So... the mileage I've been averaging in my 2008 Fit Sport (5spd) has dropped off a bit this year. I expected to get some of it back the end of summer (no more AC on), but it's about the same.

I have ~78K miles and I used to average about 29mpg on a tank (I drive relatively aggressive and a lot of city driving).

Lately, with the AC off (season) I'm only hitting 26-28mpg. I used to average 29-31 per tank.

I know it's not a major drop, but it's still not appreciated

Is there a common maint thing I'm missing that would cause this, or perhaps something in the emission/cat/O2 systems?

 

It sounds normal to me. When the air is Hot its actually uses less fuel but its getting cooler and more fuel is needed. If it is not consistent then some thing else is wrong. With 78000 miles on the engine its a good time to change the plugs and valve adjustment. Its good preventative maintenance.

 

We seeing winter blend gas yet? IDK if they do that in Texas?

 

They do that anywhere they can get away with it.

Sept 15th is the end of the EPA mandated summer mix fuel.

 

Has anyone seen E15 yet? It would be like the government to mandate it even though its not labeled.

 

There is a lot that effects MPG.

How cold is it? When you have the front defrosters on the AC compressor runs. The air is pulled though the evaporator (AC) to dry out the air, then through the heater core to warm it. Food for thought.

Do you have any mods done?

It could be due to normally engine wear. How are you on regular maintenance? oil changes, air filter, plugs, valve adjustment......? And are you using OEM parts?

What kind of tires are you currently running? They play a big part in fuel consumption. Also Make sure the are properly inflated.

 

Not that cold... it's TX

Yes, have mods... but have had them for 5 years, so nothing new. I haven't done plugs or valve adjustment... didn't think the valves would need an adjustment yet either

 

Valves need to be adjusted as needed and there is no set time. MPG is one sign. Is that the original battery? A bad battery will cause the engine loads to increase trying to recharge the battery.

 

Nope... replaced battery earlier this year.

 

Do anther relearn procedure. It should help because the conditions are different. I just did one on my car and its helped especially on how the engine starts. MPG is still the same and idles better too. I am getting great mpg and there is no improving on it.

There are a lot of variables and resetting and controlling the oil and fuel type your mpg should be constant. It only takes 1 variable to snow ball into a problem.

 

What relearn procedure?

 

As the O2 sensor gets older, it controls the mixture less effectively. I have seen that on many cars (not just Hondas).

Usually, gas mileage will drop. Because the air/fuel mixture is "richer" as a result, your catalytic converter has to work harder. Eventually it will fail with a P0420 code.

 

80K seems low for an O2 sensor to start failing... but who knows.

I would assume it's the pre-cat O2 sensor?

 

I noticed the problem happened around the same time the battery was changed. When you changed the battery the learned parameter were erased and it rebuild them based on conditions of that time. You can clear the learned parameter a few different ways. 1 with a scanner that is made to erase MIL which has a erase feature even if there is no codes or removing the battery cables. I use my ultra gauge. You should have a full tank or at least 3/4 and after you clear the memory you should start the motor and let it run with no accessories on and giving no gas. Let it idle till the fan come on is the procedure and then drive at a constant speed and releasing the gas pedal coasting. I have not done mine like this but I drove it for a while and had no problems.

I think this should help and is free so its a good starting point. If the O2s are bad you would have surging and hesitation with about a 10 to 15 percent loss of mpg with the MIL on. Also there would be more emissions and your catalytic converter would fail.

 

Yes, the it's the "upstream" sensor that actually controls air/fuel ratio. Honda uses "wide band" sensors, BTW. The cheap replacements don't last long.

Like all auto parts, usage determines how long it will last. Short trips in cold climates shorten life. Long trips on the highway (steady state) prolong the life.

The post-cat sensor monitors the effectiveness of the cat.

 

Check them all in this order: Air filter, Air in Tires or new Tires, Alignment.

Do the first two first because they are the cheapest and see if you notice a difference. Alignment would only help a little bit but go to a place that checks it for free.

I just bought my 2011 Fit and the gas mileage was not impressive, I took a look at the Air Filter and it seemed fine until I flipped it over and saw that it was covered with dirt and leaves. I put in a new air filter and the gas mileage and performance shot up, plus it shows on the mpg gauge in the dash.

 

Here is the idle learn procedure. ECM/PCM Idle Learn Procedure | Honda Fit DIY

You can reset the ecu with the ultra gauge or any other reader that has a erase feature with no codes. It works and have done it too many OBDII cars so you dont have to remove the battery cables.

Edit you can see the timing fuel trims and other parameters start over from factory specs.