As long as the method used is the same before and after, it doesn't matter. Consistency is key.
As mentioned, my Mazda (2024 Turbo, Cx 5 with ECU tuning) LOM difference from hand cal is NOT consistently the same spread as hand cal.
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Ideally, take a trip, use CC with the grounds attached, run out a tank of gas. Connect the grounds, run another tank. I simply disconnect from the body and arrange so the free end doesn't touch any metal parts. Only need to carry a 10mm socket in my case.
I will do the above on our next trip. It's a quick, clean process to do while filling up. I will hand calculate MPG and compare to the LOM as well.
FWIW, the three 30+ MPG 5.9 diesel tests, I noted:
traffic slowdowns,
CC speed,
engine RPM,
ambient temp, wind speed and direction
terrain,
starting tire pressures,
tire type,
loaded or unloaded,
which and how much fuel additives,
which ECU tune and so forth.
After 20+ years, most of these trucks have been modified over the years. Most of their modifications hurt fuel consumption. Lifted with bigger, wider tires is number one. Store bought CAI is second most mod that reduces MPG.
Detailed report of 30 MPG give validity to my claim. LOM data is not accepted as valid with the truck guys. Even the new diesels, LOM data is not consistent.
I'll list of modifications we did to our CX 5: cylinder grounds, turning vanes in the inlet air duct, variable volume CAI and tuners name and tune with some of the above driving conditions on my A to B test this summer.
30 MPG in a 20 year old Dodge, 230,000 mi, 2500 is up from 18 MPH when I first bought it 17 years ago with 30,000 miles. The fuel savings is REAL.
I want real data too to prove or disprove if grounding helps a Skyativ engine. It MAY not. LOM may or most likely will not give meaningful data.