Currently the Potential Savings Per Fuel-up states your savings with 1,2,3 mpgs and then the US Dollar Value for each. I don't think this will really give a person an idea of the amount of money they can save if they were able to increase or change vehicles that they are driving. A better example would be to show 1, 5, 10 mpgs respectively so that the user will grasp the true savings. Obviously the metric based figures could use a similar example showing similar changes if you were able to achieve them. I know its simple math, but people like to be shown the amounts even if they can add. It's just a quick helper.
We could have put 1, 5, and 10mpg changes, but we realized in testing that for most cars getting ~20mpg in the US, changing your habits and improving economy worked for easy 1-3mpg changes. Going beyond that is quite difficult (a 10% change is easy, a 25% change is major and 50% improvement is impossible) so we didn't put 5mpg or 10mpg down there.
We considered also showing an annual increase in mpg's impact instead of just the next tankful, so maybe it'd say getting 1, 2, and 3mpg could result in hundreds of dollars of savings over the course of the year.
What about taking, say, the average number of fill ups per month (that'd be cool to see!) and multiplying that out to an annual savings if your vehicle got 1,2,3 MPG better? Seems that would allow people to quantify the savings and the data to do it should be available with existing entries.
Seeing I can save $2.45 per tank of gas isn't much. However, if I'm filling up 3 times a week, every week, that's substantial annual savings (~$400).
Maybe i'm not clear enough. I'll try better to put forth my point.
In my eyes the whole point of this website is to get people to conserve and buy less polluting and better mpg vehicles. I think the best way to help a person understand savings is to really give them a working idea of what they would save and what it would take to make it a cost effective purchase.
Let's take a gas guzzler car that get's 17.5 mpgs on average. If i am shown that i can save a little bit of money by getting 1,2,3+ mpg then there is really no incentive. Your showing me that if i drove conservatively that in the end i can save $5-6 and unhappily drive my vehicle or just drive it like i stole it and not worry about those $5-6.
Now lets take that same 17.5 mpg avg car and show the person a real time savings if they were not only driving more conservatively, but a different car completely. If they were to trade in that 17.5 mpg car for a 20,30,40 mpg car what would they save? If we took the same figures and broke down a $2.79 per gallon, 294 miles driven with an base avg mpgs of 17.5, Base Real Cost of $46.87:
Base Real Cost $46.87 (all figures rounded up for simplicity so numbers aren't down to exact cents).
+1 MPG - $44.36 (Savings $2.51)
+2 MPG - $42.13 (Savings $4.74)
+3 MPG - $40.18 (Savings $6.69)
Hardly an incentive to sell my car, take a loss on trade and purchase a higher mpg vehicle. But...if you also showed your savings on a different vehicle getting much better mpgs it would look like this:
+10 MPG - $29.83 (Savings $17.04) (27.5 mpgs)
+20 MPG - $21.88 (Savings $24.99) (37.5 mpgs)
+30 MPG - $17.27 (Savings $29.60) (47.5 mpgs)
My point isn't to get rid of your 1,2,3 mpgs, but to add and give a person a better insight as to what they could save over the long haul. If you had a 17.5 mpg vehicle and could trade in for a 27.5 mpg vehicle you would save (based on 10,000 miles per year, $2.79 per gallon)
17.5 mpgs - 10,000 miles per year - $1594.29
27.5 mpgs - 10,000 miles per year - $1014.55 (savings $579.74)
37.5 mpgs - 10,000 miles per year - $744.00 (savings $850.29)
47.5 mpgs - 10,000 miles per year - $587.37 (savings $1006.92)
As you can see the savings start to equal a couple car payments if not more. You website can give people a real time update of those savings and really show them whats economically better. Loss on trade-in or savings over the long haul.