Why is my truck thirsty?

Q: In June 2008 I turned in my leased 2005 Chevrolet Avalanche, which averaged 14.6 L/100 km over 64,000 kilometres of about 75 per cent rural driving on 80 km/h roads (rarely less than 15 km each way), 20 per cent on freeways and the remainder on urban roads.

I bought a used 2007 Avalanche with 33,000 km. It has a similar 5.3 L engine but with the 8/4 cylinder shut-off system. I expected to get better mileage than from the 2005 engine but am only averaging 16.5 L/100 km after 4,000 km of similar driving, in spite of being a little less heavy-footed.

My nephew has a 2008 Avalanche with the 8/4 system and averages 13.5 L/100 km with a much higher percentage of urban driving.

Is this normal mileage for the 2007s?

Technician Peter Lokun replies:

When determining accurate fuel economy, the average fuel economy calculated by the vehicle's driver information system may be inaccurate.

I would investigate this concern by recording the distance travelled from the odometer and the amount of fuel added during a fill-up. Make sure the fuel tank is filled to the same level each time. Calculate the litres per 100 km from this information and compare your results with the vehicle's fuel economy calculation.

If your fuel economy is the same as above after verification, examine the vehicle build specifications. You would expect that your 2007 Avalanche would get close to the same fuel economy as your nephew's 2008.

There are many variables that could affect fuel economy, including:

different options such as tire size and/or axle ratio;

a vehicle equipped for towing will not give you the same fuel economy as a comparable vehicle without a towing package;

installing different size wheels or tires. If you change the size of either, you must properly program the change into the vehicle's computer for accurate fuel economy calculations.

Q: I have a 2008 Cadillac SRX. I am satisfied with the vehicle, but am a little confused with the oil monitoring system. Mobil One is being used. I have driven 10,000 km and the monitoring system shows the life of the oil is at 53 per cent. The adviser says that is okay. How safe is this over the long haul?

Technician Peter Lokun replies:

The oil monitoring system uses the vehicle's power train computers to monitor the type of driving the vehicle is exposed to.

Your vehicle has the ability to monitor the length of each trip, the type of driving (city or highway), the load of the vehicle, the terrain the vehicle is driven through and the driver's habits. Based on this information, the computers calculate the oil change interval.

Considering your vehicle is filled from factory with synthetic oil, the oil change interval is calculated based on an increased oil change interval, as recommended by the oil manufacturer.

A couple of points to keep in mind:

The oil change intervals are based on the recommended oil for the engine. If an inferior grade or viscosity oil is substituted, damage to the engine can result if you base your oil change intervals on the monitor.

Many vehicle owners change their engine oil with 10 to 20 per cent remaining on the monitor. You can't change the oil too often. More frequent oil changes with the correct oil can only help your engine last longer.

Email your mechanical questions to

wheels@thestar.ca. State make, model, year, engine and kilometres, as well as your name and address. Letters may be edited. Letter volume prevents personal replies.

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