Speedometer Calculations

Boy, good luck on that one!
55 series tires were mounted on TRX wheels (16.3"+) & I believe 70 series tires were mounted on the 15" Series 1 cars. 50 series tires on 15" wheels will definately register higher speeds on the speedo than actual, I would think your 60 mph variance would've been more. The tire gap to the wheel arch must be huge, any chance you could attach a picture??

 

Howard, just wondering why you went with the 205s on the front and 225s on the back. The original size is 215. Did you try the 225s on the front? If you didn't could you try putting them on the front and let us know if you have any problems. I currently have the Perrelli P4000 super touring in 215 70 zr 15 and really like them but have heard they are NLA. Just trying to figure out what the next set of tires will be.

 

Thanks to the GM TH400 speedo correction should be peanuts, there are many gears available for the speedo cable drive.
http://chevelleengineer.home.comcast.net/~chevelleengineer/speedo.htm

http://www.summitracing.com/search/Department/Gauges-Accessories/Section/Speedometers/Part-Type/Speedometer-Gears/Transmission-Type/TH400/

 

Ok first of all there were a few different speedos from the factory so lets try to figure which one you have. Post a close up picture and lets see if you have a true MPH speedo or if you have a KPH that has been stickered.

Once we know what you have here are the guys to calibrate your speedo:
http://www.paspeedo.com/

 

Not sure if you have the electrical or mechanical speedo but here is a fix for electronic speedos:

http://ferrarichat.com/forum/showthread.php?t=329231&highlight=speedometer+correction

 

It matters whether you have an auto-box or a 5-speed.
I got to thinking about this a little more...

With all that aside though, the RPM & speedo relationship are all performed before the driveshaft, it stands to reason that you could be running on donuts of various sizes & the correlation between the two (RPM & speedo) would still be the same. The difference, however, would show up between the speedo & your GPS (as you have found).

You'd have to calculate the % difference of the overall dia. (or revolutions per mile) of the original tires & of the new tires, & apply that to the pinion gear of the speedo cable (whether @ the tranny or the gauge end). If I'm not mistaken, the tach is electronic & the speedo is mechanical. I think the problem here is the pinion is of a small # of teeth & just a 1 tooth change (increase in this instance to slow the revs of the cable to bring it more in line with the actual speed) is going to change the results more drastically than that of your tire change.

Edit:
I can definitely understand your reason to go with a wider tire, the TRX wheels use a 245 series (a visually & measurably beefier tread).