that says max fuel mileage occurs at the torque peak, because that's the engine's point of peak efficiency, but that's obviously too high to cruise at with other than diesel engines.
There is no easy ROT that fits all engines. Best to use trial and error to find it yourself.
Optimizing engine RPM is only part of the equation. Modern engine controls keep fuel/air mixture close to optimum all the time, and four/five speed transmissions keep the rpm's in a narrow range. Lower RPM's reduce internal friction and improve mileage - but may not produce enough torque to get the rig up the hills without downshifting, which is hard on the tranny.
The most significant variables are weight and speed. You have limited control over weight (don't bring your anvil collection). Speed is about all you can control. Since drag increases four times when speed doubles, generally slower is better. However, at idle you have zero wind resistance, but you are going nowhere - thus getting zero mpg. In general - the optimal speed will be somewhere just above where the highest gear becomes usable. While towing, the highest gear (overdrive) may never become usable except on very flat roads so the next highest gear would be used.
Wind resistance is the killer, followed by weight. An old rule of thumb for gasoline motors was to cruise a few hundred rpm below peak torque for the best balance of power and economy (in the days when V8's were at torque max at 2600-3200 rpm).
Optimum mileage is a tradeoff. We can't run the Interstates at 45 mph nor would we want to. I use 62-63 mph as a good speed that features very good reaction times, etc. It also allows me to easily come to a stop.
Strictly my own observation, but optimum gas mileage occurs when the smallest vacuum space occurs behind the vehicle or the towed vehicle. The size of the vacuum space increases with speed. Example: my Airstream Excella 25 weighs about 5400 lbs but because of it's ability to slip thru the air, I get slightly better gas mileage pulling it than I did with my previous trailer which was an Award 23 weighing about 3300 lbs. The Award was advertised as being especially aerodynamic. Once I pulled a Nash 16 "square ol' box" several hundred miles and got awful gas mileage even at 55mph. Anything over about 40 mph and I was pulling a full size vacuum behind it! It wouldn't have mattered much if I was pulling a 16' trailer or a 26' trailer if it were similarly shaped.
For the record I got about the same mileage pulling the Award 23 with my V6 Tacoma as I got with the Tundra.
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2002 Toyota Tundra Let the Big Oil companies bail out GM, Ford and Chrysler
i have a 77 argosy motor home. 454 engine. It has a vaccum gauge installed as original equipment. It has a green zone which is good gas mileage, a yellow zone that's fair mileage and red zone that's poor mileage. I drive at around 60 mph. 2800 to 3000 rpm. I get 9.5 to 10 mpg. weather i'm driving across the praires or over the mountains. I gear down to second gear going over steep hills as the owners manual suggests and drive at 45 mph. That keeps the vaccum gauge in the high yellow zone to mid red zone. Also going into a head wind the vaccum gauge reads slightly higher. So if you want to get the best mileage maybe you might consider installing a vaccum gauge.
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