Locate the tire diameter specs on the manufacturer's site for both tires
then do the math. Always divide the new tire size by the old tire. You may
get a number over 100%, but that means the tire is turning that much faster
than before and is the percentage to DIVIDE the indicated speed by.

For example: if your speedometer indicates 75 mph and your tire is turning
115% of the old tire's rpm, then you actual speed is 65.21 mph. Odometer
mileage will be affected the same way.

Go here:  http://www.miata.net/garage/tirecalcold.html

With the new tires, your speedometer will read 3% faster.

Now, that assumes an ideal world where all things are exactly as they are supposed to
be.  Some tire companies vary the diameter from published specs.  And some factory
speedometers are 2-3% off anyway.  Who know, now you could be closer to your exact
speed than you were before.

Go out on the freeway and clock out 10-20 miles with the mile markers.  See how you
your odometer matches up.  In some places you'll find a radar trailer or radar
signpost on the highway, that's usually pretty accurate.  And the new GPS hand helds
are pretty accurate, too.

If you have to change the speedometer, I'm not really sure.  I think you have the
sensor on the differential.  The only way to change it that I know of is to reprogram
the computer with something like a Jet tuner.  And you would want to do that anyway
to make the truck run better.

Size---Sidewall  Radius---Dia---Circum---Revs/Mile---Difference
275/60-20---6.5in---16.5in---33.0in---103.6in---611---(0.0%)
305/50-20---6.0in---16.0in---32.0in---100.6in---630--(-3.0%)

Thanks to all who have submitted, I think I will live with the difference,
Thanks