What is relation of speed to gasoline usage?

Question

How much does your gasoline consumption go down when you reduce driving speed to, for example, 55, 50 or 45?

Answer 1

with most cars, 55mph is the most efficient speed to travel.  Although the highest gear always gives you the best mileage, less than 55mph usually puts the car in a low rpm range in which it must work harder to maintain speed.  Over 55mph, you start a battle with wind resistance.  But all of this depends on the gearing of your car.  Older cars have shorter gears, so 45mph might be the most efficient.  newer cars have taller gears and often times MORE gears (5, 6, 7-speeds), so 55mph is most optimal.

Answer 2

Your peak economy is probably at around 45 mph - there you are in your highest gear at relatively low RPMs and wind resistance is less.  

Wind resistnace is the main killer to fuel economy at highway speeds.  Because it is a viscous force, is proportional to the SQUARE of your speed.  That means your fuel economy devreases EXPONENTIALLY as you go faster.  

Example:  Lets say you drive a set distance at 45 mph and you get 40 mpg on that trip.  If you then drive the same trip at 90 mph, two things happen:

1. You get there faster, reducing the time traveled by a factor of 2.  

2.  due to speed doubling, your drag has increased by a factor of 4.  

Your also turning more RPMs but that effect is negligible compared to drag.  

So, the end effect is that for this case, your mileage will drop to about 20 mpg or half.

Answer 3

The general rule is for every 10MPH you go over 55MPH you are reducing your fuel consumption by 10-20%.

Answer 4

you'll get worse gas mileage if your goin 45mph at 3000rpm, than if you were goin 55mph at 1500rpm. it depends on what gear your in and wind resistance.