Finding and Fixing the Causes of Poor MPG
Simple Checks: Tires
I like to start with simple checks when dealing with any automotive issue. In the case of poor MPG, I like to start with the tire pressure. This is an often-overlooked item, and one that should be first on the list. Your tires are everything when it comes to your vehicle, and gas mileage is no exception. The rolling resistance of your vehicle has a direct effect on your fuel economy. If your tire pressures are low, your vehicle will have to work harder to get itself down the road. The harder your vehicle works, the more fuel it will use. It really is that simple.
The type of tire you use has a direct effect on fuel economy. Some tires are better than others; some have less rolling resistance. You might spend more for better tires, but in the end, you will likely come out on top based on your fuel savings alone.
As for what you should set the pressure at, check your owner’s manual or the tire sticker that should be located somewhere on your vehicle. There are a lot of places they put these things. Mostly I see them on door jambs, or sometimes on a sticker in the glove box. Occasionally I find them under the hood. The important thing is to find it and set your tires to that spec.
Many people make the mistake that the pressure indicated on the outside of the tire is where you want to set the pressure. This could not be further from the truth. That listing is the MAXIMUM tire pressure for that tire, NOT what you should set your tire pressures at. I also often see that people want to inflate their tires till they look round, with no bulge at the bottom. Once again, the wrong approach. That bulge at the bottom of the tire is important. It’s called the “foot” or “contact patch.” The name implies its purpose, and it is an important one. After all, you wouldn’t walk very well without your feet, or even feet that didn’t have the proper shape. Overinflated tires have the same effect. Plus, overinflated tires can be dangerous. Sure, you’ll get great fuel economy with overinflated tires, but the safety risk just isn’t worth it. Vehicles with overinflated tires are unstable and difficult to control. So please, don’t go by appearances when it comes to inflating your tires. Inflate them to the proper pressure and move on.
Here are some videos to reinforce what I’ve talked about here.
Simple Checks: Check the Oil
Here’s another simple check that can yield a big result. Engine oil is the lifeblood of your engine. If it’s low, your engine isn’t feeling very well, and as a result, it’s less efficient. A loss of efficiency equates to a decrease in fuel economy. I know your engine doesn’t really have “feelings,” but you get the point. The harder your engine has to work, the more fuel it will use. It really is that simple. Checking the engine oil is a great check if you are having issues with poor fuel economy. If you find that it’s low, top it off to the proper level and recheck your MPG.
Another thing to consider is changing your oil. Fresh oil lubricates better than old oil. As a result, your engine won’t have to work as hard to do its job. So if you’re in doubt as to when your oil was last changed, you might consider changing it if you’re having issues with poor fuel economy.
Something else to consider is using synthetic oil. Synthetic oil has better flow characteristics and lubrication ability than conventional oil, and as a result, your engine will run with less resistance when using synthetic oil. A word of caution on synthetic oil though. If your engine is old and leaking, you might forgo synthetic oil. If you switch to synthetic on an older, worn-out engine, you might see a negative result. Synthetic oil is very good at what it does, but it can’t repair a worn-out engine. In fact, it can make a worn-out engine worse. If you have an older engine, when switching to synthetic oil you might see more leaks and perhaps even have more engine noise than you did with conventional oil. However, if your engine is in good shape and doesn’t leak, you might consider switching to synthetic for better fuel economy. I often get asked if you can switch back and forth between synthetic and conventional oil. The answer to that question is, yes you can. Synthetic oil is designed to mix with regular oil, in fact many synthetic oils are really blends of synthetic and conventional oil. There are actually few fully synthetic oils. So if it’s already mixed in the bottle, you shouldn’t have any issues switching from synthetic to regular oil should you so choose.
Simple Checks: Check the Transmission Fluid
Are you seeing the pattern yet? Yep, anything that makes your engine work harder costs you fuel economy. Your transmission is no exception. Primarily I’m speaking of automatic transmissions, but you could also apply this to the manual transmission as well. If you have a manual transmission, this might not be your first check, but if you’re running out of options with a manual transmission vehicle, it’s definitely worth a look. This section mainly refers to the automatic transmission, however.
If you have poor fuel economy and an automatic transmission, checking the fluid level is one of those quick checks that can yield big results. If the fluid is low, discolored, or full of air bubbles, your transmission won’t work as efficiently. It might even start to slip, which can cost you MPG. If the fluid appears dark or old, you might consider changing the fluid. Just like changing your engine oil, this can help your fuel economy a great deal. While you’re at it, you might also consider changing the transmission filter if applicable. This will ensure that your automatic transmission is in top form and is as efficient as it can be. You can find more information about automatic transmissions in the Solving Transmission Problems article.
Simple Checks: Coolant Level and Condition
In addition to checking the oil and transmission fluid it’s a good idea to also check the coolant level and condition. If the coolant is low, or the cooling system is malfunctioning and causing an overheat, your MPG will suffer. So be sure to also check the coolant level and condition as well as the operation of the cooling fans if you have electric fans. More information on cooling system issues can be found in the Diagnosing an Overheat article.
Beyond the Simple Checks
In the next sections of this article, I’m going to get a little deeper into the diagnosis of a fuel economy issue. Once you’re done with the simple checks, you might want to head here if you haven’t solved the problem yet.
