replaced speedometer driven gear with recommended 40 tooth and still not accurate...

nitronut said
I recently installed a new speedometer cable and the mph readings were off by 20-30 plus mph. Called Mancini Racing, gave them my tire size (P225R1595S) along with the rear end gear size (3.91) and was told I needed to install a 40 tooth driven gear, which I purchased and installed. (The old gear I replaced was a 23 tooth). It still seemed off so I used the speed app on my phone and now reads accurate to 10 mph and then jumps the speedometer reading 10-13 mph OVER my actual speed when driving faster than 15 mph. Is there a rule of thumb for speedometer speed readings per gear tooth? Do I need a 42-44 driven gear instead of the 40 tooth? Any thoughts? Thanks in advance...

What is the speed that the speedometer is reading and what is your actual speed???

Of go on the highway and do a steady 60 MPH then pass a mile marker, start timing when you pass the mile marker and see how many seconds it takes to pass the next mile marker while maintaining a steady 60 MPH...

Then post it up and I can do the math to tell you what direction to go...