The funny thing is that both M-8 and T-17 have less HP than a Jeep.
Jeep takes normal 36 dmg from 1 mortar round, M3 Halftrack, M-8 and T-17 take x2 dmg (72) from german mortars (both).
This makes the Jeep 2,5 times more resistant to german mortars than an M-8.
Allied mortars are more accurate and do x2 dmg vs infantry (40), but their base dmg is only 20 and they have no multipliers vs halftracks and armored cars.