? about changing tire size

[Sigifrith]

My 94 VX takes a P165/70R13 78S tire.

If I buy Bridgestone Potenza RE92's in
165/65R13 how much will the speedo & odo be
off?

To make it more interesting, what if I go to 14" HX wheels, and step up to the Potenza's in 165/65SR14?

Bridgestone shows the 14" tires as 13 Lbs. VS 19 Lbs. on the 13" tires.

[Sludgy]

Tire speed calculator

The numbers on a tire tell you its dimensions. For example, 225/70R 15 means that the tire is
225 mm wide, 70% as tall as it is wide, and fits on 15" rims
"R" means it’s a radial tire.
Other letters are speed ratings, meaning how fast the tire can safely be driven.

Original equipment Alternative tire Alternative 2 Alternative 3
width 165 165 165 155 mm
aspect ratio 70 65 65 75
wheel 13 13 14 13 inches
width 6.5 6.5 6.5 6.1 inches
tire height 4.5 4.2 4.2 4.6 inches
diameter 22.1 21.4 22.4 22.2 inches
Diameter ratio 100.0% 97.1% 101.6% 100.3%
Indicated speed 60 60 60 60 mph
Actual speed 60 58.2 61.0 60.2 mph

[Bill in Houston]

Tirerack should show the numebr of revs per mile for your current tire and for your potential replacements. You can compare those numbers to see what kind of speedo/odo error you might introduce.

The 14" tire is lighter, but the 14" wheel is probably heavier.

[brucepick]

There are some fine tire calculators on the web. Here's my curent favorite:
http://www.wickedbodies.net/Tire-Size-Calculator.htm

Another very good one:
http://www.miata.net/garage/tirecalc.html