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Catlas_Se7en
Catlas_Se7en New Reader
4/17/19 6:38 a.m.

So I just recently bought my 1993 miata. I've filled it up twice since I got it, about 400 miles total drive. My most recent fill up was 170 miles on the tank. That's like 14mpg. That's horrible for a 1.6. Anyone know what I can do to fix this? I will say I drive it hard but not all the time. I rarely break 50mph, headlights down for the most part, tend to drive around town in 3rd to 4th gear.

Floating Doc
Floating Doc Dork
4/17/19 6:42 a.m.

Fresh plugs to start, cheap and easy. I would next look at the catalytic converter, may be plugged.

Catlas_Se7en
Catlas_Se7en New Reader
4/17/19 6:44 a.m.

In reply to Floating Doc :

Did my plugs first thing when I bought it. Then did my coolant hoses because two of them burst on me. I'll check the cat.

gunner
gunner HalfDork
4/17/19 7:00 a.m.

run a scan tool and make sure your fuel trims are in spec and your ecu doesnt think you are7 running lean when you are not, then make sure your o2 sensors are working and working correctly. The front one should bounce around a lot (in volts) and the rear one should be pretty steady(in volts). check for vacuum leaks, I use the smoke test as an excuse for a cigar(YMMV).

Edit, also change out the fuel filter unless you know it has been done recently

sergio
sergio Reader
4/17/19 7:55 a.m.

Thermostat stuck open?

Catlas_Se7en
Catlas_Se7en New Reader
4/17/19 7:57 a.m.

In reply to sergio :

I don't think so. I've filled up the coolant twice and both times it's drained into the radiator properly. (First time I filled it was after I drained it. Second was after two coolant hoses blew up on me.)

Ovid_and_Flem
Ovid_and_Flem SuperDork
4/17/19 8:08 a.m.

Keep your foot out of it.cheeky

Catlas_Se7en
Catlas_Se7en New Reader
4/17/19 8:15 a.m.

In reply to Ovid_and_Flem :

It's so difficult not to!

AWSX1686
AWSX1686 SuperDork
4/17/19 8:52 a.m.

I've had a 97, 94, 01, and now another 94. I've pretty much always gotten about 25mpg driving them pretty hard. (Haven't tracked the new 94 yet, but I would expect it to be the same.) These are all 1.8, and yours should be a 1.6, but I wouldn't think it would be much different. 

 

I would check it the thermostat is stuck open, I had that happen before and it would never get warm enough for heat, and also didn't get great fuel mileage. Pretty cheap and easy part to swap, and you can compare new and old in a pot of water brought to a boil with a thermometer in it. 

Catlas_Se7en
Catlas_Se7en New Reader
4/17/19 9:08 a.m.

In reply to AWSX1686 :

I'll check that, you're the second recommendation. 

ProDarwin
ProDarwin UltimaDork
4/17/19 9:10 a.m.

Also, you can't improve what you can't measure.  Calculate actual distance traveled vs. gallons used instead of using rough numbers.

John Welsh
John Welsh Mod Squad
4/17/19 9:27 a.m.
Catlas_Se7en said:

 ...tend to drive around town in 3rd to 4th gear.

Move to 5th gear whenever possible.  My '90 Miata accepts 5th gear as low as 35 mph.  The higher the gear, the lower the RPMs.  The lower the RPMs the lower the fuel consumption.  

 

Here's the right way to track mpg:

  • fill tank to the top.  Just accept that it full at the first time the hose clicks off.  Don't force in more.
  • reset the tripometer to zero
  • drive until the tank is low-ish and fill the tank again to the first click

You should then see something like...you have now bought 9.255 gallons of gas.  Your tripometer says you have traveled 216.6 miles. With your phone calculator, before leaving the gas stations, you can see that 216.6 divided by 9.255 = 23.4  You have gotten 23.4 miles per gallon.  

Once you have done the calculation then set your tripometer back to zero so you can calculate the MPG again next time you buy gas.  But, you have to fill the tank every time for this to work.  If your the kind of person who just ads 10 bucks and then again ads 10 bucks when it looks to be empty again then your numbers will never be accurate.  

