I agree both are accurate, and really say the same thing.

But GPM makes comparisons more straightforward between vehicles, at least in my mind, because you have to have to calculate gallons used separately for each vehicle for whatever miles you want to drive before you can really compare the 2 cars. Using GPM gives you one less step to get to that point. The math is pretty easy in your example, so lets make it a little bit trickier to compare methods:

Let's say you're comparing 17mpg vs 27 mpg cars in the car lot, and you drive 1500 miles/month.

If you had both mpg and gpm on the sticker, which would you rather use to figure out how much you save each month?

Using MPG:
1) Car A gallons used = 1500 miles / 17 mpg ~= 88.235 gallons used
2) Car B gallons used = 1500 miles / 27 mpg ~= 55.5 gallons used
3) 88.235 - 55.55 ~= 32.685 gallons saved over 1500 miles

You had to divide twice and do 1 subtraction to get here.
If you wanted to do the comparison for another distance, say 10000 miles, you'd have to do all 3 steps over again, so you could again get to the gallons used for each car.

Now lets try doing the same with GPM:

17 mpg ~=.0588 gpm (pretend these were right on the car stickers)
27 mpg ~=.037 mpg

1) .0588 - .037 = .0218 gpm difference per mile between cars
2) .02gpm * 1500 miles = 32.7 gallons saved over 1500 miles

You just did one subtraction and one multiplication instead of 2 divisions and 1 subtraction. If you wanted to do the savings comparison for another distance, say 10000 miles, you only have to multiply 10000 * .0218 calculated in step 1 and you're there.

To me anyway, this makes apples to apples comparisons much more straightforward. Sorry for the long post......

 

Yes, change sucks, but often times not changing sucks more.

It sucks having to have 2 sets of wrenches, metric for anything made anywhere else in the world in the last 20 years. and SAE for US made stuff. When I wrench on my Harley, I need both. That really does suck.

It sucks not to be able to decipher a weather forecast when I'm traveling anywhere else in the world because we don't use Celcius (sp?), and the rest of the world does.

Ditto driving anywhere else because we use miles and the rest of the world uses kilometers.

And on and on and on. We are WAYYYYY overdue to get on the same playing field as the rest of the world, and after a short (but yes, painful) adjustment period, life will go on, and be better for it (even Harley is well on their way.....)

 

thats true metric is better and more widely used. agreed with everything youve said redandy. we are long over due for a change

and you didnt fix anything with your sarcasm rich... ;] and its not unfamiliar if you paid attention in school, you learn the metric system...poor attempt at making somebody youve never talked to sound like an idiot. i didnt think i was being rude, but apparently i offended you. oh well. take your sarcasm and shove it.

guess i have no point, haha...and agree, my original post was right after a frustrating day of work and i wasn't thinking properly.

 

I'm being pro-metric system... not anti!

 

yeah, just thought you were taking a shot at me. guess i deciphered it wrong...damn internet! ;p sorry. back on topic! ;D

 

Gallons per mile figures should be 0.1, 0.05, 0.04 and 0.02 gpm. Guess you're part of that public.

That said, MPG is less useful for the public, as 10 -> 20 mpg seems like less of an improvement than 25 -> 50mpg, as there's fewer MPG in the difference. Comparing in gallons per hundred miles easily reveals the difference:

10mpg = 10gphm
20mpg = 5gphm
25mpg = 4gphm
50mpg = 2gphm

The problem is people have an intuitive grasp of what an MPG is, as they've been using the same system for decades. They won't intuitively grasp gphm right away, so the switch should probably be to L/100km like the rest of the world.