I purchased a 1991 Miata a month or so ago. It has a 95 1.8 swap in it from flyin' miata. I am currently only getting about 200 miles per tank and i drive 95% highway. I am thinking that my fuel gauge might be off because i am getting 24 mpg average over 4 tanks. Please take a look at my Google sheets fuel table. I know the fuel tank is 11.9 gallons so i must be showing empty early. I saw multiple people with over 300 miles per tank. Some claiming 400. Any help would be greatly appreciated.


https://docs.google.com/spreadsheets...it?usp=sharing

- Ben

 

To add..

I only drive about 60-65 mph on the highway and have a light foot.

 

Is it still running on the 1.6 electronics? 24mpg combined doesn't really shock me a whole lot. It's worse than i've typically done with my Miatas, but not by a huge margin or anything.

 

It is still on 1.6 electronics (AFM and ECU) but i plan on getting a megasquirt. My main concern isn't the 24~ mpg it is the 200 miles per tank. I am only using 8.5~ gallons out of 11.9 that i am supposed to have. I don't overfill the tank i just let it stop when it stops so that might be part of the fuel but that shouldn't equate to 3.4 gallons not being used.

 

I know mine originally was the same way, it would say I was well past the E but would only be about 9 gal to fill up. When I replaced my fuel pump I adjusted the float. Funny thing is now I wish it was the other way bc if it is on E I am basically running on fumes, I think I adjusted it a little too much but dont feel like taking it out again. And just fyi it isnt bad to not use that last couple gallons bc there could be trash in the tank that doesnt get to the pump unless you run it near dry. I typically fill up around 220-250miles

As for mpg, when I was running my 1.8 on the stock 1.6 ecu I was only getting about 18-20mpg(mostly around town driving) during break in so 24 isnt terrible(never got better than 28 on the stock 1.6). On megasquirt im getting about 25-26(80% city 20%hwy).

 

Well if you fill up before it's empty, you won't use the whole tank.

 

I did replace the fuel pump on it. Maybe I will adjust the float. Did you just bend the rod? I read somewhere else they just moved the gauge needle.

 

I know that... The car says it is empty but i only can fill in 8.5~ gallons. That is my issue. I know there are more gallons below empty. I am sure there is a reserve of 1-2 gallons but that's almost 4 gallons off.

 

Bring a gas can and run it empty. You'll probably find that it took 11 gallons, then you'll pull the fuel pump and realize you probably bent the float or something.

 

Yes, did just bend the rod but like I said i did it just bc it was right there, i wouldnt take it all apart just to do that. I think if I remember right if i get low enough(around 10.5-11 gal at the fillup) it does have fueling issues in cornering so I really suggest leaving it and if you feel like you need it lower just take note of the odo at E and count the miles then fill up, after say 25 miles later than usua,l and see how much you have left and pick a number you are comfortable with.

 

Yep sounds like a Miata.

 

Thanks man. I am going to just leave it as is for now.

 

Yeah that mileage is normal. you are on stock ecu. With aftermarket ecus, you can go much leaner than oem tune in cruise cells.

60-65 is still fast if you are looking for mpg. 55mph (not using cruise control, and keeping throttle position steady, slowing down on uphills and coasting down hills) is the sweet spot, but its pretty inconvenient/unsafe for other motorists on the highway.

Tire choice also has a significant impact on mpg, I went from 205 re71r tires to 185 snow tires, no other changes and picked up 4 mpg.

Do you have the oem undertray? that can have a small effect on mileage as well.

 

Well another thing is that I only drive with the top down and I daily drive with Toyo Proxes r888r tires in 215. I dont think i have the undertray anymore either... I am not solely looking for mpg per say, but the average speed on the highway is 70 and i go 60-65 so i am slower than most. My goal is to get 300 miles before filling up the tank because i drive 45 minutes each way to work 5 days a week and i feel like i am filling up all the time.

 

I fill mine when the needle points at the top left corner of the E itself. Usually takes 9.5-10 gallons there.

I get 26-27 combined with a supercharger setup though....

 

Top down lowers aero.... so it lowers mpg.
No undertray increases turbulence under the car... so also reduces mpg.

If your goal is to get 300 miles per tank, then you should probably consider getting a prius.

 

There is no reason to daily drive track day tires. Extremely illegal speeds around corners are easily attainable with econotires.

-Any toe in/out will also reduce your mpg.

-Find yourself the cheapest, lightest rims that fit over your brakes (some 14x7 fit over sports brakes) and the cheapest, narrowest econotire you can find. I promise you see a minimum 4 mpg just from that.

-find a cheap, functional oem undertray

-cut out rear bumper skin, lower your drag a bit (free if you have a tool that will do it)

-look around for a different ring and pinion. I'm using the 3.63 set, and cruise at 80mph at 3800rpm. (most expensive option, but lower rpms at the same speed is more mpg)

- there are plenty of cheap cars (1500 dollar range) that get over 40mpg (many manual hondas, toyotas). Some of them easily pass 50mpg (such as and old 5 speed manual Toyota Tercel) if you are really concerned about lowering the cost of your commute that much

That's all I can think of, good luck.

 

Sounds like a lot of work and decrease in enjoyment to make a non-noticeable economy change from a financial perspective.

Also: My car can and has done over 30mpg with 6spd/4.30 running 75mph, 245/40 VR1s and a big 'ol wannnngggggggg.

Measure away!

 

To calibrate what you have:

1) Take 2 gallons of gas in a can with you.
2) Drive until you run out of gas.
3) Note miles driven since last fillup; and needle position.
4) Put the 2 gallons of gas in.
5) Drive to the nearest station and fill up.

Now you can derive:

A) Gas tank holds how much you put in + a little less than 2 gallons.
B) From (3), you know how many miles you can drive, on your commute, before you run out of gas.
C) From (3) you know what the needle is when you should have already filled up.

BOTTOM LINE
You can drive (3) miles - 25 between fill-ups (or whatever your safe-place margin is)

 

The 1.6 cars have a smaller fuel tank than the 1.8 cars. 1.8 tank is 12.7 gallons.

You can adjust the needle or the arm on the sensor if needed.