Here are the stats according to the game:

DINO 246 GTS Novice - 5 gear - Speed: 288km/h (179mph) acc - good / han - good / spe - bad

365 GTS/4 DAYTONA Novice - 5 gear - Speed: 288km/h (179mph) acc - good / han - good / spe - bad

F50 Intermediate - 6 gear - Speed: 293km/h (182mph) acc - good / han - good / spe - good

360 SPIDER Intermediate - 6 gear - Speed: 293km/h (182mph) acc - good / han - good / spe - good

288 GTO Intermediate - 5 gear - Speed: 293km/h (182mph) acc - great / han - good / spe - good

TESTAROSSA Intermediate - 5 gear - Speed: 293km/h (182mph) acc - great / han - good / spe - good

F40 Professional - 5 gear - Speed: 300km/h (186mph) acc - good / han - good / spe - great

ENZO FERRARI Professional - 6 gear - Speed: 300km/h (186mph) acc - good / han - good / spe - great

F355 SPIDER Intermediate - 6 gear - Speed: 293km/h (182mph) acc - great / han - good / spe - good

328 GTS Intermediate - 5 gear - Speed: 293km/h (182mph) acc - great / han - good / spe - good

250 GTO Professional - 5 gear - Speed: 300km/h (186mph) acc - good / han - good / spe - great

512 BB Professional - 6 gear - Speed: 300km/h (186mph) acc - good / han - good / spe - great

Through minor experimentation done by Varuuna and I, it appears that:

• 5 gear cars are faster to reach their top speeds than the 6 gear cars
• Manual Transmission is faster than Auto (if you frame perfectly switch gears)
• seemingly obvious, the higher the top speed the better. though some other cars hit their top speeds of 293km/h faster due to a higher acceleration stat, this eventually gets caught up to by the cars that can hit 300km/h (this isn't a huge difference either, the professional cars are only that tiny bit faster overall)

So this all means if you want to be ENTIRELY optimal you would use either the F40 or the 250 GTO with MANUAL TRANSMISSION - though as stated, this isn't a huge difference