Tony, you are correct that the forces must balance but they don't have to balance over the same area. If you overinflate, the total weight of the car is balanced on four smaller contact patches. If you underinflate, the total weight of the car is balanced on four larger contact patches. If you have bigger tires, the total weight is balanced over four larger contact patches. Even within the contact patch, more of the weight is centered in the middle of the contact patch than on the edges. When you underinflate, the contact patch grows, friction with the ground grows (rolling resistance), and heat builds up...hence the Explorer problem. Think of a bullet hitting a bulletproof vest. The forces must balance but the little bullet concentrates its force in a small area while the vest dissipates the force of the bullet over a larger area. Later. 
Fastest Contour at SZ 2002 Auto-X. 10th in PAX out of 125. CEO of FOGEY(Fast Old Guys Emasculating Young-uns), Inc. Terry Haines, Chairman, Senior V.P.s: (alphabetical)JavaContour, Jet Mech, MFE, SeicoRacing, SVTSTS, Vern Kilburn. If your not a member, yet, wait a few years. I'm not just the CEO, but I'm a member, too. Working with the rank and file to get the job done right!
Historical:
63 TVR (1K in 74), 75 TR-7 (paid cash new), 79 RX-7 (zoom,zoom), 81 RX-7GSL (autobahn-driven),82 Mustang GT (autobahn-driven), 85 Mustang GT (SE Division F Stock Solo II Champ), 86 MR-2 (3rd SE Division D Stock), 88 Civic DX (had 1st born and still owned MR2) 92 Sentra SE-R (all go and no show), 98 EO SVT Contour (FTD SZ 2002). 02 Altima 3.5SE 5 spd!!!
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