# Gas mileage **URL:** \ **Category:** Maintenance/Repairs\ **Created:** [December 28, 2007, 4:32am UTC](https://community.cartalk.com/t/gas-mileage/8760 "2007-12-28T04:32:54Z")\ **Posts on this page:** 12\ **Page:** 1
**Author:** ![weeklywidow](https://avatars.discourse-cdn.com/v4/letter/w/ad7895/32.png) [@weeklywidow](https://community.cartalk.com/u/weeklywidow)\ **Post date:** [December 28, 2007, 4:32am UTC](https://community.cartalk.com/t/gas-mileage/8760/1 "2007-12-28T04:32:54Z")
is it true that your gas mileage is less when you use your headlights… i find this hard to believe… ---
**Author:** ![ok4450](https://avatars.discourse-cdn.com/v4/letter/o/e9bcb4/32.png) [@ok4450](https://community.cartalk.com/u/ok4450)\ **Post date:** [December 28, 2007, 7:24am UTC](https://community.cartalk.com/t/gas-mileage/8760/2 "2007-12-28T07:24:31Z")
In theory, yes since the electrical load applied is heavier. This makes the alternator work a shade harder and since the alternator becomes a magnet of varying intensity when the engine is running one could say a little more strain drops the mileage. There’s another scenario in which one could figure a weak battery, faulty alternator, etc. but we won’t get into that one. However, the amount of gas used is going to be so miniscule it would be near impossible to measure short of extended lab testing and even that is grasping. It’s nothing you should be concerned with at all. ---
**Author:** ![Joseph\_E\_Meehan](https://sea1.discourse-cdn.com/cartalk/user_avatar/community.cartalk.com/joseph_e_meehan/32/21_2.png) [@Joseph\_E\_Meehan](https://community.cartalk.com/u/Joseph_E_Meehan)\ **Post date:** [December 28, 2007, 10:48am UTC](https://community.cartalk.com/t/gas-mileage/8760/3 "2007-12-28T10:48:55Z")
Sure. When you have the lights on or the heater fan or the radio, you are using electrical power that the alternator is replacing and it puts an additional load on the engine. Of course you also need to look at the amount. If 0.002 mpg is important to you, then worry about it, if not don’t. ---
**Author:** ![SteveF](https://sea1.discourse-cdn.com/cartalk/user_avatar/community.cartalk.com/stevef/32/2_2.png) [@SteveF](https://community.cartalk.com/u/SteveF)\ **Post date:** [December 28, 2007, 12:33pm UTC](https://community.cartalk.com/t/gas-mileage/8760/4 "2007-12-28T12:33:34Z")
Yes it’s true, quite true. And it is not too hard to calculate the theoretical drop in mpg. One adds up the wattage of all exterior lighting and then converts this amount of electrical power to units of mechanical power. But I’ve already told you more than most people really want to know. ---
**Author:** ![refaller](https://sea1.discourse-cdn.com/cartalk/user_avatar/community.cartalk.com/refaller/32/35_2.png) [@refaller](https://community.cartalk.com/u/refaller)\ **Post date:** [December 28, 2007, 1:30pm UTC](https://community.cartalk.com/t/gas-mileage/8760/5 "2007-12-28T13:30:21Z")
If you stick your arm out the window it should decrease gas mileage as well. I just hope you know scientific notation in order to be able to calculate the difference. ref PS- Please do not let this fact from preventing you from turning on your headlights. Even if they do not help you, they help the other drivers in low light situations. ---
**Author:** ![bscar](https://sea1.discourse-cdn.com/cartalk/user_avatar/community.cartalk.com/bscar/32/34_2.png) [@bscar](https://community.cartalk.com/u/bscar)\ **Post date:** [December 28, 2007, 3:13pm UTC](https://community.cartalk.com/t/gas-mileage/8760/6 "2007-12-28T15:13:41Z")
so THATS the reason people don’t want to use their headlights when it’s raining, or just starting to get dark outside. 😛 but yeah, any actual decrease would be really insignificant ---
**Author:** ![the\_same\_mountainbike](https://avatars.discourse-cdn.com/v4/letter/t/c89c15/32.png) [@the\_same\_mountainbike](https://community.cartalk.com/u/the_same_mountainbike)\ **Post date:** [December 28, 2007, 5:17pm UTC](https://community.cartalk.com/t/gas-mileage/8760/7 "2007-12-28T17:17:30Z")
In some states it’s illegal to drive without your headlights in inclimate weather or at dusk. People do it anyway. ---
**Author:** ![Docnick](https://avatars.discourse-cdn.com/v4/letter/d/ecae2f/32.png) [@Docnick](https://community.cartalk.com/u/Docnick)\ **Post date:** [December 28, 2007, 5:53pm UTC](https://community.cartalk.com/t/gas-mileage/8760/8 "2007-12-28T17:53:09Z")
The instrument which affects gas mileage most has 5 toes! All the finetuning in the world with minor items like light on or off, will have little effect compared to careful driving. One lady poster suggested we really don’t need the 35 mpg standard since her car’s computer (Prius) tells her exactly what driving style gives the best mileage. Tape an egg (boiled if you like) to your gas pedal, and try not to break it with your right foot! ---
**Author:** ![Joe-Guy](https://sea1.discourse-cdn.com/cartalk/user_avatar/community.cartalk.com/joe-guy/32/124_2.png) [@Joe-Guy](https://community.cartalk.com/u/Joe-Guy)\ **Post date:** [December 28, 2007, 6:26pm UTC](https://community.cartalk.com/t/gas-mileage/8760/9 "2007-12-28T18:26:24Z")
Yes. Use of headlights will decrease gas mileage. That’s why I always use a flashlight instead. ---
**Author:** ![Roadrunner](https://avatars.discourse-cdn.com/v4/letter/r/d2c977/32.png) [@Roadrunner](https://community.cartalk.com/u/Roadrunner)\ **Post date:** [December 28, 2007, 7:00pm UTC](https://community.cartalk.com/t/gas-mileage/8760/10 "2007-12-28T19:00:51Z")
Heh heh. Mb, he’s makin’ a funny, notice the first two letters or his nickname? ---
**Author:** ![Craig58](https://avatars.discourse-cdn.com/v4/letter/c/3da27b/32.png) [@Craig58](https://community.cartalk.com/u/Craig58)\ **Post date:** [December 28, 2007, 8:14pm UTC](https://community.cartalk.com/t/gas-mileage/8760/11 "2007-12-28T20:14:15Z")
OK, just for fun lets assume you have your lights on for an hour and use 200 W-hr of electrical energy. That’s about 680 BTUs. One pound of gasoline contains about 20,000 BTU of useable energy (about 140,000 BTU/gal). Assuming the whole process is about 15% efficient, having your lights on will burn about .03 gallons of gasoline in an hour. If you are driving on the highway with a 30 mpg car, you will burn about 2 gallons in an hour anyway, so having your lights on could affect your mileage by about 1.5%, maybe 0.5 mpg. That’s a very rough estimate, in reality the effect is probably smaller. ---
**Author:** ![Docnick](https://avatars.discourse-cdn.com/v4/letter/d/ecae2f/32.png) [@Docnick](https://community.cartalk.com/u/Docnick)\ **Post date:** [December 28, 2007, 8:20pm UTC](https://community.cartalk.com/t/gas-mileage/8760/12 "2007-12-28T20:20:10Z")
Make sure you use rechargeable batteries!