Austin05mustang

Im trying to figure out the axle ratio for my 2005 v6 mustang, I know stock its 3.31 or something other but I now have 20 inch rims on it. Does that change the ratio? Im wondering because I can change the ratio with my new tuner to match the spedometer.

rdsx18

No it won't physically change the ratio (obv.), but if the tires are bigger or smaller it will change the speedo. To fix that there is a setting called tire revs per mile in the tuner, look up the tire size you have and put in the correct revs/mile.

157dB

Rim size has nothing to do with the speedo.
Its the overall rolling diameter of the TIRES you have mounted on them.
OEM tires are anywhere from 740 to 747 revs per mile.

215/65R16 STOCK
747 REVS / MILE

235/55ZR17 STOCK
742 REVS / MILE

235/50R18 STOCK
740 REVS / MILE

The gear ratio is on a tag on the rear axle.
Its probably a 3.55.
Jack up one tire and spin the driveshaft while
counting the wheel revolutions.
That will also give you the rear end ratio.

JimC

The V6 gear ratio in 2005 is 3.31 not 3.55. Only the Gt had an option for a 3.55 gear.

Here is a gear calculator to determine what your effective gear ratio is with a different tire size. You will need to know how tall your tires are

Edit
Sorry, looks like I can't upload an Excel spreadsheet - to the OP if you PM me your email address I can sent you the spreadsheet.

This is what it looks like though