1989 Mustang Wondering about Ideal RPMs When Driving
Hey folks. I've got an '89 2.3l, and since it has an RPM gauge, I've paid attention to it, and it has me wondering.
Is there an ideal RPM range for best gas mileage and engine life? Does lower RPM automatically equal better mileage, or is there more to it?
I notice at around 45 MPH, when my automatic transmission shifts into overdrive, the RPMs end up at under 2,000. At 65 MPH, which is about as fast as this car goes without an undue amount of coaxing/flooring/cussing, the RPMs are around 2,500-3,000. Is this range a waste fuel-wise? And is it hard on the engine?
Thanks for imparting any wisdom on this almost car-illiterate padawan.
Lower RPMs is ALMOST always better for mileage... there is a such thing as being too low...
My '87 GT had the factory 2.73 gears when I bought it... and around town the poor thing was a slug and got about 6mpg... replaced everything (literally, engine, tranny, all sensors, tune-up parts, filters, computer, K&N filter, aftermarket exhaust) still didn't break 8mpg in town... nor did it ever hit 1900rpm unless I just stood on it... I could get 28mpg on the highway all day long, and on a really good day even more than that... but in town it sucked gas worse than my old big-block T-bird... Put 3.55 gears (raised the RPMs in town substantially, usually ran 2000-2800 while moving) and doubled my mileage to 12-16 consistently, and I only lost 1mpg highway, was a great tradeoff!
If you're looking to change the RPM range you drive in, gears are the way to do it, but you've got to know what gear you've got and how big a move you want to make.
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1976 Ford Mustang II Ghia: 302 with a 600cfm Edelbrock carb, Edelbrock Performer 289 intake, Dynomax Blackjack headers, 2.5" exhaust with Flowmaster Super 44s. RJS 11-gallon fuel cell, C4 tranny, chrome 16" pony wheels, fuzzy dice, brown vinyl half-top, and painted in the tackiest color ever (harvest gold, that's why I call it "The Goldenrod").
Also have a 2003 Dodge Ram (lightly modded daily driver/tow rig/office/dining room/home away from home/workshop... I call it "The Big Blue Dawg".)
Alright, so lower RPMs equal better mileage, so I thought. But... that means I'd get the best fuel economy at about 45-50 MPH!
How... fun.
Generally true... my '75 Chevy pickup gets 16+mpg even with a 400 cubic inch engine that's built with all the goodies driving about 50-55mph... nice even 2200rpm for her, still just on the front two barrels of the carburetor; My Dodge Ram gets 28.9mpg at 50-55mph (found that out in a 40 mile construction zone, I'd gassed up right before I hit it, and it's onboard fuel mileage computer was showing 29.5mpg so I gassed up to see if it was right... damn close... and damn depressing since I get about 19mpg at 70mph!)
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1976 Ford Mustang II Ghia: 302 with a 600cfm Edelbrock carb, Edelbrock Performer 289 intake, Dynomax Blackjack headers, 2.5" exhaust with Flowmaster Super 44s. RJS 11-gallon fuel cell, C4 tranny, chrome 16" pony wheels, fuzzy dice, brown vinyl half-top, and painted in the tackiest color ever (harvest gold, that's why I call it "The Goldenrod").
Also have a 2003 Dodge Ram (lightly modded daily driver/tow rig/office/dining room/home away from home/workshop... I call it "The Big Blue Dawg".)
Alright, so lower RPMs equal better mileage, so I thought. But... that means I'd get the best fuel economy at about 45-50 MPH!
How... fun.
Not necessarily...
Without getting into complex math and crap, let me try to explain this simply and quickly.
The higher the RPM, the more gas you consume (typically). IF you mash it at a low rpm and let off at a higher rpm, you may actually use more gas at the lower rpm. For our purposes, lets assume you are using cruise control.
At 55 mph, your rpms are around 2000, right?
At 65, you're around 2400...
At 55 mph, you're using less fuel in the same amount of time, but at 65 mph you're covering 18% more distance in the same amount of time.
To find your ideal speed/rpm, simply drive at a single speed on the highway for about 20 minutes after filling up. Calculate your gas mileage. Do it again for another speed. Do it again at another speed. Eventually, you'll find an ideal rpm. But, with your car, you're probably not going to see much of a difference.
Ahh, alright, that makes sense. I had wondered about the RPM/speed to time ratio, and whether it made much of a difference.
How about the health of the car's innards? Is it going to put more wear on my engine if I habitually drive at 2400RPMs/65MPH than at, say, 1900-2000RPMs/50MPH?
Obviously, how late I am for work dictates how fast I drive most of the time. Heheh.
1989 Mustang Wondering about Ideal RPMs When Driving 
