Actual is one thing - and of course its the only thing that actually counts.
How to do the math is another thing.
Yes, as others wrote, the % increase is the amount of increase divided by the base number. When you start piling on multiple mods or driving technique changes, you could measure each increase (or decrease
) against what it was just before the most recent change. Or you could measure against your most recent number. In the fuel log, there's a comparison against EPA combined, which would be the base number.
And of course, if you start at 20 mpg and every time you achieve an improvement it's another 2 mpg, each subsequent 2 mpg gradually becomes a smaller and smaller percent. The first time it's 10%. But when you're getting 30 mpg and you increase it to 32 mpg, that's only a 6.6666%. But you still added 2 whole miles to what you get per gallon. See comments on the European system of measuring FE further down.
In my humble opinion, yes, the increases are synergystic. Maybe not in raw testing with technique and everything else absolutely the same. BUT if you coast, and you reduce the load by doing aero mods (or serious weight reduction), AND you start coasting with the engine off - - - better aero will allow longer coasts so you need to run the engine even less than you would expect from just the aero mod, or from just coasting without mods. So the mods or technique improvements help each other.
Related math - The European system is to measure fuel qty/distance, the reverse of what we do. It tells you how much gas you'll need to cover
x distance, which is potentially more useful than distance per gallon. This gives different % numbers for the same change. Take an extreme example: in US terms, doubling your distance per fuel qty gives a 100% increase in the MPG number. But the same doubling cuts your Liters/100Km in half, to 50% of what it was. You can't get a 100% reduction in L/100km unless you can run the thing on zero fuel. Don't we all wish we could.
There was an old joke about a guy who bought every MPG enhancement product on the market. Each one claimed a 10% increase in MPG so by the time he was done installing all that stuff, his gas gauge would go up as he drove down the road.
Sorry, I digress in various directions. Hopefully someone reading it found it worthwhile.