Gasoline: Does it really matter which gas you purchase?

It does.

Higher octane ratings are more resistant to “knocking”.

Engines with higher compression and/or that have forced induction (turbos or superchargers) are more prone to pinging and thus generally require higher octane to avoid detonating the fuel-air mixture at the wrong time.

Modern cars can adapt quickly to stop knocking by altering the mixture and/or ignotion timing, though doing so reduces fuel efficiency and horsepower. My car with it’s carburetor and distributor doesn’t have that luxury, so instead of pinging leading to a quick recalibration, it leads to engine damage. My 4A-LC is robust enough that the damage is minor so long as I don’t make a habit of it, but an old RX-7 will blow the apex seals and require an engine rebuild after just a couple of “pings”... which will take less than 0.03 seconds at normal cruising speeds. The RX-7 is a notably fragile example as most engines can tolerate some detonation, but it’s really not something you want to have happen if you can avoid it.

My first Corolla would not run right on 87 unless I reach under the hood and retard the ignition timing so far that it won’t make enough HP to get to highway speeds or to climb a hill at any speed. With 93-octane, I could keep the timing at factory specs and drive normally.

My current Corolla can run on 87, though it won’t get over 55 MPH, can barely make hills, and gets ~23 MPG. When I use 92-octane (WA doesn’t have 93), I have no difficulty doing… let’s call it 65, and gets ~27 MPG.

My wife’s old Saturn got 26 MPG and was a little sluggish on 87-octane; with 93 it went up to 28.5 MPG (enough of am improvement to actually lower the $/mile despite the added fuel cost) and was slightly peppier.

It might, but it is easy to find out. Put in 87. If the engine knocks when you accelerate, you need higher. If it doesn’t, you don’t.

@josie True. Trial and error is the best way to find out what works best.

If your car runs good on the “cheep Stuff”, use it. If you have a high performance car that you push to the limits you will want the higher octane. Octane is a measurement of how much the gas/air mixture can be compressed before igniting. the higher the number the more compression it can take before self-igniting.

Most cars have a compression ratio of about 8:1, if you have a high performance engine the compression may be higher and therefore need a higher octane to prevent the engine knock associated with pre-ignition .

The engine knock is very destructive to your engine and should be avoided. Most cars will get slightly higher gas mileage with the high octane. But to see if it is worth it. Fill up with the lower one, and calculate, NOT MPG but rather Cents per mile for each. Most cars will be close to break even on cost per mile.

From what I understand it depends on the car. My Acura said use premium, and the salesperson told me it was fine to use regular unleaded. I always did, and it was always fine. Supposedly in cars like Corvettes and Porsches there is something about the engine that you have to put in higher octane? That it can really mess up things. People who seem to know what they are talking about have told me this.

Buying the high test encourages more of it to be produced and it takes more resources to make a gallon of it and drives the cost up. Given todays economy I wonder if there are really many people buying better gas than is needed.
There are things that can be done to an engine if it knocks on the cheap stuff…discounting high performance machines, a regular family car shouldn’t knock on regular.

Being naive and a believer, I actually follow the advice in the manual (the only one I read of the many lying around the house).

Rule of thumb: read your owner’s manual. You probably won’t get a lot of benefit from using gasoline with a higher octane rating than what it recommends. Be careful and make sure you are reading the correct entry as many cars will have a number of optional engines and the manual will probably present you with a list of engines rather than just the one that is in your car.

But there’s more to gasoline than just the octane rating and % ethanol added, which is typically all the quantitative information a service station pump will give you besides the price per gallon. If your automobile is a recent make from BMW, GM, Honda, Toyota, Volkswagen or Audi, the owners manual may say something about Top Tier Detergent Gasoline. ‘Detergents’ in this case are chemical additives put into the fuel to prevent deposits from clogging the fuel injectors. If you’ve seen a Shell commercial or been to a Shell station you’ve probably seen the ‘Nitrogen Enriched’ marketing slogan. These detergents are what that is referring to. But detergents in the gasoline are by no means unique to Shell or any other retailer.

EPA regulations rolled out in the ‘90s set a minimum level for detergents in gasoline sold for use in passenger cars. As this article will relate, this minimum was well below the level that most retailers had established through experience going back to the ‘80s when fuel injection started to come on the scene and problems with deposits clogging them were noticed. The general response of the industry to the regulations was to cut detergents down to the minimum to maximize profit. Automobile makers noticed an uptick in fuel injector clogging issues and a group of them eventually did an end-run around the oil industry standards bodies and set their own standard which is the aforementioned Top Tier Detergent Gasoline. The website will tell you which retailers have gotten on board with the standard. (Some retailers may go above the EPA minimum while choosing not to participate in this arrangement. I think BP falls into this category.)

(Full disclosure: I work for an additives company. I used to buy the cheapest regular gas I could find. But being aware of these issues now I stick to vendors who have adopted Top Tier when I can. And I will especially prefer those I know buy the most from the company I work for!)

Note that I used to be able to run on 87 until we started using Ethanol :/

@woodcutter What do you recommend for an old (‘85 with 247k miles) Toyota? And altering the ignition timing is out.

@jerv put a match to it.

@woodcutter Given how many American cars I’ve had blow engines, break transmissions, or otherwise fail completely, and how few problems I’ve had with Toyotas, I think I’d rather just keep buying the Premium gas :p

@jerv Well at 247k that motor’s probably not going to go a whole hell of a lot more, then you can throw another in that might use regular gas.

@woodcutter Three things there. First, I’ve seen 4A-series engines break 400k, mostly the older ones. Second, my engine choices are limited to engines that are either weaker or would require a total refit to handle fuel injection. Third, my first Corolla was exactly the same way despite having far fewer miles on it, as have other vehicles with the 4A-LC engine. I suspect that it actually has more to do with the use of ethanol instead of MTBE, since older cars were never designed for the stuff; other pre-OBD cars I’ve owned had similar issues with E-10 that disappeared whenever I filled up somewhere with the old gas (back when that was stol an option).
Too bad there are no newer cars that can match my rig for reliability or repair costs :/

With all the road salt there every winter and accompanying body corrosion it seems almost a moot point to hang on to an old car even if the engine seems ok. I lived in NH my first 18 yrs and other times intermittently, and even with z-bart / Rusty Jones the salt killed cars fast.

Or you got real slick with a pop rivet tool and tin snips.

@woodcutter My first one was back in NH, and it had far more rust on it at age 12 than my current one has at age 27. My current one was and is a WA (state) car though, and we don’t salt here.

However, back in NH, I owned a couple of A2 Golfs, so yes, I know more than I care to about bodywork.