speedo question

April 10, 2010, 07:28:35 PM

hi guys, i just switch my bird over to 15 inch gt mustang rims. i had 14 inch rims. how much of a speedometer diffrence is there? seems like alot because i just dusted my wife up a giant hill... now she hates me:mad: ive got a digital dash if that helps. any info might save me a speeding ticket:burnout: thanks, dwight

speedo question

Reply #1 – April 10, 2010, 10:58:02 PM

The speedometer difference depends more on the diameter of the new tires and not necessarily the diameter of the rims. If you did proper plus sizing with the tires then your speedo will remain close to what it was before. Plus one sizing means dropping the aspect ratio of the tire which is the middle number when you increase the rim size. If you increased the size of your tire diameter by getting tires with the same or higher aspect ratio as your 14" tires, it causes your speedo to read too slow.

For instance, if you change from a 205/70R14 tire to a 235/60R15 tire, when your speedo reads 60 MPH you are actually going 61.9 MPH.

If you do proper plus sizing going from a 205/70R14 tire to a 235/55R15 tire then your speedo reads 60 MPH, you are actually going 59.7 MPH therefore maintaining the accuracy of your speedo within an very acceptable ratio without getting the surprise of a speeding ticket.

Follow this link here below and plug in your original tire size vs. the tire size you are using now. It will calculate the rotation of the tire and indicate the speedometer variance.

http://www.miata.net/garage/tirecalc.html