 

I find that due to gearing, and the joy of driving, my '90 Miata rarely gets more than 25 mpg.  

Ovid_and_Flem
Ovid_and_Flem SuperDork
4/17/19 9:47 a.m.
Catlas_Se7en said:

In reply to Ovid_and_Flem :

It's so difficult not to!

If I hadda guess you're having fun wrapping it out in first second and third around town.  You gonna play you have to pay.wink

AWSX1686
AWSX1686 SuperDork
4/17/19 9:50 a.m.
Ovid_and_Flem said:
Catlas_Se7en said:

In reply to Ovid_and_Flem :

It's so difficult not to!

If I hadda guess you're having fun wrapping it out in first second and third around town.  You gonna play you have to pay.wink

I drive mine hard every day and get a consistent 25mpg.

volvoclearinghouse
volvoclearinghouse PowerDork
4/17/19 10:03 a.m.

While fuel economy will improve somewhat with lower RPMs, dogging the engine in a high gear doesn't necessarily mean better MPG.  In fact, it can hurt it.  I've found that holding slightly higher RPMs in my Mazda 3 in each gear tends to improve my fuel economy.  The engine's sweet spot for efficiency per kW seems to be in the 1800-2500 RPM range.  But by the "shift reminder" I'd be dogging it at 1500 RPM in 6th a lot more often.  

Robbie
Robbie UltimaDork
4/17/19 10:11 a.m.

How large is a miata 1.6 gas tank? 170 miles divided by 14 mpg is 12.1 gallons...

I don't remember ever putting more than 10 gallons into mine. But I also remember mine didn't have great range (because of small tank) and filling up about every 200 seems about right.

Edit, Google says Miata fuel capacity is 11.9 gallons. That would be if you ran it completely dry, which is quite the hassle. You might want to re-evaluate where you buy your gas (since they might be charging you for more than you are getting)!

kazoospec
kazoospec UltraDork
4/17/19 10:40 a.m.

I did better than 14 mpg at a track day.  Something is definitely wrong with the car, the math or both.  The other thing I'd check is your tires.  Are any of them ridiculously low?  Do any show any weird wear patterns?  It doesn't explain 14 mpg, but low pressure can bleed off an mpg or 2.  A bad alignment can do the same.  It might be a combination of a bunch of factors.  

 

Robbie
Robbie UltimaDork
4/17/19 10:52 a.m.

Also, wrong size tires can throw off your odometer. If the tires are bigger diameter than stock, you are travelling farther than the odometer says. (and you are accelerating more slowly than you could!)

BoxheadTim
BoxheadTim MegaDork
4/17/19 11:41 a.m.

IME track day/HPDE mileage in an NA6 or NA8 is somewhere in the 14-15mpg range. And I don't think  you can squeeze 12 gallons into the tank in an NA even when it's completely dry. But I've been wrong before.

Catlas_Se7en
Catlas_Se7en New Reader
4/17/19 11:51 a.m.

In reply to kazoospec :

I do need a serious alignment. To go straight, my wheel is like 45 degrees to the right.

Catlas_Se7en
Catlas_Se7en New Reader
4/17/19 11:52 a.m.

In reply to Robbie :

I'm on 14s, specifically 14x5.5 American Racing AR24s.

BoxheadTim
BoxheadTim MegaDork
4/17/19 12:03 p.m.

In reply to Catlas_Se7en :

What tire size?

Catlas_Se7en
Catlas_Se7en New Reader
4/17/19 12:06 p.m.

In reply to BoxheadTim :

195/x/x I personally don't remember the other two off the top of my head and I'm not able to check at this time.

ProDarwin
ProDarwin UltimaDork
4/17/19 12:08 p.m.
Catlas_Se7en said:

In reply to kazoospec :

I do need a serious alignment. To go straight, my wheel is like 45 degrees to the right.

Does it track straight?  It may just been a wheel centering issue.

Ovid_and_Flem
Ovid_and_Flem SuperDork
4/17/19 12:09 p.m.

In reply to Catlas_Se7en :

Your speedo may be off dependingon aspect ratio..

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