Top speed estimates don't match

Today I was experimenting with modern hypercars in Automation and created a carbon fiber coupe with a 1,411hp engine. I noticed that whilst the estimate in the gear setting popup said it could do 306mph, the top speed in the graph above was only listed as 237mph, and wouldn’t go any higher if I shortened the gearing to remove the apparent overdrive. I haven’t refined the car yet but I am using sport compound tyres and ESC + LC for driver aids. What could be causing this discrepancy?

Is there much downforce? IIRC the gearbox popup estimate doesn’t take that into account. Well, it’s just an estimate, helpful for initial gearbox tuning when you don’t have all the rest of the car engineered yet.

I have since deleted the car but I’m pretty sure I used a flow optimised undertray and not a strong downforce one; also that’s likely not the cause of the issue because when I tried using all-wheel drive, the top speed estimate in the upper box increased to 273mph, but likewise wouldn’t budge when I adjusted the gearing length and was still lower than the top speed estimated by the slider popup.

More than likely in that case then it’ll be that you don’t have enough grip to put down the power at that speed, thus the car doesn’t accelerate past a certain point. You can see that in the second bit of the gearing or tyre graph where it shows speed vs acceleration or something and if grip is limiting your acceleration then you should add more downforce to compensate.

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Or wider tyres. I’ve just noticed that with what I suspect to be a 1400 hp engine the 0-100 time is as much as 4.8 seconds, while I’d expect sth more along the lines of 2.8s with wide enough tyres.

That might be the case then; I put a 1,200hp version of the same engine into a heavy sedan and it accelerated to top speed just fine with both RWD and AWD. With downforce being added I might even need to use turbocharging to achieve the big three hundred.