How Fast Ya Goin’? *

On the way to Atlanta on Sunday Donna and I were in the middle of the pack, with us, the lead car and the caboose all having CBs. After clearing the Augusta area and getting into the part of the Interstate where there are 10 miles between exits, John in the lead car calls out, “How fast are you going? I’m showing 72 MPH.” To which I replied, “Seventy-five.” Rudy from the back said he was showing 72 as well. John said he would bump the cruise up to 75. After settling in at that speed (the limit is posted as 70) he asked for another check. Once again Rudy was right in line with John at an indicated 75 MPH. My speedo read 80. Why the difference? I’m riding on the same size wheels and have the exact same tire model as John, while Rudy has 17″ wheels & tires. Was my speedometer calibrated differently because my car came from the factory with 16″ wheels while John’s 99 came with 15″ wheels and Rudy’s did come with the 17 inchers?

Yesterday I asked Russ to bring in his GPS so we could take a lunchtime run and see how far off my speedometer is. Back towards the end of last year when Russ first got his GPS he brought it along on a lunch trip to Hardee’s while I drove. He showed me one of its neat tricks was that it could calculate your speed when moving. Over a portion of road that is marked at 55 and my speedometer was reading 60, his GPS said I was going 57 MPH. Today’s drive produced some interesting results. Just like last year on the stock tires when the speedometer read 60, the GPS said 57. We then got on the Interstate from our secondary road. At 70 indicated the GPS told us I was actually traveling at 66 MPH. And just like Saturday, 80 MPH on the speedo translated into 75 MPH. To get an actual 80 MPH ground speed the speedometer was reading 85. While we where going that fast I put the pedal to the floor just to see what it would do. Unfortunately it was up a slight grade to our fast approaching exit, so the GPS reported a max speed of 96.2 MPH. At that level the speedometer was one tick over 100 or 102.5 indicated.

Using today’s figures I show that my speedometer is reading about 6.5% low. If I use the Miata.net Tire Size Calculator, going from 205/45 x 16 (tire diameter of 23.3in) to a 195/50 x 15 (tire diameter of 22.7in) results in a 2.5% low reading. Was the GPS accurate? Are my 195/50 x 15 tires actually 22.7 in diameter? Did Mazda calibrate the speedometer differently for different wheel sizes? What is their tolerance on the speedometer readings from the factory? Does anybody really know what time it is?

*An unprofessional driver on an open course. Don’t try this at home!