speed

i have a 1946 chev car and can not get over 45 mph

More info would be helpful !

First thing that would occurr to me would be either the ignition timing is retarded a few degrees, or that the centrifugal advance mechanism inside the distributor is stuck - and not advancing. In such a case, 45 mph would be about the top speed. With the distributor cap off, you should see two weights that swing out as the distributor turns. Usually the distributor shaft that you see (with the lobes on it for the ignition points) rotates a little on the inner shaft. This mechanism needs to be 'free'. There might be a place under the rotor, at the end of the distributor shaft, for a drop of lube each 10,000 miles, or 5 years, whichever happens first.
I dont know the advance setting for your 'cast-iron-wonder. If it pings when trying to accelerate in 3rd gear at 40 mph - then its too far advanced. On my 32 Chevy there is a steel ball embedded in the flywheel, and a hole on the flange of the block at the bell housing, where a strobe light will show the steel ball going by - if the timing is set right. It can also be used to static time the ignition.
Being a 46, unless its been rebuilt since new, it has cast iron pistons in it. Chevy didnt go to aluminum pistons until 1950!

If power is pretty good at low revs, the lack of advance is a reasonable suspicion. Of there simply is low power at all engine revs, look for incorrect valve timing/adjustment or a blocked exhaust system.

Does it stop at 45 mph for lack of power or lack of gear?

Judging by your pm and your other post I'd guess that it isn't revving up like it is supposed to and will probably act up in neutral as well. I'd try to differentiate between fuel problem or spark problem with extra fuel,timing light,listening.choking,turning dist, ect.