It even easier then that.
You can tune a car without even saving the tune.
Just tune it, and when you’re done instead of clicking ‘save’ you press escape (or whatever ‘back’ is on your system). Then the game asks if you want to apply the changes you made, you confirm, and thats it.
The tune is applied but not saved, and thus doesn’t count towards the 500 tune limit.
Guessing the only DISadvantage to doing it this way is if you want to race a car stock, and you use the upgrade to “reset to default” or whatever that option is, if you want to put back your tune you have to re-create it at that point, right?