silverado engine maintenance

I have a 02 chev silverado with 110 000 km. Is there any engine maintenance I should do? A local radio show mentioned changing the thermostat and O2 sensor every year but my manual says nothing
of this and plugs are not do until 166 000 km. I still have original air filter but the gauge says it is still very good and the gauge is working properly. I consistantly get 12.5 mpg combined city/highway with the 4.8L and this seems very low to me (EPA estimates 18 city/22 highway). I feel I should do something but like I said the manual has nothing scheduled. Transmission was rebuilt 3 times so it has fresh filter and fluid.
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There used to be a radio commercial for some product -- I forget what, and that's supposed to be a frequent problem with clever commercials; but I digress -- that started out by asking people, "Would you rather play this tuba full of mayonnaise, or..."
My humble guess is that that's what your engine is doing, if you have 110,000 km (70,000 miles) on the original air filter. I'm impressed by the range of the memory map in the engine computer, but am reserving judgements on the efficacy of the maintenance telltales.
It is more plausible that the plugs are still decent but it probably won't hurt to change them. Yank one or two and see what they look like. Think about the maintenance interval of the plug wires too.
I basically never preventively replace an 02 sensor -- just wait'll it throws a trouble code. (By the way, On-Board Diagnostics II scanners are your friend -- there's even at least one now that doubles as a driving computer of sorts, so you can see what sort of mileage you're getting under different conditions). The thermostat I replace during routine replacement of radiator hoses and coolant, whenver that either comes up on a schedule or strikes me as a good idea, whichever is in nicer weather.
I'm rather bothered by the idea that you've gone through three trannies in 70,000 miles. Que limon! Or are you using it to pull/haul really heavy things all the time? That too reduces mileage a lot.
Anyway, start with a new air filter. They're pretty cheap, and engines suck a lot of air, and the air is dirtier than most of us realize. Even if you still have other problems to look for, that alone will help.
Best of luck, --Joe
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The tranny problem was the sun gear reaction shell. About the air filter. The box has a vacuum detector on it which detects the slightest suction that would be caused by restricted air flow from a dirty filter. I removed the unit and with the slightest suction from my mouth the gauge moved into the red/replace area. In the truck it is staying in the green area. This tells me the air filter is allowing plenty of air thorugh it. I do not haul or tow anything. I do have 235/85-16 tires on which are 4% taller than the stock 245 70 16 but I factored that in when computing my mileage. The engine runs and idles perfectly.

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