I think what my challenger is after is more of a net mpg gain as measured at fuel-up time. Not the instantaneous mpg or gph during the coast. But after thinking about it, this would be hard to accomplish wouldn't it. There would need to be a test route that allows coasting and that is long enough to allow a measurable amount of gas to be pumped at the end of it.
The conditions would need to be similar for each test, which would be something like:
1. Fill the tank
2. Drive the route using in-gear coasting
3. Refill the tank and record data
4. Drive the route using in-neutral coasting
5. Repeat 3
6. Repeat 2 and 3.
Yes, I can see why nobody has done this.
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