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Is driving faster to one place more gas efficient or driving at a slower and constant speed?

Okay, my mom bought a new Lexus RX350 I told her that if she drives slower and at a constant speed, you'll save more gas. But she thinks that if you drive faster and get to the place you want faster, you save more gas. Which is correct? And does every car have its optimal mpg level?

Public Comments

1. It depends on the average RPM your engine is running at and how hard it has to work to keep the car moving. If you're driving 20 MPH and the car stays in a lower gear, then you are going to use a lot more gas than driving 55 MPH in a higher gear. For Manual Transmissions: The lower RPM will get you the best gas mileage, but if the driver doesn't know how to shift gears then you'll lose all of that. For example, some people shift their car all the way up to high gear on the highway even if they aren't really going that fast. Just because a car has a five-speed doesn't mean that you always should get it into 5th as soon as possible. Some people drive their cars too slow in high gear and you get some jerky stuff from the transmission and you are actually using more gas trying to move the car like this.
2. I'm including a site talking about emissions measured to tell you how efficiently your car is running, and really the more emissions the more gas is consumed. Jack rabbit starting and braking can increase consumption by as much as 40%. The idea is to accelerate smoothly and slowly, and get into high gear as soon as possible. There's also a chart that puts top efficiency at about 55mph. Meaning if the speed limit is 75mph, but you want to save the gas, then you go under. I don't advocate speeding though since that will get you a ticket at best, injuries and such at worst. Safety is worth more than the price of gas. They also have several other things to do to improve gas mileage, from tire pressure to not running the AC. I know that I was told to only fill up at night when the gas it cool since the expansion of gas in warmth makes you buy less for the same amount... (basically... warm gas takes up more space and so you buy less when it's expanded, cool gas takes up less space and so you buy more.) Add: Double-checked on finding a site with more accurate fuel models, and they STILL say that even with the more gas efficient cars of today (and... the Lexus Rx350 is not one of them, with a rating of about 20mpg... at about the best...) cars lose their gas efficiency once they accelerate above 60mph. So the older model of 55mph being the best speed for gas use still stands. Not saying it couldn't change. But not in this case, and not in most cars yet.
3. I think as long she drives a constant speed, that'll use less gas. I don't think it matters much if she drives fast or slow. But I would say that new cars get better gas mileage at higher speeds, even around 70 mph. You're correct that every car has it's own optimal mpg. And it used to be that driving slower around 55 mph got better gas mileage, but I don't think that's true for today's cars. She could test her gas mileage by driving fast after a fill up and slowing down after getting gas again and compare the gas mileage.
4. You are correct. Unfortunately your mom is one of those wormen who are completely ignorant of the basic laws of physics. Just let her drive her car and don't argue with her. It's her money after all.
5. the faster you drive, the more fuel your use. every car will have test figures stating the MPG
6. Depends what you consider slow and constant. If you ever noticed HIGHWAY is always better MPG then City. So if you think 55 is fast then you are wrong about slow being better. Yeah part of the problem in city is Start and Stop, but the other thing is that even at a constant rate cars tend to be best between 40-55, or at least used to be.
7. Its like painting , say your going to paint something and a gallon might not quite be enough. If you paint really fast you will be done before you run out of paint, where as if you paint slow the paint will run out long before your done<