Top speeds, laptimes and lot of this site runs off the data published in videos and spreadsheets produced by broughy1322.
Broughy1322 on YouTube
Broughy spends countless hours testing each and every car so we don't risk wasting our hard ground money without knowing better. If you find his videos useful, please consider becoming a patron.
If you'd like to know more about Broughy's tests, please visit these links.
How Top Speed Values Are Calculated Accurately
Top speed information is gathered from data published in videos by SD1ONE.
Some of the information and images come from GTA WIKI.