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MetroMPG 09-18-2017 01:38 PM

"MPGiata" modding thread: aiming for 50 MPG with a 1990 Miata. Update: success, 55 MPG!
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You may have already seen my initial Miata thread, which documents the saga of buying and repairing this fun old turd bucket to make it road legal. Time for a fresh thread that focuses on the ecomodding!

The car:
  • 1990 Mazda Miata MX-5
  • Engine/trans: 1.6L (116 hp), 5-speed
  • Drag coefficient (random search result): 0.38 top up; 0.40 top down
  • Frontal area:
  • Curb weight: ~2k lbs, depending on options
  • Odometer: 216k kilometres (134k miles)

MPG potential:

EPA says: 22 city / 28 highway / 24 combined

AntiochOG did a 50 mpg fill using pulse & glide (but P&G is not my plan): 50 mpg! (1990 Mazda Miata)

The top 3 Miatas in EM's garage:
Fuelly shows a couple hundred owners of the early 1.6L cars are averaging 27 mpg US. Half a dozen owners of the 1.6L "NA" model report averages of 40-45 mpg.

The goal:

50 MPG (US) / 4.7 L/100 km, in my typical summer driving environment:
  • relaxed cruising, almost always taking the scenic route or back roads;
  • but fun DWB ("driving without brakes") in the twisties!
  • 80 km/h (50 mph) max. posted limit most roads
  • primary driving techniqe = driving with load (when not affecting traffic)
  • but also engine-off coasting toward some turns & stops, or on steep enough descents that allow me to keep speed.
  • top will be down most of the time (despite worse aero), since that's one of the main reasons I got it!

Results so far:
1) 37.5 mpg US = 6.1 L/100 km: first fill last year, cool autumn driving (10-15 C = 50-60 F) with no mods other than tire pressure;

2) 43.6 mpg US = 5.3 L/100 km: 2nd fill (1st fill of this year); warm summer driving with some mods in place;

3) 44.2 mpg US = 5.2 L/100 km: warm summer driving with even more mods in place;

4) 49.9 mpg US = 4.7 L/100 km: so close! Continuing ideal warm weather, still more mods!

5) 51.0 mpg US = 4.6 L/100 km: Success!! 2/3 ideal warm weather, still more mods.

MetroMPG 09-18-2017 01:38 PM

Completed mods:

(in no particular order)

- MPGuino fuel economy computer

- 50 PSI
- zero-toe alignment

- cleaned & regapped spark plugs (the gap was 0.010 wider than spec!)
- air conditioning delete (removed components)
- power steering delete (belt removed)
- ignition timing advance
- air dam, with...
- partial grille block
- half size radiator, and...
- one of two factory cooling fans deleted
- smooth wheel discs
- taller gearing (larger diameter rear wheels/tires)
- side mirrors delete/relocated inside
- LRR tires on the front, narrower than stock: 165/64R14 Dunlop Enasave
- wiper delete
- synthetic motor oil: 66% 5w30 + 33% 10w30
- weight reduction (goal: 100 lbs / 45 kg is attainable)
- MAF / open loop AFR adjustment
- synthetic lighter weight transmission oil (50% lightweight GM Syncromesh + synth 75w90)
- synthetic differential oil

- WAI - warm air intake

(More info & pics to come.)

Future mods:

- engine kill switch (been using the key)
- coolant/block heater
- reduce disc brake drag (springs are missing; service calipers)
- rear fender skirts
- fairing ahead of rear tires
- fairing behind rear wheels
- fairing behind front wheels
- belly pan
- decklid extension/spoiler?
- weight reduction: power steering pump delete
- lower it a bit?

Dream mods (but probably not)

- targa style aero bodywork aft of the seats
- mini boat tail!
- ???

MetroMPG 09-18-2017 01:42 PM

(post reserved)

MetroMPG 09-18-2017 03:33 PM

possible parts car
My mechanic says he's buying a 1990 Miata for its drivetrain (to swap into an MG). The body is shot (rust).

He asked me: "Need any parts?"

What stuff would you grab from an ecomodding angle? Off the top of my head, I couldn't think of anything.

What stuff should I grab purely to have spare parts on hand?


Edit: I need a driver's side sun visor!

MetroMPG 09-18-2017 03:58 PM

He's keeping the water pump, but the radiator is a possibility. I already replaced mine, though.

Unfortunately, the rear end was the same for all of this generation. A taller diff would be fantastic. Plus it'd let me run the slightly smaller diameter & narrower LRR tires on the back too.

Daschicken 09-18-2017 04:43 PM

You should also be able to adjust the idle speed on that NA miata. The bros NB miata is too new for that. AND we can't adjust the ignition advance! Enjoy the adjustability of your old miata! His NB miata idles at 900-1000 rpm.

freebeard 09-18-2017 07:34 PM

That's an ambitious goal.

It needs to be aerodynamically perfect. The killer is the top down driving enjoyment. Maybe chop the windshield and add a metal tonneau cover and headrest fairing?

That said, your list doesn't include a rear diverter.

For the rear wheel fairings, I like what I call the Bonnevette:

Starting just inside the tire they flair out to flush with the outer tire face at the back. Wheelwell turbulence is separated from the underbody flow and thrown outward into the ambient airstream. They would also form the outer fences of a diverter bell with a 4 angle roof.

MetroMPG 09-18-2017 07:44 PM

Diverter bell!


4th fill-up tonight: 48 MPG. :) [Correction! 49.9 mpg! -- see post #18]

I would like to be able to cruise a little faster than I have been, though. Mods will continue! Average speed may (will probably) increase.

And it looks like another week of warm weather coming, so the cold won't mess with my progress just yet.

kurzer 09-19-2017 03:38 AM


Originally Posted by Daschicken (Post 550263)
You should also be able to adjust the idle speed on that NA miata. The bros NB miata is too new for that. AND we can't adjust the ignition advance! Enjoy the adjustability of your old miata! His NB miata idles at 900-1000 rpm.

to do the idle, u have to connect 2 pins in the engine bay. this will show the real setting. but there is a trick. if the rev is falling to slow while shifting, u can reduce the idle air. drops it to fast, rise it a bit.

i hope u will set the mixture somewhat leaner, as i suggested in the the other thread. u will love it, i promise. miatas running verry rich in open loop.

Baltothewolf 09-19-2017 06:31 AM

Metro, see if you can get your hands on a 4.1 rear diff from a 94-99 car, it will lower your RPM by like 5%. They can be had for cheap. You may he able to trade someone for free who wants your 4.3 for track use.

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