SPEEDOMETER/ODOMETER Accuracy

Very unlikely, I should have thought. Try checking the speedo against a GPS readout from a satnav / satnav program on your smartphone.

 

Yes. I'm impressed with the accuracy on these mobile devices once again.

 

Try calculating or looking up the diameter of the original 17" wheels and tires and comparing with the diameter of the 20" wheels and tires. Direct ratio of what happens to your speedometer and odometer. As far as I know, there is no such thing as an auto correcting speedo/odo and certainly not on a 1999 Ferrari.

 

Just expanding on what Terry has said. The 456's speed is taken from left rear wheel, so the diameter of the new rear wheel is the key. Check your handbook for original tire specs., but the Workshop Manual for the 456M shows a 17" rim with a "285/40" tire. That gives an outside tire diameter of approximately 26 inches.

This is using a formula (285 x 40/1270) plus 17

Or just use this rough online calculator:

https://tiresize.com/height-calculator/

What numbers do your new tires have? Your new tires may have the same outside diameter (i.e. bigger wheel rims, but lower profile tires), so the readout will be the same.

If the rims are 20" and the new tires are, say, 285/30, this gives an outside tire diameter of approximately 26.73 inches. i.e. 0.73 inches bigger. This will introduce a speed error factor of 26.73/26 inches = 1.028 times or about plus 2.8%. Since Ferrari speedos normally underread, your speedo will be closer to true speed.

Divide new diameter by old diameter (26") to get the error factor.

Let us know if you have problems with the calculations

 

Sounds like you are slightly oversize. The inaccuracy is absorbed by speedometer calibration at lower speeds. Your new rear tires may indeed be 285/30. Perhaps Michelin Pilot Super Sport 285/30ZR20?

 

You have rear tires which are way bigger in diameter than OEM 305/30 19 tires, so naturally the speedometer would be way off. 305/35 20 tires are 28.4" in diameter and OEM tires are 26.2" in diameter. That is an 8.4% difference. Unless you have larger tires on front to make up for it, I am surprised you do not have ABS issues. Anything over ~5% difference front to rear diameters from OEM (pretty much the same diameter F&R for 456s and Maranellos) can give ABS problems.