# What's the most efficient speed to drive at? **URL:** \ **Category:** Cecil's Columns/Staff Reports\ **Created:** [October 11, 2013, 3:14pm UTC](https://boards.straightdope.com/t/whats-the-most-efficient-speed-to-drive-at/670987 "2013-10-11T15:14:24Z")\ **Posts on this page:** 8\ **Page:** 2
**Author:** ![watchwolf49](https://avatars.discourse-cdn.com/v4/letter/w/e9c0ed/32.png) [@watchwolf49](https://boards.straightdope.com/u/watchwolf49)\ **Post date:** [November 30, 2013, 3:27am UTC](https://boards.straightdope.com/t/whats-the-most-efficient-speed-to-drive-at/670987/22 "2013-11-30T03:27:08Z")
Best gas mileage is with a Tesla … any speed. ---
**Author:** ![Rick](https://avatars.discourse-cdn.com/v4/letter/r/9f8e36/32.png) [@Rick](https://boards.straightdope.com/u/Rick)\ **Post date:** [November 30, 2013, 4:46am UTC](https://boards.straightdope.com/t/whats-the-most-efficient-speed-to-drive-at/670987/23 "2013-11-30T04:46:31Z")
> [@watchwolf49](#): > > Best gas mileage is with a Tesla … any speed. Nope sorry that creates a divide by zero error. ---
**Author:** ![watchwolf49](https://avatars.discourse-cdn.com/v4/letter/w/e9c0ed/32.png) [@watchwolf49](https://boards.straightdope.com/u/watchwolf49)\ **Post date:** [November 30, 2013, 2:34pm UTC](https://boards.straightdope.com/t/whats-the-most-efficient-speed-to-drive-at/670987/24 "2013-11-30T14:34:50Z")
> [@Rick](#): > > Nope sorry that creates a divide by zero error. The limit as gas consumption approaches zero gives a mileage that approaches infinity. Anyone of the Church of the Most Holy Sir Issac Newton will tell you that IS infinite miles per gallon of gas. Dividing by zero is not an error, our initiates here at the monastery typically say it is not defined. In of itself, a Tesla gets the best gas mileage … but that’s not to say that salmon aren’t getting ground up in hydroturbines for every mile driven. ---
**Author:** ![drs1969](https://avatars.discourse-cdn.com/v4/letter/d/dec6dc/32.png) [@drs1969](https://boards.straightdope.com/u/drs1969)\ **Post date:** [December 9, 2013, 12:48am UTC](https://boards.straightdope.com/t/whats-the-most-efficient-speed-to-drive-at/670987/25 "2013-12-09T00:48:28Z")
> [@postpic200](#): > > It depends on a lot of factors, but in general the sweet spot tends to be between 40 - 45 mph. > > I seem to remember back in the 1970’s there were government studies that showed that [Home - Transportation Energy Data Book Transportation Energy Data Book](http://cta.ornl.gov/data/tedb32/Spreadsheets/Table4_28.xls%E2%80%8E) (a newer study that included some older data). It was during the 1970’s “fuel crisis” and cars were much bigger and about as aerodynamic as a brick and the government was trying to force people to save fuel, but even the government felt the public wouldn’t drive that slow on the highways so they went with the 55 mph speed limit. They wouldn’t go 55 either! Dad got three tickets the first year. He drove in his job, 6-8 hours each way. ---
**Author:** ![Cerealbox](https://avatars.discourse-cdn.com/v4/letter/c/7ab992/32.png) [@Cerealbox](https://boards.straightdope.com/u/Cerealbox)\ **Post date:** [December 11, 2013, 9:10pm UTC](https://boards.straightdope.com/t/whats-the-most-efficient-speed-to-drive-at/670987/26 "2013-12-11T21:10:43Z")
I heard recently that cars are most efficient at around 55 mph because that’s how the government measures mpg. It was in the context of how that standard has caused cars to be terribly inefficient when it comes to gas because people rarely drive at that speed and how using a single speed at which to measure fuel efficiency has caused cars to be designed so that they get terrible fuel efficiency driving even moderately off from the speed at which it was designed to be most fuel efficient at. Edit: this was in a podcast with some woman who had written a book about wastefulness in industrial design, from the perspective of someone who took a very green approach to things, and whom I didn’t entirely trust to be politically neutral, but who none the less seemed to have her basic facts down. ---
**Author:** ![Xema](https://avatars.discourse-cdn.com/v4/letter/x/9de053/32.png) [@Xema](https://boards.straightdope.com/u/Xema)\ **Post date:** [December 12, 2013, 4:49am UTC](https://boards.straightdope.com/t/whats-the-most-efficient-speed-to-drive-at/670987/27 "2013-12-12T04:49:01Z")
> [@watchwolf49](#): > > Best gas mileage is with a Tesla Or any diesel-fueled vehicle - these, too, use no ‘gas’. Of course, just like the Tesla, they require a source of energy. ---
**Author:** ![Oldfarmermac](https://avatars.discourse-cdn.com/v4/letter/o/f19dbf/32.png) [@Oldfarmermac](https://boards.straightdope.com/u/Oldfarmermac)\ **Post date:** [February 16, 2014, 4:44pm UTC](https://boards.straightdope.com/t/whats-the-most-efficient-speed-to-drive-at/670987/28 "2014-02-16T16:44:06Z")
A lot of newel cars have instantaneous mpg readout and a quick informal poll of acquaintances with such cars indicates that the absolute max mpg usually falls between 25 and 45 mpg. Below 25 it is almost impossible to drive in top gear in a modern stick shift and an automatic just won’t shift into top gear at such a low speed. Above 45 aero drag plays a bigger role in most cases with new car because the engines are about as efficient at low speeds as at higher speeds.The most aerodynamic cars of all are the ones which will achieve their mpg max at higher speeds. In the case of older cars the engine may be more efficient at higher speeds and mpg may max out at a somewhat higher speed because even ten or fifteen year old cars are far more aerodynamic than really old cars built back when gas mileage was mostly an afterthought. ---
**Author:** ![sbunny8](https://avatars.discourse-cdn.com/v4/letter/s/c77e96/32.png) [@sbunny8](https://boards.straightdope.com/u/sbunny8)\ **Post date:** [February 17, 2014, 12:22am UTC](https://boards.straightdope.com/t/whats-the-most-efficient-speed-to-drive-at/670987/29 "2014-02-17T00:22:47Z")
For an ICE (internal combustion engine), different points on the power curve are more or less efficient than other points. For any given gear, there is an optimum point which gives the best combination of engine RPM and vehicle speed. In top gear, most cars reach this point somewhere between 40 and 60 mph. For an EV (electric vehicle), efficiency is pretty much the same at any RPM. The controlling factor therefore is indicated airspeed. The slower you go, the less energy is wasted pushing air out of the way. Optimally efficient speed is probably around 10 mph. Take the Mitsubishi i MIEV electric vehicle, which has a combined EPA rating of 112 MPGe (miles per gallon-equivalent ;they figure 1 gallon of gas is equivalent to 33.7 kWh of electricity). I own one of these EVs. I have found that driving half the speed gives twice the range. At 64 mph, your fuel light will be blinking after 40 miles. Slow down to 32 mph and your fuel light won’t start blinking for 80 miles. I’ve never had the patience to see what would happen if I drove all day at 16 mph. I imagine the car might go 160 miles, which would take me 10 hours. [Previous page](https://boards.straightdope.com/t/whats-the-most-efficient-speed-to-drive-at/670987.md?page=1)