"MPGiata" modding thread: aiming for 50 MPG with a 1990 Miata. Update: success, 55 MPG!

[MetroMPG]

^ Never heard of it!

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Weekend mods: not much, other than I replaced the transmission & differential oil.

I did some reading on the Miata forums, and Redline MTL synthetic is used by many to improve cold shifting because it's supposedly thinner than other 75w90 oils. Should slightly reduce energy consumption too.

Of course I couldn't find any locally.

So I did what I did with the Firefly, which also calls for 75w90 in the transmission: I bought some GM Synchromesh oil (which is also used in 75w90 cars). I compared its viscosity against the old stuff that was in the Miata's transmission, and the GM stuff pours twice as fast. I ended up mixing 50% Synchromesh with 50% synth 75w90 gear oil.

The car no longer has the "hard to get into 2nd gear" problem when cold.

Replaced the diff oil with synth as well.

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EDIT! Oh, I forgot I also did a tiny bit more "lightweighting": took off another 5 lbs or so. (Off the car! The car!)

[MetroMPG]

Interesting - it seems like your car has one of those automatic engine stop/start systems!

Is that a BMW?

Twice:

Is that black cardboard on your car?

(Explains what coroplast is...)

[Baltothewolf]

Originally Posted by MetroMPG

^ Never heard of it!

---

Weekend mods: not much, other than I replaced the transmission & differential oil.

I did some reading on the Miata forums, and Redline MTL synthetic is used by many to improve cold shifting because it's supposedly thinner than other 75w90 oils. Should slightly reduce energy consumption too.

Of course I couldn't find any locally.

So I did what I did with the Firefly, which also calls for 75w90 in the transmission: I bought some GM Synchromesh oil (which is also used in 75w90 cars). I compared its viscosity against the old stuff that was in the Miata's transmission, and the GM stuff pours twice as fast. I ended up mixing 50% Synchromesh with 50% synth 75w90 gear oil.

The car no longer has the "hard to get into 2nd gear" problem when cold.

Replaced the diff oil with synth as well.

---

EDIT! Oh, I forgot I also did a tiny bit more "lightweighting": took off another 5 lbs or so. (Off the car! The car!)

You really should have put the Ford Motorcraft stuff in your transmission, it's THE trans oil to use. I have seen people complain about cold weather shifting that gets completely solved with it. Oh well.

[AdrianD]

Originally Posted by MetroMPG

The car no longer has the "hard to get into 2nd gear" problem when cold.

I was about to mention the API GL rating but since you have no problems with 2nd gear, all good.

[kurzer]

hard to shift? watch this:

https://www.youtube.com/watch?v=iesJ64TlidQ

[MetroMPG]

Not generally hard to shift - just resistance going into 2nd gear when cold.

It's been A-OK since doing changing the oil.

[kurzer]

the point is, in almost every used mx5 the shift turret is open and fresh oil and new seals are a good idea.

[MetroMPG]

MPGuino showing typical numbers as of this AM:

49.9 MPG over 39.2 miles. That's in 2 days' use, about a 1/3 mix of sub/urban to 2/3 secondary highway driving in hot (25-30C) weather.

UNFORTUNATELY, my MPGuino is a bit buggy.

It randomly resets itself. That 39 mile reading is one of the longer stretches it's gone without a reset. Sometimes it resets multiple times over the course of a short trip, and sometimes (rarely) it completely freezes, requiring a hard reset (unplug).

I keep meaning to check for bad solder spots, but haven't gotten around to it yet.

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I posted a dedicated MPGuino / Miata installation thread.

[MetroMPG]

I finally got around to comparing the rolling resistance of the 3 different tire options I have on hand for the Miata.

Available tires are:

• 185/60R14 GT Radial Maxtour - this is what came on the car. I'd never heard of them before, but they're described as a long treadlife touring tire with low rolling resistance.
  ...
• 165/65R14 Dunlop Ensave - these the stock LRR tire from a 2014 Mirage. They're nearly new (owner swapped tires shortly after buying). Really only suitable for using on the front wheels, since the diameter is slightly smaller than the OEM tire size, so using them on the rear wheels would send gearing in the wrong direction for fuel saving on the highway (where I mostly drive).
  ...
• 185/65R15 Motomaster Touring AW/H - these are tires from a major manufacturer (not sure which one), re-labelled and sold at Canadian Tire stores. They came on the 15 inch Honda alloys my neighbour gave me that I put on the back of the car for higher gearing/lower RPM.

Goal of my little test: find out the best combination for lowest rolling resistance and highest gearing.




Test method: all tires @ 50 PSI; low speed coastdown test, from a dead stop held/released by parking brake only; down a slight grade onto a level roll-out.

Tire combinations tested:

(average of 3 runs per combination)

1) 14 inch GT Radials on the rear, 14 inch Enasaves on the front.

Results: 76 feet

2) 15 inch Motomasters on the rear, original 14 inch GT Radials on the front.

Results: 110 feet

3) 15 inch Motomasters on the rear, 14 inch Enasaves on the front.

Results: 154 feet

So through sheer dumb luck, it looks like the hoped-for combination of the Enasaves on the front + 15 inch Honda wheels on the back is the way to go. Despite the so called "LRR" properties of the GT Radial Maxtours, they dragged down the rolling distance when tested.

[freebeard]

I realize it's 'run what you brung' testing.

If you had four of the Enasaves it would be interesting to try all four, since the coast-down-from-stop test factors out the gear ratios, if not rolling diameter.

I ask because I have front-wheel drive and 185/60-14s now, and there's a whole world of 4x130 fitments.