I am pride, to suggest mine first one vehicle. With tuning.
 
1980 BMW M1 replace primo
step one
1. open openIV
2. mods/x64e.rpf/levels/gta5/vehicles.rpf
3. replace primo.ytd primo_hi.yft & primo.yft

step two
1. mods/x64i.rpf/levels/gta5/vehiclesmods
2. replace primo_mods.rpf

step three
1. mods/update.rpf/common/data
2. replace handling.meta


Step four. 
Done and have fun

Friendly regards,
Bart Servranckx.