One way to get around this problem is to do what I did with Panzergrenadiers ... create a Panzergernadier sftype which combines Infantry sftype combat values with Halftrack movement values. That way, when the Halftracks are lost, the Infantry goes with them ... so you never have the situation where the tanks are slowed down to infantry movement rates. Of course, the (realistic) downside is that Panzergrenadiers consume Halftrack replacements.
For Panzergrenadiers, this is certainly reasonable ... there are so few, relative to the overall infantry strength of all units, that having Halftrack replacements for them doesn't unbalance things, infantry wise.
(Realistically, I'd strongly suspect that Panzergrenadiers would largely be destroyed with their Halftracks, or, in a pinch, the unit commanders would simply put any without Halftracks into commandeered trucks ... since this can't be done in the game system, I believe the Panzergrenadier sftype is a reasonable compromise).
I also created Motorcycle Recon sftype ... infantry with truck-like movement (tweaked somewhat) using infantry replacements ... because I don't believe Truck replacements really include Motorcycles and, of course, the Motorcycle Recon elements are quite small overall. I am considering creating a Bicycle Infantry sftype with an enhanced movement better than infantry but less good than cavalry, for the same reason.
There is no reason at all why this should not be done with Motorised (Truck Borne) infantry ... create a Motorised Infantry sftype ... Trucks with infantry combat values ... and have it use Truck replacements. Again, since there are relatively few Motorised Infantry elements in the game, this should not unbalance things.
Author, Space Opera (FGU); RBB #1 (FASA); Road to Armageddon; Farm, Forge and Steam; Orbis Mundi; Displaced (PGD)