Speedo is off 10-15 Mph

You have a 348 Spider, right? Have you gone from stock 17" wheels to something larger? Different rolling circumference?

Your description of the problem does not sound like the typical speedo sensor malfunction. I'm guessing it's either the tire sizes or the speedo head.

 

I found about a 10% variance in speedo readings going from the P-zero's to Bridgestone Fuzions: exactly stock tire size, but guess the actual rolling circumference differs a fair bit. Speedo with the Bridgestone's is now accurate at 70mph, but only off by 10% at theoretical speeds.

Anything change?

 

Likely the transmission has been changed. There were two different gear ratios in that type tranny. Your speedo is probably calibrated for the original one. No real solution available short of swapping speedos.

 

I have the same exact problem.
My tires are pirelli P0's factory sizes.
My car is completely factory with 25K miles. For the past 20K miles of ownership (5 years), the speedo has always been around 10 miles per hour to fast.
I had spoken to other 348 owners that have had the same problem.

 

Fortunately the top of my steering wheel blocks the speedo so I never look at it....

But seriously, mine reads consistently fast as well, more like 5-10 mph. Probably just marketing.

Cheers
Kev

 

Mine read fast with stock wheels and tires, probably 7 mph off. With my aftermarket set up with very slight diameter change its off by about 10.

 

My speedo in the F355 was 7 mph off. Dealer fixed it for free when they installed the tubi exhaust.