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I have the front passenger side and rear driver side tires at 34 psi.

The other two wheels are around 28.

How much is this affecting the mileage on the car? It is a 2004 Cadillac SRX V8, and the valves are stuck on, even when using a pliers. Oil change guys said we need to take it to a tire place to get them off and possibly replaced.

Can I wait on this, or should we get them evened up right away?

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FYI: those metal valve stem caps have a risk of welding themselves on. If you'd like a nice set, best to go with plastic. –  Bob Cross Mar 6 '12 at 13:30
    
How did he check the pressure if the valve caps are stuck on? In-vehicle sensor? –  dotancohen Mar 22 '12 at 17:51
    
Yep, in car, see comment below. –  Matt Mar 23 '12 at 13:30
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up vote 3 down vote accepted

I would get them changed straight away, not necessarily for the mpg, but in order to be safer on the road. In an emergency situation, where you need to brake really hard, there is a real chance that the car will pull uncontrollably to one side.

As sorting tires is cheap and quick, I would suggest doing it now.

And to specifically answer your question, the fueleconomy.gov website gives us:

You can improve your gas mileage by up to 3.3 percent by keeping your tires inflated to the proper pressure. Under-inflated tires can lower gas mileage by 0.3 percent for every 1 psi drop in pressure of all four tires.

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I guess that's part of the problem... They're the tires that tell you pressure electronically inside the car, so what we were told is that it may cost a lot to get the caps/valves off and replaced. Will definitely look into it, though! –  Matt Mar 6 '12 at 14:13
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