If you are reaching the redline, or are just about to reach the redline in the highest gear, it is advisable that you Increase the final gear ratio and therefor lengthen the gears allowing the car to reach higher speeds. Some cars only allow you to choose the gear ratios presets:

• Short - less top speed
• Long - more top speed

By lengthening the final gear ratio you are also negatively impacting the acceleration to an extent, so make sure your gear ratios are dialed in just right.

You want to have some headroom before the limiter on the longest straight for when you are in a draft of a car ahead.