Hi, think this is my first post ever, but long time lurker so please be gentle 
A quick bit of background that may help with my question ref indycar/nascar problem. I have been playing forza since 4. And I think one of the older generation (40+) not that that particularly matters. I usually spend circa 10hrs per week ‘playing’ (understanding partner and no children) Anyhow the issue I have is this.
I play the game (Single and multiplayer) with no driver aids so full clutch etc. And use full damage /unbeatable with limit aggression off ( not that I think this option makes much diff) for single player. I tend to do longer races with at least one forced pit stop in the the race setup option. And use whatever class/car/track that takes my fancy that evening. So everything from the 2cv to the f1 car. I tend to race 1/2hr ish per track so anywhere between 5 laps for the ring and 50 ish for ovals etc. What I have found though is when racing nascar/Indy on the ovals I can just pick a car (std set up no homologation / tune etc)and by lap 10ish I am clear of the field and it is just a case of not crashing etc to win. Surprisingly I am not interested in winning all the time. As I do enjoy the whole getting through the field fighting fairly for places etc. Even if it means finishing last. If I crash etc oh well pit stop it is. Better luck next time and that will teach me for missing a corner / clipping that car. On any other track/s I can really struggle to win races and need to tune/upgrade my car to stand any chance of podium. I don’t believe I am a brilliant driver or anything as in leagues I am only in professional and on rivals / leaderboards I am usually well behind the top times. And std multiplayer normally finish circa mid field unless I have been lucky. So in short is their anyway to make nascar and or the Indi races harder? Can I dumb down my car without it being a ‘silly’ tune /build etc so it keeps its basic characteristics or force upgrades on the Ai cars to make them a little quicker? Nothing I seem to do in the race setup seems to change the Ai cars specs 
(although I except I may be missing that 'vital option/ selection) when I try and do this.) I would also like to avoid having to go down the no pit stops option for the Ai car/s route as like the ‘chaos’ / mix it gives the field when racing. I would also like to avoid a mixed field of Ai cars eg nascars and super cars etc.
One final note/s when using nascars/indycar on other tracks I am back to my usual needing to tune them etc to have any hope of a decent finish… Oh and I am not a ‘bash my way to the front’ player even when racing against the Ai so I can’t use that as an excuse to why the oval races seem so much easier either . Finally apologies if I have bumped any of you while racing online I am very sorry it is never intentional. And thanks to all the genuine racers on line that make this game so much fun! Thanks in advance for any help.
About the only thing I can suggest is figuring out what kind of lap times the drivatars are running, and dropping gearing to kill top speed and slamming the downforce to kill cornering speed (at Indy and Homestead, max it for Daytona) until you’re running the same times, which you’ve probably thought of. Other than finding a group that runs ovals, that’s all I got. Good luck.
Oval racing is not a strong point for Forza…if thats the only place you are winning handily, thats not bad…
Hi both, thank you very much for your replies.
Will keep trying with the gear set up and wings .
As this currently does seem the best option.
Try running a handicap on your grip level in the rear to loosen the car until your at their speed. The computer tends to brake late and this will let you brake late with them while losing your time on exit with a lack of off corner grip. Might need to turn front grip down as well if your scary loose and still to fast for them. Multiplayer ovals seem to attract a weird group of people that love running around Daytona under caution and don’t believe in tuning or holding a line. It is however pretty sweet if you can get into a homestead/indy lobby where people can handle being side by side long enough to get things rolling. Individual results may vary, good luck.