CarTalk.com Blogs Car Info Our Show Deals Mechanics Files Vehicle Donation

91 octane?!

So I turned in my car for one with a smaller payment. But I turns out that the new one requires 91 octane or higher. We have established that I am cheap. So can I put a half tank of 89 octane in it and a 1/4 tank of the 96 octane and let them average out?

Where can you get 96 octane pump gas? I guess you could put in half a tank of 89 and half a tank of 93. But you would only be saving about $1-$2 a fill up. Frankly my time is worth more than that. So I’d just use 93 octane and forget about the mixing nonsense.

If you think it’s worth your time, you can mix octanes and they’ll average out. I believe some of the pumps do that, actually.

I suggest pumping the higher octane first. If the gas doesn’t mix thoroughly until you drive around a bit, that might put the higher octane nearer to the fuel pickup.

I’m also curious where you can find 96 octane.

Isn’t that something you should have checked BEFORE you bought the car?

No. Just use the required gasoline. Your car wouldn’t way that if the engine weren’t designed, engineered and built for it.

If the $1.20 per week is really a financial concern I think there are other issues.

Most cars that “require” a higher octane fuel, will run just fine on the lower octane swill… It’s just their computers will dial back some timing etc, so you will lose some performance… You may not be able to tell the difference, or on your car you might?? It all depends on the driver… However unless you are always running the car at 100% then no damage will accrue and you can put the cheap stuff in.

With that said I had a friend in highschool who had a 87 Buick Skyhawk (basically a Buick cavalier)… Any way he insisted on running high test in it all the time, one day the station was out and he filled it with regular (which should have been fine for this car)… The car had become so used to high-test it ran very poorly on the regular… In fact so poorly I could hear the car idling wrong a 1/2 block away, he put high test back in ASAP and it ran fine again. What I am saying is that every car is different, and if you are this cheap try the regular and see what happens.

What car do you have?

Depending on the specific model and how you drive you might get by with 87. More information please.

The pump mixes the octanes for you automatically when you choose the mid grade.

I believe you meant to type 93 octane rather than 96.

Yes, you can mix octanes to average 91 octane. This plan appeals to my personal sense of economy, so I support your intent. Of course, it makes more sense to find a station that offers the mid-grade.

gs, you’re thinking of cars that “recommend” premium. cars that “require” premium actually do require premium, and using lower-grade gas can damage the engine.

I don’t know why companies do that. Same with my truck and my wifes car. Both require 91 Octane. I can buy 93 Octane or 89 Octane. I just fill up with 93 octane.

Sunoco used to have several octane levels available, but I’m sure that they stored only 2 or 3 underground. The pump mixed them to provide the octane level you desired. Maybe some Sunoco stations still do.

Shadow… The only cars I know of the require premium and mean it are high compression mussle cars (new cars can have high compression and still run regular), and highly boosted turbo cars like the WRX… Is this what you mean??

I know that BMW/MB/Audi all say they need it but I know many who run regular and the car runs fine. It may cost some power as the timing gets pulled back a smidge… But no damage is done…

Just as a for-instance, the TL requires premium. There have been guys who try to cheap out (which really makes no sense. You’ll drop 40 grand on a car, and whine about $1.20 a week extra to fuel it?) and they end up pinging.

The computer can only retard the timing so much before it’s out of wiggle room. If you’re still pinging at that point, you cause damage. No, it won’t blow your engine up in 50 miles, but it will shorten the life of the engine.

The manufacturer doesn’t care how much you spend on gas. They have no vested interest in requiring premium unless the engine really does require premium.

The only reason newer cars can run on regular when premium is required is because of the knock sensor. The knock sensor senses when the car is knocking and then retards the timing…which results in lower performance and gas mileage.

<b>You'll drop 40 grand on a car, and whine about $1.20 a week extra to fuel it?</b>

Some cars with higher compression ratio need the octane higher to burn correctly at the higher pressure. Otherwise pinging results. Pinging is a poor detonation condition. Normally this will not hurt the engine but it does hurt mileage and power. Modern computers will try to minimize the issue but a a poor performance curve. There is no clear damage to the engine from pinging aside from performance. This is a big myth and a lot of folk here drink that koolaid. But it is just unscientific.

I’d like to see you link to a study proving that, euryale1, because pinging is known as predetonation, or, the gas mix in the cylinder firing before the piston reaches the top of the travel. A lot of damage can be caused that way through over stressing the piston components.

Make sure it’s not costing you more money to switch from premium to regular gas.
Many cars suffer a drop in mpg when switched from their required premium to regular gas.

Take the following example:

  • Regular gas cost $3.80/gallon. Premium costs 5% more at around $4.00/gallon.

  • If your car gets 20 mpg when driven with premium, but drops 5% to 19 mpg when driven with regular, then you’re not saving any money switching to regular gas.

  • If your mileage drops below 19 mpg, then your switch to regular gas will be costing you more money.