I’m getting some unusual performance in my attempts at making classic muscle cars and other vehicles in game. The top speed ends up being higher than historical vehicles, but the acceleration isn’t as fast. I’ve tried trading some speed for acceleration, but lowering the maximum speed on my gearbox doesn’t do anything for the car. I’ve also tried using wings and other aerodynamic modifications to increase the downforce and try to increase grip, but that doesn’t seem to do anything for my acceleration. In terms of the specifications I’m aiming for, it’s the larger muscle cars (I’m using the large 1965 body) with engines of around 427 cubic inches and higher displacement (I’m using a 465 cubic inch engine) for the classic muscle car, and a the mid-1980s Corvette with the more modern looking mid-1980s mid-engine vehicle body.
So what era are you comparing yourself to, and what are your 0-60 and 1/4 mile times?
Acceleration times are largely tied to the gear spacing, as well as top end (final drive ratio). Final drive will limit top speed, but the gear spacing will determine how fast (or economically) you will get there. Tire width and weight distribution will also dramatically effect acceleration times. Weight itself is also a consideration, but if you’re talking real muscle cars, there’s not a lot you can do about it short of completely gutting the interior. (it’s the only thing you can actually do in game for weight reduction while maintaining any sense of legality for street use) In the real world, there are a lot of other things to reduce weights, but those definitely were also not done from the factory.
Also, as nialloftara asked, what are you comparing to? Factory times are not what you’ll find on NHRA-type lists, as those are mostly aftermarket setups for the strip/street or even strip only setups.
It’s a 1965 model car with a zero to 100 kilometers per hour time of 7.1 seconds, and a quarter mile time of 15.06 seconds at 169 kilometers per hour. The top speed is 241.5 kilometers per hour. It has a wing and front lip tuned to produce near even downforce at the back and front of the car, and has sport tires. The engine is a 7.6 liter that produces 383 horsepower at 4000 rpm and 517 pounds of torque at 3500 rpm, and it has a four speed manual transmission.
Open diff or automatic locker?
That’s with an automatic locker. It helped lower the time by around a second or so.
You widened your rear tires as much as possible? Rim sizes as small as 12 or 13 inch would still be period correct and allow a fatter tire.
That seems to have done the trick. I was using 15 inch wheels. With some other modifications I now have a zero to 100 kilometers per hour time of 6.4 seconds, a quarter mile time of 14.46 seconds at 172 kilometers per hour, and a top speed of 241.0 kilometers per hour. The engine outputs 395 horsepower at 4000 rpm and 524 pounds of torque at 3600 rpm.
This seems to be about where the car should be with production tires.
Acceleration has always been very finicky with me. I find that setting the engine’s rev limiter roughly 500Rpm above the pique horsepower helps sometimes. Lowering the top speed doesn’t always make a car quicker, so you’ve got to play around with that. Try making a difference in the spacing of the gears as well.
Raising the quality of the gearbox and the tires are variables that also make a big difference.
If you’re really desperate, i’d go for weight reduction by lowering the quality of the interior, entertainment, and safety. I’d not expect to get THAT quick on old muscle cars though. 
Having done more research into historical designs, it seems that most engines of the era were rather oversquare, especially American designs. I was going for undersquare designs to try to increase low end torque, but I suppose if I have over 60% tire slip at launch that isn’t really do much good. Does anyone have better luck with oversquare engines in the earlier eras when tire technology isn’t as good, or at least not end up taking too much of a hit to acceleration?
Are you using a solid rear Axle BTW? The extra rearward weight can help counter the wheelspin.
As for under/over square you are already making pretty good power and under square is saving you some nose weight, I wouldn’t worry about it unless you are looking to recreate engines.
The effect in the game is minute, but have you tried soft springs in the rear? You get more weight transfer that way which could help depending on your setup. I remember a while back I was able to improve my acceleration by 0.1 second, just by softening the rear springs and raising the ride height.
Weird,I didn’t think they’d added that. I thought the weight transfer was only calculated laterally with the roll angle.
Could’ve been a bug for all I know. But it’s worth a shot! 
edit: Direct from source… viewtopic.php?f=14&t=6851&p=75665&hilit=weight+transfer#p75665 Weight transfer is calculated for acceleration after all. My suggestion still stands. May not make a huge difference as I noted earlier, but hey, nothing beats a trial.