Sounds like you calibrated it incorrectly.
I had some issues early on with my throttle. For some reason when I floor the throttle my pedal does not always send the same signal and that result is that sometimes I will get less than 100 % throttle when floored.
Using the calibration I pressed the throttle down to about 1/4-1/2 inch from the bottom and set that as 100% so now the throttle works great and when I put pedal to metal it is always giving me 100%.
The calibration is not per car. It is simply so the game can set the min and max range of your controls.
It is possible to mess up the calibration phase if you do not do it properly, especially the steering.