Tire size/ distance is way off..?

[ejp2fast]

Stock Festiva tire size is 165/70/12
My setup is 195/45/14

From http://www.miata.net/garage/tirecalc.html
My speedo is 0.9% fast - when my speedo says 60, i am really going 59.5.
Makes sense.

But on a recent trip (202 miles according to google), My odo showed me going 216 miles.. hmmmm....

So 216/202 is 1.0693 leading to my speedo is 6.93% too fast. Thats quite a big difference in mileage..

Am I missing something? I can't seem to figure this one out.. arrrgh.

Next step- go side-by-side on the freeway with someone in another vehicle and just really see if my speedo is 6.9% too fast.
That means my mileage calculations are not 1% off, but more like 7%...

[htchbck]

Google was wrong once before... lol. But remember, what your speedo reads and what your odo says can disagree with one another. The odo is direct drive, but the speedo is not. It can get "off" over time due to the spring stretching and probably other factors too.

[ejp2fast]

Yeah, good point on the spring stretching.. I've done this run before in another car multiple times, so i can vouch for the 202 miles from point A to point B (203 actual in another car 4 years ago). When i first got the car, it had a broken speedo cable, so it's been replaced, plus my auto to manual tranny swap... so i've tweaked a bunch of things..

[blkfordsedan]

Just a couple things to consider...

The way that the miata.net calculator figures the tire circ. is geometrically correct, but erronious in real world application. To be accurate, you need to go to the tire manufacturer's website and use the "revs/mile" that they state for that particular tire. Since the tire sidewall will sag and bulge between the axle and the road (and the flexing action that takes place in the tire dynamics), the actual revs/mile will almost always be more than if you calculate the circ. of the tire. What's more, different tire designs have a difference in the amount of variance...i.e. a 10ply truck tire or low profile performance tire with stiff sidewalls will vary less than a tall profile and soft riding 2ply radial. It is actually slightly more accurate to measure the distance between the axle center and the road and use that as the radius to calculate the revs/mile. Make a mark on the bottom of your tire and on the pavement, then roll the car back until the mark comes around to the bottom again and measure the distance on the pavement. If you compare that to what you would calculate based on tire circ., you'll be amazed at the difference (depending on the tire).
I used this technique to calibrate the electric speedo on my '97 F250. The difference was substantial, but when I calibrated it based on the actual measured revs/mile, and not the calculated, the speedo and odometer were dead on. When I compared my measured revs/mile to that listed on General Tire's website, it was within a fraction of an inch in 2 revolutions.

Also, like htchbck said, the speedo needle in my Festy has "slipped" on the peg (or something like that) and now reads consistently 5mph off....but the odometer is pretty much dead accurate when compared to GPS. Go figure.......