Sunday, October 08, 2006

Saving Money On Tires: What's the catch?

This is the continuation of my earlier post on tires. They were intended to be consecutive, but there were a couple of entries in between, that I posted in response to events.

If you use a different size tire than the vehicle's original equipment, there is a catch. However the effect is small.

A smaller tire means the wheel it has to turn faster to go the same speed, which means slightly more wear & tear on all the moving parts, up to and including the engine. And since the RPMs are higher, your gas mileage will be a bit less. The speedometer will read a little too high, and your odometer turns a bit faster. It's like driving in a slightly lower gear, so you also get a bit more power going up hills.

Of course if you get bigger tires, everything is reversed. In particular, note that the speedometer will read a little too low, and you'll need to back off on the pedal a bit to keep from getting a ticket. (This happened to someone I know.)

Again, the effect is small -- but all in all, if I had it to do again, I probably would have tried to get the next larger size tire, rather than the next smaller size.

The tire sales rep told me the speedometer should be off by less than 1 MPH. In my tests it looks more like 2 MPH. And you definitely need to test it. (You should test your speedometer anyway, but especially after getting any new tires.) Just get out on the highway and watch the mile markers. Time how many seconds it takes to go one mile, and divide that into 3,600 to get MPH.

For example, if it takes you 52 seconds to go the mile, then your speed is 3,600 / 52 = 69.2 MPH.

If you don't have a companion to wield a calculator while you're driving, the alternative is to keep adjusting your speed until it takes one minute for the mile. Then you're going 60 MPH, and just compare your speedometer to see how far off it is.


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