O2 Replacement / Poor mileage

I've had my Suburban ('98, K1500, Vortec 5.7, 121,000 mi) for about 6 weeks and am getting really poor mileage :frown: , 13-or-so city, 14.5 highway. So far I've done:
Autolite Platinum plugs
Cold air intake
Taylor ThunderVolt plug wires
Cap & rotor
Tires; had Merit All Country LXTs, replaced with Uniroyal Liberator A/Ts, LT265/75x16, 10ply
Tire pressure is 38-40psi
New loaded calipers, rotors
Rear brakes seem to be adjusted properly with no significant drag
Had tranny fluid exchange, both diffs serviced, cooling system service
Also have replaced all 4 tie rod ends, pitman arm, idler arm & bracket.

I've been driving her really easy too to see what the best mileage will be. It's not throwing any codes. Would it be a good idea to replace the 2 upstream O2 sensors and see if it helps?

Is it necessary to replace the 2 downstream sensors as well? I've been told they don't affect the air/fuel mixture, just measure the effectiveness of the cat. If I've been told wrong please let me know.

Thanks much in advance!!!!
Jeff

I think that 14mpg in the city is max mpg you could get . I get 16mpg driving around my rural town.18-19mpg on highway.at 100,000 mi the o2 sensors upstream of the cat should be replaced. take one out write down the number and get the same sensors. my vehicle uses denso 234-4650 on all 4 .if you don't change these upstream sensors you put the cats at risk of getting plugged. downstream o2 monitors effectiveness of cat operation, this is in the repair manual. also check or replace fuel regulator the vacuum line going to it could be sucking gas into the intake manifold lowering mpg and damage to cats.