Tires for my Geo...
 

Well, wet, cold, sometimes snowy, sometimes icy weather has arrived. Here in northern Los Angeles county desert, we are so far removed from the weather you might find in Santa Monica, or Beverly Hills, that we might just as well be in Kansas. Driving to Tehachapi, I might as well be driving to Iowa or Michigan, only difference is that we use gravel instead of salt (except in the parking lot at work). If tradition holds, since we had a lot of fires in the mountains, we'll have a really wet winter, with lots of mudslides. Fortunately, that shouldn't affect ME...

Anyway, after about 50k miles, the front tires on my Geo are getting ready for replacement. There is still tread, in an academic sense...Jean Shepherd's "Old Man" would probably consider them good for another year or to. (Watch "A Christmas Story" if you are not familiar with the reference.) This car specifies 145-80R12 tires, and since nobody has had those, I've been using 155-80R12. I've been getting them from "America's Tire Store" here in Lancaster, last price was $29.99 each +mounting/balancing/road hazard. When I first got the car, they were $21.99 each. And they say the cost of living has not risen!

Anyway...when I went to purchase new tires, the tire store was out. They've apparently found some at another store, FedEx has supposedly had them since Wednesday (it is Saturday as I write this). I checked Kuhmo's website, and they apparently no longer manufacture 12" tires.

It looks like I'm going to have to change to 13" rims and tires. Now to my question: how easy is it to change the speedometer drive gear so I can keep my speedometer calibration?

The difference between a 155/80/12 and a 155/80/13 tire is 4.6% so at 60 MPH your speedometer would be about 3 MPH slow. I guess what I'm trying to say is unless you just wanted to change the drive gear you could just always stay 2-3 MPH below your target speed and you should in reality be right on it. Of course this would also make a difference of 4600 miles on the odometer every 100,000 miles. I'm including a tire size calculator so if you want to make some comparisons on different sizes. Going with a larger tire will also give you a bit taller gearing which might help your fuel efficiency.

https://www.miata.net/garage/tirecalc.html

Looks like 175-70R13 is the closest I can find available for a reasonable price...guess I'm going to have to get over to the local junkyard one day soon, look for some wheels, and hopefully there's a car like mine there that I can do some test-fitting on. S'pose worst comes to worst, I can wait for Auto Club to have one of those speedometer check events again...