Efficient alignment
 

I could use some opinions on alignment numbers.

I haven't taken the MX-5 in for an alignment yet. It tracks straight, but it's a fair bet that the previous owner had a fairly aggressive, perhaps even autocross-oriented alignment on the car, based on how much front camber there is.

Factory specs, per wheel:

Front:
Caster: 5.5° to 7.5°
Camber: -1.7° to 0.3°
Toe: -1/16" to 1/8" (ideally 1/32" toe in)

Rear:
Camber: -2.7° to -0.7°
Toe: -1/32" to 5/32" (ideally 1/32" toe in)


Flyin' Miata recommends the following for aggressively driven street cars:

Front:
Caster: 8.0°
Camber: -1.8°
Toe in: 1/32" per side

Rear:
Camber: -1.8°
Toe in: 1/32" per side

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I take corners at rather high speeds, so I'm looking for a reasonable compromise. I'm thinking of dialing the caster back just a bit, perhaps 6°, and setting camber to something like -1.5°. What I'm not certain about is toe. I can see a reason for having just a bit of rear toe in, but since the car is propelled by the rear wheels and it ought to have a tendency to toe-in anyway under power. What would be the purpose of slight front toe in? Thoughts on zero toe, front and back?

Zero toe in back is normal, zero toe in up front is considered "in spec" on most vehicles.
I ask for zero toe in on the leaf when I get it aligned.

Toe in is stable, toe out is unstable (the vehicle can't figure out which side to follow).

On solid rear axles, toe isn't a problem because it is very close to zero.

Camber tends to cause good cornering, but at the expensive of tire wear. So it is very important to get the toe very close to the target value if you want good tire wear.

A nice new tight front steering system might get away with zero toe for a while. Test stuff and see if the system flexes, and how much. The how much is twice what your setting should be for a side if your preventing the other side from moving. Ditto for the rear.

Camber is a more personal thing since it loads the steering. I like lots of negative in the F250, but not much in the golf because I feel it makes the steering heavy and less responsive. Positive makes steering bump twitchy on some cars, neutral makes the tie rods flex erratically and any play is magnified.

My $0.02. YMMV and I don't worry about wear as much as handling or traction since my tires tend to sun rot

I would keep some toe in on the front (in the middle of the recommended spec for toe in), and use the minimum recommended toe in on the back.

Toe in is required because suspension flexes at higher speed from tire drag. This will cause it to be zero toe in at highway speed or during braking. What you don't want is toe out at speed. That leads to very twitchy and dangerous steering. Aftermarket racing suspension is much stiffer than stamped steel factree components and you can get away with less toe in than stock since there is less flex.

If you hypermile at 50 mph or less, you could get away with zero toe in front and back.

As far as caster, more positive caster is more stable at speed but leads to harder (effort used) steering at low speed. Camber, IDK. I like zero just for tire wear, especially if you are running maximum tire pressure for gas mileage. There is some advantage with having camber cornering, as it allows the maximum size contact patch on the road.

I had a street rod shop do a four-wheel alignment on my Type III notchback, and it had the most responsive steering of anything I've driven. I think it was the rear toe, but IDK if it was plus or minus from zero.

High caster is supposed to give better steering feel and reduced camber when the wheel is turned. I would keep max caster. -1.5 camber is not bad for tire wear at all, I would be okay with that.

Not sure about toe, but my uninformed opinion is to err towards 0. If factory asks for toe in, I would personally feel the urge to reduce the toe in.

For what it's worth, the G1 Insight aliment specs, which I'm sure are no-compromise economy specs, are zero camber, zero toe in the front, and very slight toe in at the rear.

It sounds like that's the way.

Farm tractors have massive positive front camber. I guess it makes for straighter furrows?

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The only piece I'd add to this is, I'm often taking corners at the very limit of grip. Having some negative camber likely evens out tire wear, but I don't have a baseline yet. I suppose if I lose some rear-end traction from having zero toe in at the back, I'll simply scale back the speed around corners slightly, and it'll be fine, but I'm a little wary of making the car too tail-happy. The compromise for being able to set the rear toe to zero might be a little more rear camber than front, and/or a stiffer front sway bar, so the rear is less likely to get out from under me.

They do that so that the footprint of the front tires is small and can fit in a furrow. The alternative is to space out the front axle so each is in its own furrow.

So, like a slot racer then?

And what would you say the target value should be, to keep tire wear low, accounting for dynamic loads and rubber bushings that deflect?

That wold be case dependent, varying with suspension geometry and [possibly] roll center height.

Solid vs trailing arms vs A-arms; anti-dive and anti-squat; and &tc.

Go by the vehicle manufacturer's published target values for toe. Ignore the tolerance - or at least divide the range by half!