Correct tranny fluid / tire size

Recently I replaced the 5 speed in my 633 CSi with a
Getrag 260 010 091. The old one was a 260 010 090.
Well, first in the mornings (mid 60*s) it grinds the synchro's going into 2nd gear and snaps going into 3rd. until it warms up. I put 85W-140 gear oil in it and wondered if this is the correct oil. :dunno:
Second, the gearing in this tranny is higher, I will need to go to bigger tires and wheels for the speedo to read correctly, it is off by 7 mph @ 70. I dont know what size tires were on the car I got the tranny out of, they were gone. :banghead:
Mine are 24" in diameter, How much taller do I need to go??? I know I will have to go to a 15" wheel already.

a) the trans needs, ATF, 30W non-detergent oil or RedLine MTL [though this tends to seep through older seals].

b) you didn't mention if the speedo reads too high or low but this would have nothing to do with the trans. ratios as the speedo is electronic and senses axle output speed and trans. or final drive ratios don't effect it.

c) the OEM size would have been TRX types which were closest to the earlier 14" wheels. You'll have to calculate roll-outs with various sizings to correct your speedo..........most on-line stores/manufactures have the spec's. listed.

Best to you and stay in touch.........................jeff b.

BUt... which fluid, ATF & 30W motor ooilk have different properties for heat absorbtion.

Heat transfer isn't the issue, your concern should be compatibility with the syncro's which all three will address. ATF and MTL [manual trans lube] are very well suited and many perfer the 'feel' of MTL but as I said, it tends to seep more than DEXTRON III......avoid Type F as it's friction coefficient is too high I believe. The straight 30w {NONE DETEGENT to avoid foaming] also will work well but the shifts won't feel as good as with the other two............jeff b.

p.s., I was wrong about the TRX sizing, there 390's which is just OVER 15".