These add or subtract from your core car design. Imagine you have 20 design points in all eight of your design attributes.
Say your engine supplier is Toymotor which is +10 Handling / -6 Fuel Economy
This makes your design:
20 Acceleration
20 Braking
20 Cooling
20 Downforce 14 Fuel Economy 30 Handling
20 Reliability
20 Tyre Economy
If you use Bridgerock as tyre suppliers as per your original question this modifies the values as follows:
20 Acceleration 25 Braking
20 Cooling
20 Downforce
14 Fuel Economy
30 Handling
20 Reliability 17 Tyre Economy
Finally just to complete the answer, if you choose Chavon as fuel suppliers, they give +2 Fuel Economy and no negative which makes your design:
20 Acceleration
25 Braking
20 Cooling
20 Downforce 16 Fuel Economy
30 Handling
20 Reliability
17 Tyre Economy