skidpad G 0.91

They're OK. Not terribly accurate by absolute numbers, but great for measuring changes. They can also be moderately useful when using the same instrument on a number of different cars. The real problem is they are not calibrated to a traceable standard (like a NIST standard), so their absolute numbers could be off more than you would expect.

Skidpad numbers are subject to a lot of variables too. The diameter of the circle, the coefficient of friction of the road, road temperature, tire temperature, who's driving, etc, etc. The only way they mean a lot is when you can either correct for conditions or you can test in the same venue with the same conditions after you've made a change.

A .91 is perfectly believable for a basically stock car with upgraded tires.

depends on the meter. car and driver recently did a comparison on all the low budget units vs all the high end stuff they actually use to test. see their site for details and find out how accurate the exact meter you used was.

Even then you won't know. Every unit has its own sensor. Each is unique. Each has a slightly different response, so even if C&D tested one, and it turned out OK, the one you buy might not be OK. That's why all serious instrumentation is calibrated on a regular schedule.

Like I said, the unit is consistent with itself, so you can use it to compare before and after pretty effectively. Just don't compare your numbers to C&D or R&T, or any one using a calibrated instrument. Your absolute numbers might be off significantly.