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Does the LS400 call for high octane fuel? Be aware that higher octane gas has marginally less energy per gallon, so all else being equal, your fuel economy will be slightly worse. Many newer vehicles can advance the timing a bit and (partly) offset the extra fuel costs with higher octane gas, but it's almost never cost effective.
To answer your question though, the device you're looking for is the "MPGuino". There's a subforum dedicated to it here in the instrumentation section.
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