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DEAR CAR TALK: Back in the '70s, we were often told it was best for a car's engine to always use the same brand of gas. Some sources even said to always use the same gas station or even the same pump.

Have engines evolved now to the point where this doesn't matter anymore? It would be nice to no longer have to maintain a mental list of acceptable gas stations in my area, or to stress over finding one before my tank runs dry on a road trip.

Is this still true? Was it ever true?

— John

DEAR JOHN: Not and never. It is not still true. And it was never true. Your car's engine was designed to run on any brand of EPA approved gasoline, and the idea that it "got used to" a certain brand or a certain station or pump is an old father's tale. So we're here to set you free, John. More or less.

Here's the more or less part: Modern fuel-injected cars run best (over a long period of time) on what are called Top Tier gasolines. Those are gasolines that have a higher level of detergent additives than the EPA requires.

The whole Top Tier thing was started by a group of manufacturers that felt that gasolines weren't doing enough to keep their advanced engines clean. So they pestered a bunch of major oil companies into raising their standards. And that's how Top Tier Gas got started.

Top Tier gas has extra additives, especially detergents, that reduce deposits in the fuel system and help keep expensive parts like fuel injectors clean. And today, the vast majority of major brand gasolines sell only Top Tier gas. Those include well-known brands like 76, Arco, Amoco, BP, Citgo, Chevron, Exxon, Mobil, Philips 66, Shell, Sunoco, Texaco, Valero and others you've heard of. Plus some you may not see advertising, like Kirkland Signature Gas from Costco.

To see a full list of all the brands that sell only Top Tier gas — or to print out the list and stick it in your glove box — go to

And that's all you need to consider when buying gas, John. It doesn't matter which of the Top Tier gasolines you buy or if you mix them. It doesn't matter what grade you use (use the grade recommended in your owner's manual). It doesn't matter which pump or which particular station you buy from.

We recommend you buy from the station with the lowest prices and the cleanest restroom and use the pump that no car is currently parked in front of. And as long as it's one of the Top Tier gasolines, your car will be happy.

DEAR CAR TALK: I've always loved the columns by you and your brother. You recently answered a reader who wanted to know why his Prius' mileage dropped after he got new tires. You suggested his new tires may have been "higher rolling resistance" than his original tires.

I agree that's possible. But I would also add a "measuring error." I do not believe the Prius is able to adjust the calculated MPG for tire wear (or tire size, for that matter). If my calculations are correct, 3/16 inch of tire wear decreases the circumference of the tire, and results in 2% "higher" MPG.

When you buy new tires, you "lose" that advantage. For most cars, you would hardly notice this because of their lower MPG, larger tires, less precise displays, etc. But we Prius owners are obsessed with our fuel numbers, so your reader noticed it.

— John

DEAR JOHN: I think you have it backward. You've always hated the columns by me and my brother. Actually, I think it's the "mileage error" that works the other way.

When your tire's circumference gets smaller due to wear (or due to compression from excess weight in the car, or other reasons), you travel less distance with each rotation of the axle. That lowers the "miles" part of your miles per gallon.

Think of an extreme case. Let's say you had huge tires on your car — tires that had a circumference of 1 mile. OK, it'd take you a year to fill them up with air at the gas station, but just imagine it for a minute. For every rotation of the car's axle, you'd go 1 mile. Wow. All else being equal, you'd get incredible mileage with those tires, right?

On the other extreme, if you had little, 1-inch roller-skate wheels on your car, the axle would turn and turn and turn, and you'd go very little distance, and get poor mileage. So, in theory, a larger tire should increase your mileage, not reduce it.

However, if you're relying on the car's internal computer to measure your mileage (rather than, say, mile markers and actual fuel measurements), you might be right, John. The car's computer may show decreased MPG on newer, larger tires.

Here's why. The computer is programmed to calculate distance based on how many times the axle rotates with the factory wheels and tires. So let's say Toyota calculated that it takes 1,000 turns of the axle for that Prius to go 1 mile. When your tires wear down, it takes less fuel to turn the axle 1,000 times. Why? Because you're not actually going as far.

So the car's dumb computer thinks you've gone a mile (even though you haven't), and it says: "Hey, we've used less gasoline to go a mile! Yippee!" Then, when you replace the tires and make the car's factory calculation correct again, you may actually see your car computer's mileage decrease a bit.

I would argue that, in real life, once you get into overdrive, you'd get better mileage with newer, larger tires. But we may need Nobel laureates in physics to resolve this for us with any degree of certainty. It's very confusing.

So, John, whatever you do, please don't write to us again with any more questions.

Ray Magliozzi dispenses advice about cars in Car Talk every Saturday. Email him by visiting:


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