Huh. My iPhone seems to be pretty accurate compared to my garmin 60CSx. What app were you using to show your indicated speed?
best way to calculate it is take your-
new tire size / original tire size = difference ratio
difference ration x speedometer speed = actual speed
here's the example on my truck-
33/27 = 1.22
1.22 x 55mph = 67.1mph
The iPhone is a very unreliable speedometer. Have you ever noticed that sometimes when you're moving and watching your GPS that the blue dot stalls and then jets to catch up to where you ended up since it's last blip...that's basically how the speedometer apps work;
60mph
2mph
90mph
7mph
32mph
67mph
0mph
200mph
been there, done that
I use a speedometer app on my EVO (android phone). Its very accurate, and is also my GPS. I use it to track mileage on long trips, check elevation, speed, etc. I dont use it all the time in place of my trucks speedometer though.