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JASON REED/JASON REED/REUTERS

QUESTION: I'm a little surprised that the manufacturers don't make more of an effort to let Canadians know that the mpg numbers reported in American publications are based on the smaller U.S. gallon and, therefore, under-report what we would expect in Canadian units.

I understand that this also applies to our Canadian vehicles' IP trip computer displays when we switch to the non-metric display. I was once told by a GM Canada engineer that its vehicle computers record in U.S. gallons and just convert to the correct litres/100 km values for the metric display. Imperial gallons are not considered at any point. I've had guys tell me they're disappointed with their fuel economy based on their vehicle display. I always suggest they do a manual calculation and add they may be pleasantly surprised.

Ken

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ANSWER: The difference between the American and English or Imperial gallon is about 20 per cent and has always led to confusion.

In 1824 the British adapted the Imperial measure in which the gallon is based on 10 pounds or 277.42 cubic inches of water. The Americans had adopted a system where a gallon was comprised of 231 cubic inches of water. As a result, the U.S. gallon is 83.3 per cent of the Imperial gallon; put it another way, the Imperial gallon is about one-fifth or 20 per cent greater in volume than the American gallon.

What is pertinent to your question is how the conversion is done. I believe our switch to metric helped in this regard. Before that, the two different gallons did indeed cause confusion when comparing ratings.

But now that the computer that converts the speedometer and odometer units from miles and miles per hour to kilometres and litres/100 km does so in the appropriate units. By that, I mean that if the speedometer/odometer are in mph - i.e. an American vehicle - the conversion will be from the smaller American gallon to metric units and the number would be about 20 per cent lower. If the vehicle was made for sale in Canada and equipped with the federally mandated metric speedometer and odometer, the conversion will be between the larger Imperial gallon and the metric measure.

DIESEL VERSUS GAS

QUESTION: What is the difference between a diesel and a gasoline engine. I'm in the market for a new car and commute 180 kilometres each day - 90 each way so fuel economy is an issue.

Dianne

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ANSWER: To keep it really simple, a gasoline engine depends on an electric spark to ignite the fuel/air mixture; a diesel engine uses the heat and pressure of compressing or squeezing the mixture to cause it to ignite.

The gasoline engine thus requires a complete electrical system while the diesel does not.

The diesel has to be built much tougher to withstand the forces involved. Thus diesels are more expensive to make, but require less maintenance and last longer.

Diesel fuel is harder to find and in most locations more expensive than gasoline.

So I suggest you base your decision on the relative highway fuel economy ratings of the vehicles under consideration. While diesels are more efficient in stop-and-go driving that advantage narrows considerably at constant highway speeds.

You don't mention how often you trade, but unless you are keeping your vehicles for very long periods - i.e. hundreds of thousands of kilometres - the long-life advantages of a diesel may not be sufficient to offset the higher initial and per litre cost of diesel fuel.

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