That's weird, something's not right, or your tires are just really sticky. The '97 Tahoe I just picked up has the 3.42 G80 as well, and it breaks the 255/70s loose instantly. The only one I had that wouldn't spin was the '99 Yukon I just sold. I didn't have it long enough to figure out why, but it ran perfect and got great mileage.
I Don't know either, but I get around 20 MPGs and its not a hot rod, so it don't get treated rough very often, so it doesn't bother me, the 96 on the other hand can be a handful if given too much skinny pedal and its 3.42s and 315s
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Now that I think about it, I could smoke the 275/60s that were on it when I bought it.
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