New aftermarket tires affect Chevrolet Silverado's fuel economy

John Mahler

SPECIAL TO THE STAR

Q: My daughter has a 2007 Chevy Silverado pickup which originally had Firestone LT245-70-R17C tires.

The door sticker shows the following pressure requirements: normal load 45 p.s.i., full load 50 p.s.i. These tires were replaced by Hercules 265-70-R17, six-ply.

She uses the same pressures as indicated on the door but now her gas mileage is down 20 per cent from 550 km to 450 km a tank, normal load.

What should the pressure be now?

A: As a general rule, fuel consumption figures go up when you switch from original equipment tires to aftermarket. But there are a few issues with your choice of tire change.

First, your original tires have a diameter of 30.5 inches, while your new tires have a diameter of 31.6 inches. This makes for a speedometer/odometer error of 3.7 per cent reading low. That also makes the odometer read low.

Second, your new tires are almost one-inch wider than your old ones. More width equals more friction equals more fuel consumption.

Last, you have gone to a tire that has much higher load-carrying capacity. Higher load requires higher pressures. Your old tires at 45 p.s.i. had a load capacity of 912 kg your new tires at 45 p.s.i. carry 1,023 kg each. At the maximum inflation, 50 p.s.i., they can carry 1,120 kg, but the vehicle will be very stiff if it is not heavily loaded ? probably much stiffer than your old tires.

This higher pressure is the only thing that will improve fuel economy; less pressure increases fuel consumption. If you can live with the stiff ride, pump them up.

Q: I was committed to buying winter tires and checking the manufacturer's date when I bought a new 2008 Honda CR-V LX AWD last December.

It came on Bridgestone Dueller H/T tires (P225/65R17) but the tread didn't look very impressive and winter tires could not be found, so I had four Michelin Cross Terrains (100T) installed before I took delivery.

The Honda dealership charged me an extra $1,040 (there was a $60 credit per tire, which it kept).

I couldn't find the manufacturer date but the dealership said this was a 2008 model so they would have been made in 2008. Did I do the right thing for snowy Kingston?

I believe I was overcharged for them. Was I? Were they made in 2008? And what winter tire and size should I look for before the autumn rush?

A: First, the good news: the Michelin tires you bought are fresh ? they are a new size introduced in 2008.

All Michelin tires will have the date code at the end of the DOT number, which is found on only one side of the tire, probably the inside side wall ? which is hard to see when the tires are on the car.

The Michelin Cross Terrain is an excellent three-season tire but is not an ideal choice for the snowbelt you inhabit.

In fact, Michelin has this disclaimer in bold print in its price catalogue: "IMPORTANT: While all-season tires are designed to provide reliable performance in moderate winter conditions, the use of four winter tires is recommended for optimal performance."

This disclaimer ? at the bottom of the page ? is specific to the Cross Terrain tire. So I wish another choice of tire had been made.

List price for the Michelins is $250 each ? at independent dealers they are usually discounted 10 per cent ? so I fail to see how the dealer arrived at $1,040, especially in light of the $60 per tire credit allowed for the Bridgestones. At least the $60 credit is reasonable.

For next winter, I suggest you get a set of Michelin Latitude X-Ice winter tires. They are excellent in heavy snow and ice and are the correct size for your vehicle.

Q: I own a Kia Sportage 4WD, which came with a smaller spare tire. Are you saying that if I have a need to use it temporarily and in accordance with the instructions on it (maximum speed 80 km/h), this action will cause damage to my vehicle? If so, why would it come standard on the vehicle?

A: I cannot speak specifically about your Kia because I am not familiar with its AWD system.

If your AWD is not automatic and you can drive your vehicle in 2WD mode, it will not be an issue.

But it is a generally accepted fact that running an odd size tire on an AWD system can cause damage ? although not necessarily.

I would ask the Kia folks about the tire situation and just how far and for how long it can be run.

It may turn out that your "spacesaver" spare will be fine. These are used to save money, save space and save weight by car manufacturers.

Email tire questions to John Mahler at thetireguy_1@hotmail.com. Please include vehicle's make, model and year, tire brand and size, as well as your name, address and phone number. Volume of mail prevents us from answering all queries or providing personal replies.

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