# Gas Mileage Questions **URL:** \ **Category:** NYSpeed Automotive\ **Created:** [August 11, 2006, 1:28pm UTC](https://forums.speedlife.net/t/gas-mileage-questions/15836 "2006-08-11T13:28:15Z")\ **Posts on this page:** 1\ **Showing post:** 14
**Author:** ![VANDIT](https://yyz2.discourse-cdn.com/flex034/user_avatar/forums.speedlife.net/vandit/32/4977_2.png) [@VANDIT](https://forums.speedlife.net/u/VANDIT)\ **Post date:** [October 23, 2006, 10:09am UTC](https://forums.speedlife.net/t/gas-mileage-questions/15836/14 "2006-10-23T10:09:25Z")
> [@longuyen88](#): > > I thought that if you leave it in gear when coasting you save more fuel because the wheels are driving the engine? Because if you’re in neutral, the engine needs fuel to stay at idle, but while coasting in gear, there is high vaccum, which means less fuel? Reguardless of rpm, the engine only uses fuel with on the throttle, and when it’s at idle, correct? > > correct me if I am wrong, because i always leave my car in gear. not exactly… fuel trims are based on 2 things, long term and short term fuel trims… #1 rule of thumb, idle speeds (in a closed loop pcm situation) always require less fuel, #2 if your going 70 in 5th. your map, maf, tps, o2, all will read that your engine is at a higher rpm, if your moving at 70 at 1500 your car will use more fuel than at 800 at idle. #3 injector pulse width will increase only when tps is increased\* #4\*- ALTHOUGH if your are not on the gas, and in 5th gear at 1500 you still have more air entering the manifold because irreguardless of tps, when the engine is running the throttle plate is ALWAYS open just a little… the maf will see this, and hence increase injector pulse… the difference is slight, in a full tank testing you may only see maby .001mpg better, your talking about a few hundred rpm’s here, not 9grand… — a little ASE test prep questions, i hope that helps --- _[View the full topic](https://forums.speedlife.net/t/gas-mileage-questions/15836)._