Wondering what kind of gas mileage other people are getting with the Mazda 3 with a 2.5 engine, auto. I'm getting around town from 19 to 22 mpg and 24 on the highway during a trip. Been told by the service dept that this is acceptable, I'm not happy with these averages.

 

Over the last several years I've been keeping an informal tally of fuel economy from the four engines (2.0 MZR, 2.0 SkyActiv-G, 2.3 MZR, and 2.5 MZR) installed in the Mazda3 (two of which are also available in the Mazda5). Sorry to say, your fuel economy for a Mazda3 2.5 Automatic is pretty much in line with the vast majority of reports.

 

Thanks shipo, also I had taken my car to a shop to test the engine, what was found is the air-fuel mixture is far to rich, the first test showed a value of 10, the Mazda Techs told me it should be 14.2. I then installed a cold air intake and retested the engine, this helped a little by raising the value to 11, which is still to rich. Have you heard of anybody getting Mazda to set this value at the correct level??

 

A few comments:

• I'm not sure who told you that the ratio should be 14.2, but that is dangerously close to stoichimetric (which can be 14.1:1 or lower depending upon the oxygenation of the fuel and what other additives are in the mixture), and running a mixture that lean, even in a modern engine, can lead to rapid engine failure.
• That the air fuel ratio changed when you installed the CAI isn't at all surprising, however, modern OBD-II closed loop systems "learn" pretty quickly. If you check now I suspect you'll find your ratio is right back where it was before you put the CAI in place.
• So, what is the correct ratio? It depends, and is dynamic based on a huge number of environmental and engine related factors, not the least of which is fuel composition.
• If you look at the Rich side and Lean side Lambda points for "pure Octane" (which has a Stoichiometric ratio point of 14.7:1), you will see that max power occurs at roughly 12.6:1 and max fuel economy occurs at roughly 15.4:1. Those numbers need to be adjusted down by somewhere between 0.6 and 1.0 for typical pump fuel with a 10% Ethanol content, and that brings you right back in the neighborhood of what you're seeing.

 

Food for thought - I average about 23 mpg and I live in Los Angeles.

 

Mine gets a tick over 39mpg on the highway if I keep it below 60mph, if I go average highway speeds (80mph) I get 23mpg and if I go even faster to the fastest my 3 can go (118mph) it gets 11mpg on average when I can manage that speed for any distance, in town I get about 14mpg. Oh on an edit......... I should mention I DONT have the 2.5, just have the 1.6L just as a comparison.

 

Wow, that's scary.

• If I keep my 2009 Mazda3 i 5-Speed at or below 60 I get about 42 miles per gallon.
• If I keep it between 75 and 80 I get between 29 and 30 miles per gallon.
• If I drive exclusively around town I get about 28 miles per gallon.
• I haven't had the opportunity to drive my car at Autobahn speeds, but when I picked up my U.S. spec 2002 BMW 530i 5-Speed in Munich via the European Delivery program, I was able to get 22 miles per gallon at a sustained 120.

 

Shipo, I will be meeting with the Mazda Rep. soon, what should I ask him, you said something about 10% Ethanol content, is this something I need to have them set the engine to run on? My MPG problem has to do during highway travel, I get at posted speed limits 23 to 26, but stays closer to the 23 mpg.

 

There is no need to have anything adjusted, the "closed loop" nature of the engine control system should automatically adjust the air to fuel mixture to be appropriate for whatever fuel is in your tank at any given moment. The thing is, compared to what I've see reported by other drivers, your fuel economy for a Mazda3 s GT 2.5 with an automatic transmission really isn't all that far out of line; I really doubt there is much your dealership can do for you.

 

If your fuel economy has dropped significantly from its stock rating and all other causes have been ruled out such as vacuum leaks, exhaust restrictions, etc. then it is a good possibility your oxygen sensor is to blame.
Fixing a serious maintenance problem, such as a faulty oxygen sensor, can improve your mileage by as much as 40 percent.
We have seen an improvement in fuel economy of 15-18% by replacing an oxygen sensor on a 91 Ford Mercury with 127,000 miles on it. Remember that it takes a while for the air fuel ratio to adjust when making a change such as a faulty O2 sensor. So when making this modification, note that it may take a few weeks for your fuel economy to reach its new optimal level (also re-set computer for re-learning).
As a last resort you could put on a throttle body spacer (search!) which gets mounted between the throttle body and the intake plenum (pic example below). These lean out the air/fuel ratios typically from .4 to .7 (values for my V6 3.0 litre) which in your case will give you better mileage and power. As Shipo said air fuel ratio in the power band should be just under 13:1 (for naturally aspirated cars---no turbo boost). Normal driving is stoich 14.7:1 ish.
When changing O2 sensors or throttle body make sure you re-set the ECU.
I would run some fuel injector cleaner thru as well since you are rich. Red Line Synthetic Oil - Gasoline Fuel Additives - SI-1 Complete Fuel System Cleaner

THIS PIC is for my 2006 Mazda 6 V6 3.0 ltr. It is made by Steeda. You order the one for the Ford Fusion as far as Steeda is concerned. They are interchangeable. For my air/fuel ratio requirements I cut thickness down in half.