If there is no booster/buffer in the pedals, then you can't generate the sensor signal; you need to generate a resistance for it to work right.
Here's a I2C digital potentiometer with 10 kOhm range: http://www.digikey.com/product-detail/en/MCP4261-103E%2FP/MCP4261-103E%2FP-ND/1635797
It actually has two pots, so you can do both gas and brake with the same device.
And it will work with both 3.3V and 5V Arduinos.