12 to 13 inch tire, MPH difference?

[88Adventurer]

Got a 93 automatic Festiva Tuesday night. It has 13 inch tires on it. Does anyone know what the MPH difference is between the 12 and 13 inch? And does anyone know the correct tire pressure for a 175/70/13 tire? Thanks

[mattswabb]

use this calculator

http://www.1010tires.com/TireSizeCalculator.asp?action=submit

[88Adventurer]

Thanks, man! Very handy

[25Horseplay]

Here's another calculator that can compare size's.

http://www.net-comber.com/tirecalc.html

[88Adventurer]

Nifty websites, thanks. I had been wondering about the mph difference when the speedometer said 65 and I was passing a lot of people on the highway.

[bobstad]

12" to 13" rims on Festiva

I'm running 13" '87 Mazda 323 rims on my '91 Festiva L and multiply times 1.03 to get what I feel is the closest to accurate speedometer and odometer readings.(with 155 80R-13 Toyo Spectrum tires)

My feeling is that with the stock 12" rims the speedometer/odometer was reading ridiculously high.(it seems the 13" rims are much closer to accurate than the 12" as if they'd built everything for 13" and then cheaped out and put on the 12"s at the last minute?)

I've been using roadside digital radar speed clocks and five mile odometer checks to try to come up with what I feel is most accurate.(there is a strech of about six miles between Eureka and Arcata here that had lots of accidents so they lowered the speed limit from 60 to 50 and installed about a dozen fixed radar monitors with speed indicators that I often drive through several times a week)

Is there anyway to get speedometer drive gears with different numbers of teeth so that the speedometer/odometer is more accurate?

Someone once suggested I must have parts from an automatic transmission Festiva to have this happen with the speedometer/odometer, but I can't see how that would make any difference and which doesn't seem very likely anyway.(though if this is true of my car I'd think it would be true of all Festivas)