I started commuting 65 km (40 miles) each day to go to work. This got me intrigued with the question of which speed I should drive in to get the best mileage. The theory is straightforward: at low speeds, the driving is inneficient because the engine must use a set minimum amount of gas to function, so driving at 10 mph is not significantly more efficient than at say, 15 mph. At high speeds, various resistance forces increase exponentially. So you just need to find a sweet spot between these considerations, which for most cars seem to be about 90 km/h (55 mph). All of this is explained in more detail in this great HowStuffWorks article: http://auto.howstuffworks.com/fuel-efficiency/fuel-economy/question477.htm But this and other websites I consulted fail to consider an important aspect: driving faster than 90 km/h does decrease fuel efficiency while it runs, but also gets you to the destination faster, meaning the car spends less minutes running than it otherwise would. To illustrate, consider a commute of 90 km: at 90 km/h = commute takes 60 minutes, with fuel consumption X at 120 km/h (75 mph) = commute takes 45 minutes, with fuel consumption Y But true fuel consumption at 120 km/h = Y - 1/4 X (which is 1/4 of an hour you don't spend driving at 90 km/h) Why is this aspect seemingly not taken into account when calculating fuel efficiency? It's apparently assumed that you're driving on an infinite road, forever, rather than eventually getting somewhere. Finally, since there are more possible speeds than just 120 km/h and 90 km/h, can someone devise a formula that is able to take this into account for all speeds, calculating thus the true most efficient speed to drive in?