wheel size

[tomtomterrific]

Gents:

I'm new to the forum having just purchased a 1981 Spider VIN # zfaas00b48183107

It's in very good shape and runs like a champ. It has 13" wheels/tires and the speedo reads about 8-10 mph high when cruising around 70.

Could this car originally have had 14" wheels/tires?

Thanks!

Tom

[wikkid]

I don't know about 1981 original wheels, but 14" wheels can fit. In fact, check outthese 15" wheels for a Spider.

[Tobi]

The 14" have a lower profile so the diameter is about the same as the 13" wheels/tires.
The Fiat speedometers often show a little more. I am running 185/60 13, so a 13" with a lower profile and I expected the speedo to be way of and it is actually very exact (who knows why). Sometimes people replace the transmission or the rear end for something from a different year and get a wrong measure on this way.

[wikkid]

Somewhere out there in the etherinterwebz, there is a tire size/speed calculator web page, that tells you what your speed difference would be between your original tire size and a new size. I think it may have been a Miata-related site, but I'm not sure.

Well, the above doesn't (re-)read as a very helpful or useful bit of info.... but it is out there!!!

[Tobi]

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

[red107]

1981'S came with 13's and 14's. As far as the speedometer being off. Most are off a few MPH. I also remember reading that speedometer on fiat used a percentage and the faster you go the more inaccurate the speedo is going to be. I moved to 15's w/195/50/15. My speedo reads 2mph low at 60 actual spd 62 . 5 mph low at 70 actual spd 75 and 8 mph low at 80 actual spd 88.. I used GPS to determine how fast I was really going. 88 was the fastest I have been GPS was mandatory on long hwy trips for me.

[baltobernie]

Whatever wheel/tire combo you use, the closer the circumference of the tire is to 73", the more accurate your speedometer and odometer will be. Larger tire = speedo optimistic and vice versa.