Actually street/sport/race tyre difference in cost is always the same unless it’s power bulid tune, but stock tyres are different and have different pi cost to upgrade to better tyre, btw most of stock tyres are overall slower
Tested multiple cars, and with balanced tunes, ad long as u have enough grip, street/sport/race tyres have similar pace / lap on Barcelona short alt. But stock tyres are usually 1s off pace. Same for suspension, roll cage and arbs, as long as u upgrade it to min street/sport (not all cars have street parts available, and even in this case stock parts are usually garbage) u can be completive.
The only problem is Forza aero, its just too op, and in most cases u can gain lap time by installing Forza aero (only le mans is exception)
Street/sport handling+tyre have advantage in start, and on most of tracks u can disappear at beggining, and other players won’t be able to catch up at all to you cuz they loose to much time on start. (Well as long as u have good tune).
Just tip for all players, if u want to have completive tune all u need to do is to install
Sport/race roll cage
Sport/race arb
Sport/race suspension
Race transmission (most op part of tune, u can use m/c without using clutch)
Race diff 100/0 it
Race aero/wide body (use min/min or max/mid setup depending on track)
And some broken engine swap or aspiration swap (look at pwr)
Install min street tyre, and setup car in a way where handling stat is as close as possible to acceleration stat, weight reduction should take priority.
Also try using same CP cost tyre width to make car balanced, if u install 200cp front and 350cp rear, car won’t turn due to massive understeer (well it probably can be countered with some weird fine tuning, but as long as u follow this guide and use same level of handling upgrades, u won’t need to fine tune your suspension/arb, all u need is to change final drive length in transmission.
And also part about ultra big rims with heavy rims, lap time gain is so small ~0.2s per lap, that I don’t thing it’s worth using lol