I apologize if that issue is known as solved here, I am new to this forum and I will like to share my discovery with people who are dedicated to GT series. Amount of horsepower are actually kilowatts at some cars. You need to use factor 74.6 % to calculate kilowatts when adjusting power. Some of the cars have also messed up gearbox with wrong gear rates, so you will have to repair them. You can find correct gear rates on http://www.automobile-catalog.com/browse.php. Example 1: Ferrari F40 If you repair gearbox with fully customizable transmission with realistic gear rates: I 2.769 II 1.722 III 1.227 IV 0.963 V 0.767 Final drive ratio std: 3.63 And if you lower the power on 74.6%, so you can see 352 bhp (F40 has 352 kw), then Ferrari F40 will reach more or less realistic top speed 327 kmh. Example 2: McLaren MP4-12C I 3.98 II 2.61 III 1.9 IV 1.48 V 1.16 VI 0.91 VII 0.69 Final drive ratio std: 3.31 Power: 74.6% You will reach 353 kmh. That, according to Wikipedia, is realistic top speed of MP4-12C Coupe. The Coupe (in reality) can do 218.61 mph (352 km/h). https://en.wikipedia.org/wiki/McLaren_12C#Performance Example 3: Maserati GranTurismo S I 3.21 II 2.05 III 1.44 IV 1.11 V 0.9 VI 0.76 Final drive ratio std: 3.29 Power: 74.6% Top speed: 298 kmh - realistic and entirely correct top speed. Example 4: BMW M4 Warning: This car has DCT gearbox, so you will have to find gear rates for DCT and not manual gearbox. Here they are: I 4.806 II 2.593 III 1.701 IV 1.277 V 1 VI 0.844 VII 0.671 Final drive ratio std: 3.462 Power: 74.6% Top speed: 301 kmh. This is 9 kmh more then theoreticaly M4 could reach (292 kmh), but still very close. Example 5: Ferrari 599 Fiorano I 3.154 II 2.176 III 1.565 IV 1.185 V 0.935 VI 0.78 Final drive ratio std: 4.18 Power: 74.6% Top speed: 335 kmh. Much more realistic then before. However, there are still problems at some cars like 288 GTO and Cizeta V16T. GTO can do 326 kmh and Cizeta is reaching 339 kmh.