A tail pipe mounted wideband O2 is not as accurate as one mounted right after the engine.
Too many variables introduced before the end of the piping.
For one. The fact if there is a catalytic converter on the car. If so that's a noticeable variable. (now a 4 gas sensor MUST be placed before any cats!)
Then there is the effect of temperature loss, stagnant air in the mufflers/resonators, et cetera. Albeit minor but more erratic variances.
Tail pipe is definitely better then nothing but it's results are not as accurate/consistent and normally reads a little leaner then it truly is.
Most "in ground dyno" shops likely use a tailpipe sniffer because of ease of use.
