Stupid and I know it


#1

Two identical cars leave for a destination 100 miles away. One drives 70mph and the other drives 55mph.

The faster car arrives at destination earlier than the slower.

By virtue of driving for a longer period of time does the the slower car burn about as much fuel as the faster?

I drive a truck for a living and my personal car is a gas-licking Toyota Yaris. I was just curious.


#2

I have a Yaris also. If I drive it 100 miles at 55 mph I’ll get much better mileage than at 70 mpg.

It’s not the amount of time you’re driving that makes the difference. It’s the amount of gasoline you’re using to obtain a certain speed that counts.


#3

I poked a stick in this hornets nest a few months ago myself.
Many variables here from one vehicle type to the next ; engine rpm, final drive ratio, etc for the individual truck.
–here’s the hornets nest poker…in MY ONE truck ( not yours and not either of my other two ) I see better round trip mpg at 82 mph -vs- 75. DON’T START ARGUING YET people cuz I know I’d get even better mpg @ 55mph , but I really am not inclined to go that slow for the trip I take weekly to Albuquerque and back ,zzzzzzzzz.
But in many cases you’ll find an engine’s “sweet spot” and realize better mpg at slightly faster speeds than others >> not all faster speeds… IT TAKES TRIAL AND ERROR EXPERIMENTATION and isn’t true in all cases, or all vehicles.
—But generally, if you can drive just above the shift point of your highest gear ( so it doesn’t continually shift down again ) you’ll see the best mpg for YOUR vehicle,------- ie; highest available transmission ratio -&- lowest available engine rpm will result in your personal best MPG at speed.


#4

Will you get wetter if you run through the rain or(decreasing your overall time in the rain,but hitting more rain drops)or walking in the rain?


#5

The front of you gets wetter, the top is drier running…
The top gets wetter, the front is drier walking…
hmmm.


#6

All else being equal (road, weather, etc), the car going 55 will use less fuel than the car going 70. The biggest factor at highway speeds is wind resistance, which increases exponentially as speed rises. It takes much more power (and fuel) to push a car through the air at 70 than it does at 55.


#7

I agree. Besides, the one doing 55 will get sucked along by the vacuum of all the other cars whizzing past him.


#8

Based on your example, both cars will be operating in their highest gear with the TC locked up. All else being equal, you have a dominant, non-linear variable in the equation; drag. Therefore, the relationship cannot end up being linear. Since drag is exponentially greater as speed increases, the faster car will burn more fuel than the slower one.


#9

JMHO, but it depends. This argument came up some months ago in which I disagreed that faster always mean higher fuel consumption.
Gear ratio. torque band, etc. makea a difference.

My Lincoln Mark gets better mileage at faster speeds (been verified more than a dozen times) and of course was told I was full of it when I mentioned this.
At the time I mentioned that I was going on an out of state trip to CO and would maintain an accurate log and post back when I returned.

At a steady 55-60 MPH the car would get around 25.7 MPG and around 70-75 MPH it gets 27-27.1 MPG. The same gas tank fill method was used each time (fill on slow, round to next nickel) and this was verified not only by the dashboard MPG indicator in the Message Center but also by comparing the tripmeter against the gallons required to fill the tank. They always matched each other.

The car has 2:73 gears in it and I attribute the lower mileage at slower speeds to the fact that at 55 MPH the engine is almost lugging since it’s only turning about 1200 RPM.

If my car had 3:73 gears then it might be a different story.
(Any my old BMW 100/7 motorcycle would get better mileage at 70 as compared to 55 MPH. Same thing; the bike was a dog at 55 and would not start breathing well until about 70 MPH. Also verified on a number of cross-country trips. Since it was non-electronic in nature, blame this on dogging the vauum diaphragm carburetors.)

So I don’t think there is a one size fits all scenario.


#10

wind resistance does not increase exponentially. it increases with the square of velocity.


#11

A quantity that is squared is raised to an exponent of two.

I have no doubt that going 55 will use less fuel tha going faster - if you are in the same gear. mcparadise is spot-on.


#12

Ok4450, I think you are right. Some cars, like yours, are geared so high that the engine reaches it’s most effecient speed at a higher rate.


#13

A quantity that is squared is raised to an exponent of two.

“Exponential growth” is e (Euler’s constant, 2.71828…) raised to some power. It grows more steeply than x squared.


#14

I attribute a lugging or near lugging engine, or an engine hovering just outside the powerband as one that is straining a bit and affecting the manifold vacuum; the same characteristics as an engine going uphill or passing someone.

In a prior discussion here it was stated that a possibility for the better mileage could be the fact that the Mark of mine lowers itself by about an inch at highway speeds. This would make it more aerodynamic and therefore the mileage would improve because of that; not the speed itself. This is not the case for 2 reasons.

  1. The car lowered itself at around 55 MPH. (not word “lowered” not “lowers”)
  2. Due to aged and leaking front air bags a couple of years ago I replaced the air ride with a “normal” suspension (T-Bird). The ride height is the same no matter what speed the car is going.