The current system allows final drive to be changed upon upgrading to a 5 speed but not before, then specific gear ratios to be changed from 6 and up, plus all gears tunable on the “drift 4 speed”, which is only availible on specific cars.
First, final drive tunung should require a separate option in the differential upgrade section. final drive has nothing to do with the transmission, it is purely the differential.
Second, there should be custom transmissions for ALL gear counts from 3 to 10, or ideally, a new transmission option which replaces all of the gear count options and instead allows the addition and removal of gears in the tuning menu. Either of these would allow gear tuning for any gear ratio and any gear count. This transmission would give the max PI penalty for the gears, hopefully avoiding the problem some cars have with transmission upgrades where the base setup lowers PI by reducing the top speed while the final drive allows the speed to be increased back to normal (see the Raesr Tachyon which is dropped to A class by the drift transmission slashing its speed from 250mph+ down to about 90mph, but the final drive can be extended to get that 250mph speed back).
Fro example, the Dodge Charger Daytona famously ran over 200mph with a 4 speed transmission in race spec. In its current state, it is physically impossible for the car with the stock engine to go 200mph with a 4 speed, instead forcing you to upgrade to a 5 speed to unlock final drive tuning to change the speed at all. (it also doesnt make enough power compared to the race spec, requiring a supercharge to match the Naturally Aspirated race engine, but thats a less annoying issue)
This is also the case with many old cars with less than 5 gear, and is particularly annoying given several cars such as the ford GT40 have fully tunable gear ratios from stock allowing them to keep their gear counts while also being tuned to have the correct gear ratios. Rally cars like the Ford Fiesta XR2 or Race cars like the Dodge Charger mentioned before are even more fustrating as I can look up the specs and find the exact gear and final drive ratios for them online which I want to replicate in game but cannot because their 4 or 5 speed requirements.
I hope my suggestion for a single transmission with high PI cost and complete tunability seperates this suggestion apart from those just wanting another trans options.