pooley

Sometime last week I ran over a small bit of tire, heard a small knock as I went over it but thought nothing of it, figured it hit the exhaust. Since I last filled up, I noticed that I'm getting much worse mileage out of my tank. I usually can get about 50-60 miles before the needle hits the first quarter mark. This tank however, Ive gotten 35 miles and I just passed that mark. I checked under the car last night with a flash light, no leaks, no smell of gas, everything looked normal, then today on the way home I heard scraping when I was at speed. Pulled over to take a look, the front undertray was broken, almost split in two and hanging down towards the front, so Im guessing it must have been scooping up a good amount of air and causing a lot of drag. Does it sound like that could be enough to cause the loss in mpg? Might it be a loose o2 sensor?

EDIT: more info
42,xxx miles
Starts immediately cold and warm
New plugs about 3,000 miles ago
Fuel system cleaner (Redline SI-1) 4 tanks ago
Oil changed after the 2nd tank following the fuel cleaner
No changes in driving style

ASH8

Hi M8 and welcome to our club...

I really doubt a damaged or missing black plastic under-tray would cause roughly a 50% drop in MPG.

However, being in two pieces and letting it just hang there would not help drag efficiency.

Check Air Filter Element...@ that mileage I would change it.... I think in USA it is 40K schedule to change anyway.

MAF Sensor could also need a clean....last resort.

As a matter of more urgency...
I would also look at renewing your 4 x Ignition Coils and Spark Plug Wires/Leads.

There are a few options with the original 4 x Ignition Coils, stay with OEM, they have been updated in the past year N3H1-18-100B9U Coils @ about $320 a set of 4.

Or go with BHR's Coil Pack which are about twice the price, but in most cases have a longer life span, they also require some minor modification to their mounting/wiring, but is an excellent product sold and distributed by one of our most trusted vendors and a great guy name Charles (Ray) from BHR.

Or if you want to stay with updated original Mazda Coils contact Paul @ MAZMART, again another fantastic Vendor here....both will look after you.

So definitely check/renew Air Filter and update 4 x Coils.

If you leave this condition (of Ignition Coils) too long you could be cooking your exhaust CAT which can lead to major issues if not looked into.

I would also look into regularly adding a pre-mix (good 2 stroke Oil) to your fuel (gas), as all Rotaries love that little bit of extra seal lubrication.

fyrstormer

I still need to do my plugs. Stupid things, always costing money. On another note, after April my car will be >2/3 paid-off. Starting to get tired of making payments at this point. I'll be glad when that's over, so I can start saving money to spend on repairs instead. >_<

Anyway, if he's only getting 35 miles to a quarter of a tank, that's appalling. A quarter-tank is about 3.5 gallons. My first guess is that the tank wasn't properly filled in the first place, because any mechanical inefficiency big enough to cut the gas mileage in half should make the car feel extremely sluggish and heavy compared to normal.

My second guess is that it's spring in the USA, and he probably did change his driving style to "celebrate" the warmer weather, just like every other male with his ***** still attached, he just doesn't realize it.

pooley

Living in south texas, we dont have "spring", its been hot here for over a month already. I can guarantee you that my driving style didnt change one bit. I probably didnt fill the tank full like I normally do.

That being said.

Today I hit 210 miles on the same tank as when I noticed the low mpg, and i still have approx 2 hash marks left on the tank. So whatever the issue was, is likely resolved. I removed the under tray and changed the oil again. Havent had chance to get a new air filter yet but it is on my to do list. Ive changed it before so its not critical as of yet. The ignition coils are on my list, once money comes along.

Thanks for the responses!

fyrstormer

Probably just underfilled the tank then, yeah. Might not have been you anyway, it might've been the automatic cutoff on the fuel dispenser. But any real problem will reoccur on more than one tank of gas.