Percentage Versus Actual Increase
 

I'm curious about the relationship between estimated increases in mileage. Most often, I hear the expected/anticipated increase in percentages, but I'm not sure what the relation is to actual.

For instance, if modification X is estimated to produce a 10% improvement in fuel economy, and X is performed on vehicles A (~ 20 mpg) and B (~ 30 mpg), would A see a 2 mpg improvement while B sees a 3 mpg improvement? Basically, do these modifications become more substantial as the vehicle's base mpg increase? tiy

It really depends on the vehicle; which is why an A-B-A test is needed. A mod that increases fuel economy by 5 % on car A will not necessarily yield the same efficiency on car B. It is not a linear function per se.

There are many factors at play and each vehicle has its own specific characteristics.

Your math is correct - a 10% improvement is 2 MPG for A and 3 MPG for B. Keep in mind, however, that if A and B are driven the same number of miles, A will save more gallons of gas.

Also, a mod that saves 10% on one vehicle may not save 10% on the next vehicle.

many mods are for aerodimance gains, so a 10% reduction in aero drag for a truck is huge MPG gain but the same aero gain on a econo box is not as big of a MPG gain

And I guess that is what I'm getting at. I see most modifications listed with a % increase in mileage, but that doesn't, necessarily, seem to be the best way to state the effectiveness of a modification. However, generally speaking, it does seem like certain vehicles are more affected by certain modifications and less affected by others.

The other thing I guess I'm trying to figure out is whether these modifications are interdependent or affected by other modifications on the same vehicle. So, to continue with the example that I started above, would the effectiveness of modifications x, y and z on vehicle A produce the same percentage/actual gains independent of the other modifications. For instance, if x and y both result in a 10% increase on car A (~20 mpg), would they both result in 2 mpg, or would one be 2.2 mpg, or would they both be 2.2 mpg, or could they in combination result in even higher numbers? In a word: synergy.

One word to answer your one word:

Testing.

-soD

Yeah, I know that is ultimately the case. I guess I'm just looking for a way to more effectively plan what I do. Some of the modifications I'm planning can be fabricated/done cheaply, but most often, there will be an investment. If an aeromod will cost me $100, the difference between 2 mpg and 4 mpg is significant in terms of how long it will take the mod to pay for itself.

The effect of mods is case dependent. For example, let's assume two vehicles that get the same MPG. One has low aero drag and high driveline friction, the other has high aero drag and low driveline friction. The same reduction in aero drag will have little benefit on the first vehicle and a large benefit on the second.

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.

Take two cars of the same model, with only the engine different. Say a V6 and a 4-cylinder. Now apply the same aerodynamic mods to both cars. It will make the same aerodynamic improvement on both. The engine gives a different baseline mpg, but that is independent of the mod's effect. You'll see the same (or very similar) percent improvement in both cases, but it's a different absolute mpg gain because of the different baseline.