Actually knocking/pinging is caused by the fuel detonating outside of the normal cycle, if it becomes severe it can cause catastrophic damage to the engine. The lower the the octane the easier it is to ignite, with high compression engines you needed high octane to suppress the detonation until proper spark timing. Have seen the results of pre-detonation on a 13:1 compression 350 Chevy (friends car), piston had a hole melted clean through it and trashed the motor. The only good thing about "Gasohol" is that it absorbs water like gas line anti-freeze does because it's basically the same stuff.

As far as the OPs CX-7, I'd use what ever Mazda reccomends.
24 m.p.g. doesn't sound bad for vehicle. (I read 20 city - 28 Highway)
and Yes, keeping the tires up will help a lot more than you think.

I doubt ethanol can cause a knock alone. In fact it's octane is like 115 or so. They blend fuels for your area and the pump octane is pretty meaningless. Those crooks will sell what ever they can get away with. The tests are almost never performed to back up their claims of octane.

Modern cars had the compression reduced to inhibit pollution but at a tragic cost of increased fuel. It would have been much better to simply demand better fuel mileage and in total would have reduced consumption and pollution.

Top fuel cars run alcohol don't they? For good measure they throw in nitro methane. Indy cars run alcohol don't they?

You could run 100 alcohol and set fuel and compression pretty darn high.

You are right, I read more about it now. Either way, higher octane fuel is usually necessary for turbocharged cars to prevent detonation.

The way I drive, the turbocharger is rarely if ever used.

I plan on using the mid level octane gas.

I have talked to others with turbochargers and they use regular gas.

And they have had no engine problems after many miles.

Technically 89 octane and up is ok, so in the USA mid level gas (89) is ok.

Mazda recommends premium but does not require it.

It is worth knowing whether this is true or not. With the knock sensor, there is a chance that the more expensive fuel will produce a worthwhile return (usually not enough for there to be cost parity between using less of the expensive fuel and more of the cheaper one, but, sometimes, it just about gets there), but, without the knock sensor, you really don't have much chance.

The slightly more expensive fuel may well contain more/better detergent additives, and that may help over the medium/long term, and starting/initial emissions may be better, so if the costs are close on a short-term basis, you might think that's enough.

The car doesn't really measure the octane value of the fuel (you'd be asking which one), but the tendency of the motor itself to knock.

@metaschima
My averages are about 45 mpg around town and 55 mpg on a longer run (according to the dashboard readout, which, when audited, is optimistic by about 2 mpg). Of course, I'm in a different continent to you, which makes a difference (actually, probably three or four differences, but they add up).

Be happy I get 13ish

It does have a knock sensor.

It uses a peizo system that measures sound.

What car do you have that gets such good mileage ?

Yeah I want to know the car as well. Make sure to measure the actual gas mileage, not what the dashboard says.

I do not have a dashboard mpg meter.

First tank got 18.9 m.p.g. for combo of city/highway driving.

My friend has a dashboard mpg meter.

Very inaccurate.

With a 6 cylinder engine, I saw readings of 53 mpg. No way.

I would call it useless.

i) AFAIK, they all do. (Piezo, that is). That is it is not the only system that could work, but it is the one in widespread use.
ii) Ford Mondeo (probably Fusion to you) but not a model that you would have had.

Do you? Most people don't do anything about correcting miles driven and don't measure over a large enough volume of fuel used. So what do you do? You'd need to take care of both of those for your claim of exact mpg. And while it does not affect your mpg figures you may need temperatures to know what they mean.

If you're going to argue about 1 mpg. But for rough "now" estimates just computing using miles driven and gals used is probably fine. Just do it for more than 1 or 2 tanks of gas.

You need to learn to have faith in people instead of criticizing them.

I was a chemist for 25+ years.

Chemists are very, very, exacting.

You are a person who is patient and calm. :-)