[This followup was posted to alt.autos.ford and a copy was sent to the
Larger circumference (the distance around the tire) would make you go
faster and farther than what the speedomter/odometer says. It may also
reduce your acceleration a bit since it's like running a higher gear.
Typically, you want to stay within +/- 5%. If it's an electronic speedo
(very common, even if the gauge itself is analog) there are boxes that you
hook in-line with the VSS (Vehicle Speed Sensor) that can adjust for the
difference. Also, a taller tire will raise your center of gravity,
increasing your chance of a rollover in an accident.
Here's one web site that calculates out tire size differences, at least
for P-metric type sizes. For some of the truck inch-based sizes, there are
probably truck-oriented sites.
A few others a Google search found:
Finally, check the tire manufacturer's web sites. They often have the
exact circumference/diameters for a particular tire. There can be a bit of
variation between calculated and actual circumference/diameter.
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