Lost 8 MPG why?

I have two 2004 Mazda Miatas in Massachusetts. They always got 32-33 mpg with mixed driving. I hadn’t check my mpg lately until I noticed I wasn’t getting as far on a tank of gas. Sure enough I am now getting 24-25 MPG (22,000 miles on car). I did recently replace the plugs with factory originals, but put the old plugs back in to make sure it was not the new plugs, it wasn’t. I did not disturb any other wires or hoses during the plug change. I took it to my dealer and without looking at it they told me they knew exactly what the problem is. They told me its the “Winter Blend Gas” which has different additives that cause poorer gas mileage. This simply does not ring true to me, especially as I had friends test their car with no such drastic loss in their MPG. Any suggestions as to what could cause this? I’ve jacked up the car to check for brake drag, none. It is not running on 3 cylinders, the oil is at proper level and car heats up properly. I also recently put in FI cleaner, with no improvement. Stumped?

One of the first things to check is tire inflation pressure. I live in MA and with the cold temps now here I checked my tires and they were down almost 10lbs! Some loss over time, some due to the cold. Things tend to add up. The winter gas blend, low tire pressure, longer warm up times etc.

Well it is winter. I know between the winter driving conditions and the reformulated gas they use in the winter my gas mileage drops about 2-3 mpgs. 8 mpg is a BIG decrease.

As turbo mentioned…check the tire preasure. It could also be a stuck caliper. Is it pulling left or right???

You say you have two Miatas…are they both experiencing this decrease in MPG, or just one of them? Winter blend gas will cause a decrease in mileage, but 8 MPG’s is more than I would attribute to the gas alone. A clogged fuel filter will affect your mileage as well (as will a dirty air filter, for that matter), so as suggested by others maybe you’ve got a combination of small things working against you. That fuel filter is worth looking at though.

My Miata always lost mileage in the winter. Winter fuel cold temperatures and shorter average drives all contribute to it. If you have the hard top adding it would also add a little weight, but maybe better aerodynamics might counter that one.

Well my other miata is currently not registered for the winter. The brake calipers are not dragging. I did check the tire pressure today and while all tires looked fine they all read approximately 25psi, so i increased it to 40psi. I will change the air filter and look into a gas filter. Does it sound reasonable that it could be a themostat?

If it heats up, then it’s not the thermostat. Have an alignment done and see if it was any good before you did it. They will show the results. You want the toe-in to be correct.

Yes. Engine takes way to long to warm up or NEVER warms up completely- easy to see how that could affect mpg. Also- have you been idling for long periods recently? You wouldn’t believe how much that can affect mpg. Like they say above, probably a combination of factors. If you figure out what it is please post back.

Well, It does warm up as quickly and efficiently and the car does track as straight as it did before. I am putting in an air and fuel filter now, will have to see what happens at the end of this new tank of gas.

Are you using the fuel octane recommended by the car maker? Even if you are, try one tankful of gasoline one octane higher. Aging engines can need a higher octane fuel. The engine warms up quickly; but, does the engine computer know that? The engine coolant temperature sensor (cts) tells the engine computer the temperature of the engine. It could be giving the engine computer a lower figure. The spark plug wires (it’s not coil-on-plug, is it?) could be allowing some misfires. The engine computer won’t set a misfire code for misfires under 2%. Watch for the check engine light to flash during acceleration, or hill climbing. If it flashes, that shows misfires.

Well I changed the thermostat, and still no avail I am down 9 MPG. The service engine light has never come one. It was scanned and no codes, I looked at the fuel filter but an '04 miata requires a special tool to remove it.

Besides the more volatile Winter Blend, is there any chance that you’ve changed brands (or the supplier has changed formulation) and you’re burning E15? Ethanol contains less energy per gallon (thanks, Iowa!) and gets worse mileage. It might be required as a wintertime oxygenator. Don’t overinflate your tires, but keep an eye on pressure as the mercury goes down. And of course, the usual suspects: dirty filters, longer warmups, too heavy oil for the season, longer idling, shorter trips, untuned engine, bad thermostat (stuck open), snow tires,… What prompted the plug change – was the engine having problems? 4 years/22K miles is probably a little early to need to replace the wires. Did you keep gas purchase records by any chance, so you can see if the dropoff in mileage was sudden or gradual?

There is something that many people don’t understand about a scan of the engine computer: The scan is a check for signals from the sensors that are outside the permitted range of values. A temperature signal of -40F degrees is a value which is within the permitted range of output from the sensor; but, if the temperature is actually +40F degrees, the computer doesn’t know it. A trouble code would set, and a check engine light would turn on, for a sensor which has a signal outside the permitted range. // That’s why we advise anyone to actually measure, with a digital multimeter (“voltmeter”), the electrical resistance (ohms) and voltage of the sensor(s). Or, take the shortcut, and just change the sensor (here, the coolant temperature sensor). It’s inexpensive. We don’t summarily advise, without testing, expensive sensors, or other parts. // The ball is in your court. Take, and interpret the sensor readings, or…?

Mass. has been running 10% ethanol for quite some time. The drop off was all of a sudden. I changed the plugs due to the fact that twice it took 5 seconds to turn over not the typical 1 second. It was after this change that I started checking mileage again.

Hellokit, by the coolant temperature sensor do you mean the thermostat? I did try changing that to see if it was stuck open.