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Let's say I pull up to one of those multi-selection gasoline pumps – regular, mid-grade, premium – gasoline pumps after the previous purchaser has selected regular grade gasoline. I select the premium grade. Do I really receive premium gasoline from the get-go, or do I receive a few litres of regular grade gasoline as the switch-point selection location (below the ground or somewhere in the pump itself) has to flush out the volume of regular grade gasoline remaining in the hose from the previous purchaser? I know I'm paying for premium grade from the get-go. How do those selection pumps work? Conversely, the purchaser using the pump after me may select regular. Does he receive my premium gasoline remnants until the regular gasoline flushes through? – John

Good question. For an answer I asked the folks at Shell who say you get what was left in the hose from the last time it was used – regardless of the grade involved.

"The hose and dispensing nozzle are always full of fuel," Shell says, adding that the residual volume in the hose from the last use depends on the diameter and length of the hose but is commonly about half a litre.

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The only time such a small quantity would be of concern would be when filling a very small tank, such as a motorcycle.

Shell says that, as long as the premium purchase is more than 10 litres, the total sale will be "on-spec for octane and meet the 91 AKI standard." It added that Weights and Measures Canada, which monitors and inspects pumps and controls the related regulations, approved the use of "single hose blender pumps" many years ago.

If this still concerns you, shop around, some stations use separate hoses and dispensing nozzles for high-test.

Please send your automotive maintenance and repair questions to globedrive@globeandmail.com

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