Ern
The RainForce tire size is Simply 185R . Being Euro sized , that is an 80 profile, so it is Original size. I have seen some use the
195R in the same tire . This will give you a slightly higher tire while increasing the width only 10 mm............

There was an optional ZF 5 speed tranny available [ very rare] that had an Overdrive 5th gear that some liked for high speed cruise/rpm benefit.
And some 113 guys opt to grab a differential from a 108, 4.5 [280SEL] , giving the car a 3:27 rear [ this equates to approximately the same R's/speed as the OD has in top gear.]
I have done both, but again, prefer Original.....