With gas approaching $4 a gallon and diesel well north of that, everyone is interested in squeezing every last mile out of every fillup. We’ve all heard some of the common tips to save gas, like combine trips, slow down, etc. Most of them are true. Here’s my list, with some explanation from an engineer’s point of view:
- Slow down. The aerodynamic HP required to go twice as fast is not 2 times the HP, not 4 times the HP, but 8 times more HP! It increases as the cube (raised to the 3rd power) of the speed. Now, not all the gallons of fuel you use goes into overcoming aerodynamic drag, there is also rolling resistance, driveline losses, engine friction, accelerating/decelerating, going up/down grades in the road, etc. But, if you don’t have to be somewhere quick, slow down.
- Drive so you don’t have to use your brakes. Every time you touch your brakes, your are turning the the kinetic energy of your moving vehicle into heat. It takes gasoline energy to get your vehicle moving, you don’t want to be wasting it turning it into heat. This means to watch the traffic lights and start coasting if you can’t make a light. When exiting the freeway, back off and coast down to a slower speed. However, be ready for some dirty stares from the driver behind you who will race up to that red light and then slam on the brakes.
- Keep the car moving, don’t stop it completely. All drag racers know its much harder to accelerate from a standing start than a rolling start. If you are coming up to a traffic light, it may be best to use some brake if it means you can keep rolling when traffic starts to move again. This goes hand-in-hand with the point about brakes above.
- I’m all in favor of alternate energy sources. Ethanol has some good performance possibilities. But for saving money, ethanol is a complete waste. Ethanol only has 65% of the energy per gallon of typical gasoline. Here’s one link: http://forum.onlineconversion.com/showthread.php?t=233 That means it is only worth 65% as much as gasoline. Or, if you get 20 MPG on gas, you will likely get only 13 MPG on ethanol. Now there are some adjustments that can be made with ethanol which can bump that up some, but you still take a huge hit on MPG. At $4 gasoline, you should only pay $2.60 for ethanol to be equal to gasoline, and we know ethanol is much higher than that.
- Do anything you can to make your vehicle lighter, roll easier, and more aerodynamic. This means get unneeded stuff out of the trunk or pickup bed and keep the tires inflated. I wish I could say what would make your vehicle more aerodynamic, but aerodynamics is not an “intuitively obvious” endeavor. For example, most credible work on pickup truck tailgates show that you want to keep the tailgate up for better aerodynamics (here’s one: ecow.engr.wisc.edu/cgi-bin/get/me/363/ghandhi1/pickup.pdf or just google “tailgate aerodynamics”). The factories have made huge improvements in aerodynamics and it is unlikely there is anything you can do to improve it and keep your vehicle safe to operate.
- Do anything you can to take load off your engine. Turn off anything electrical in the car you don’t need. That electricity is coming from gasoline through the alternator. Headlights are a safety issue, so keep them on. Turn off the AC if not needed. Most defrost modes will turn on the AC to dehumidify the air for defogging, so don’t use that if not needed. Other things like synthetic oil and clean air cleaners help marginally.
- Keep your engine running well and efficiently. For you stockers, that means keep it tuned up. I’m a big believer in the factory calibration is probably best for all around performance and fuel economy. So I don’t endorse any chip changes or calibration mods. For street performance, it means don’t go to huge carbs but do go to EFI, don’t go for huge cams with no idle vacuum, don’t use huge intake runner and plenum volumes, etc. Your engine may get great BSFC at 500 HP of .390, but cruising around when you are only using 10-40 HP, it’s getting BSFC’s of about 3.900, 10 times worse.
- When cruising, keep engine RPM low. The lower the RPM, the less internal friction in the engine. With a manual transmission, shift into a higher gear quickly, even skipping gears works. I many times go 1-2-5 if I don’t have to accelerate in 5th. However, don’t go so high that you have to lug the engine, sending the engine control system into power enrichment mode. Also, modified engines with big cams and big intakes don’t like lower RPMs, so you will have to keep the RPMs up some.
- Don’t idle the engine. Allthe gas going in is just being used to overcome engine friction. Idling just makes it convenient for you to get going from a stopped condition without starting the engine. Now, obviously you don’t turn the engine off in traffic, primarily for safety reasons. But if you are in a line at McDonalds that’s just not moving, turn the engine off. The rule of thumb I’ve heard is if you are going to be stopped for more than 30 seconds, turn the engine off. The fuel used in idling for 30 seconds is about the fuel required to restart the engine. Also, don’t warm up the engine by standing idling. Get going cruising and accelerating slowly til the engine warms up. Obviously you don’t accelerate slowly into traffic in front of that speeding Mack truck.
Most all these principles are applied in much more detail in our Fuel Economy Calculator. With it you can actually see how much you would save by shifting to a higher gear, slowing down, proper tire inflation, etc. It also lets you street performance guys and gals see how some of your engine, clutch/converter, and transmission mods affect cruising fuel economy and what can be dont to bring it back. Here’s the link to it: http://www.performancetrends.com/fuel_economy_calculator.htm