I haven't had a spare in my E350 at all. Its a cutaway model, so I don't have room under the back - the fuel tank is there.
I do have a tire plug kit, 16-ton bottle jack, small compressor, and the confidence of H/4800# rated tires (on the rear at least).
No flats yet. 
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95 F350 Crew 5spd/4.10, SuperChips tuner, solid flywheel, T444E waterpump, 205° stat, Locked F&R, 9.00R16 Michelin XLs
97 E350 SRW cutaway, E4OD/D70/4.10, 19.5s on the rear, deep trans pan, batteries inside, 1800W inverter, manual glowplugs
84 C30 Suburban, 6.2/4-spd, 3.21s
Other rigs I goof around with regularly:
93 F350 4x4, 7.3 IDI turbo, 5spd, 4.10s
90 F250 4x4, 7.3 IDI, 5spd, 3.55s, D60 swap, straight pipe, turned up fuel screw
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