Here’s why gas is sold in gallons but engines are sized in liters


Have you ever wondered why car engines are sized in liters and car parts are measured in millimeters, yet gas is sold by the gallon and distance driven is measured in miles? Blame America’s use of United States customary units, or USCS.

This is a companion discussion topic for the original entry at https://www.hagerty.com/articles-videos/articles/2018/09/20/metric-system-history


Not too many years ago, we had 289, 302, 351, 390, 427, 428 and 429 ci Fords, 327, 350, 396, 427 and 454 ci Chevys, 383, 440 and 426 ci Mopars, and 389 and 455 ci Pontiacs! The hay-day of muscle cars! Nobody spoke of liters back then!
Chuck. '63 SWC Vette / '70 ElCo.


One of the reasons that the metric conversion act of 1975 failed was that some people viewed it as part of a global conspiracy to create a one-world state. But most of us simply saw no reason to change a system that wasn’t broken.


Pontiac actually seemed to be an early-adopter of the metric naming in the 1960s, no doubt influenced by John Z DeLorean:


Ford also participated with their “7-Litre” Galaxie badging, most likely due to their LeMans racing connections in the latter half of the decade:



There are two sizes of gallons and many sizes of teaspoons. A litre is always the same.


Do you know why it is called the Imperial system of measurement?

Because it is a ROYAL PAIN IN THE ASS.


It’s really very simple. No mfg wants people to notice the twin turbo 4 cylinder is only well under 200 Cu in. Or the hot V-8 is 260 or so. Americans do not now how big a liter is, but it sounds good. “4.6 liters or under 300 cu in”. Want to get a dumb look from someone - - tell them your cars engine size in quarts. Promise you it will work


The metric system has been inconsistent and unreliable, that’s why it’s not accepted in the US.
As an example, Nissan and Toyota fasteners do not cross, despite both being “metric”. They use different pitches for both fine and coarse threads, so you need twice or three times as many metric bolts in stock as if you used American sizes.
As for “liter”engines, a 5.7 liter could be a 348, 349, 350, 351, etc. not exactly precise. And no one remembers 7.0 liter musclecar motors, they are actually 427 cubic inch.
The 6.5 is a 400, although the conversion comes out to almost 397 cubic inches.
It’s all just marketing, no actual consistent representation of engine size.


The Boss 302 Mustang and the Z28 302 were both made to meet SCCA regulations limiting displacement to 5 liters.

One highway in Arizona is trapped in the metric system

The true fun lies in working on vehicles and equipment built between the late 70’s and mid 90’s. The screwball American manufacturers couldn’t do it all them selves so anything bought from the world market is metric and the stuff we built continued in our single minded system. Glad the manufacturer finally realizes we live in a global market. Can’t wait for the day I can trash 1/2 my tools and keep only my metric “spanners”


Nobody has mentioned the ultimate juxtaposition of metric and imperial measure, ISO Metric tire code (i.e. P215/65R15).

And lets not forget highway I-19 in southern Arizona witch is unique among US Interstates, because signed distances are given in meters.


Yeah…The Metric System…Give me 2 kilos of ham and a 1/2 kilo of cheese…see that will never work…the more we say America First, the more something change to the so-called Global Market!!! it will always irk my as just as bad with cars with only letters and numbers in their name MKX, QX4, CT5 …Is it cheaper??? Lincoln Continental , Sedan Deville Fifth Avenue…Thats a lot of script to chrome plate and hang on the side of a fender…no matter what, we lose our image when we try to be like the rest…lets all stop after work for a liter of beer…WHAT!!!


Ah, the wonderful world of metric conversion.

How many plane crashes were the result of filling the fuel tank in litres and not gallons? Would that be US gallons or imperial gallons … here we go.

In Canada they tried converting the lumber. The result was that everything ended up too small and they dropped that plan.

In Canada we no longer measure mileage in miles per gallon … now it is now litres per 100 kilometers . How handy!

Let’s not forget about tire pressure. They tried converting PSI to kilopascals, so if you were targeting 32 PSI you would need thousands of kilopascals. They stopped that foolishness.

At this point I really don’t care as long as we are all having fun!


In airplane fuel do not they go by weigh?


There are countries that use metric and those that put a man on the moon. Think about it.


Plus the venerable 409


metric system is somewhat useless…

metric gasoline sales are another way to rip off a consumer, add 2 cents to a liter and its almost 8 cents per gallon, but the canadians don’t complain, because nobody complains about 2 cents.

the fahrenheit system is much more user friendly than celsius, try converting 70* and 71* to centigrade and count the decimals, to tell the difference…when you just want to know how warm outside it is.

whats easier to say, that’s a mile away? or 1.6 kilometers, how tall are you? 5 feet 10? or 1.778 meters?

I have always maintained Americans are smarter , with our system, because we know how to convert…the rest of the world has to guess.


A liter is 61 cubic inches…give or take a little bit. Don’t assume everyone doesn’t know.


A few years ago when my very “car guy” nephew was talking about his FRS and how fast that it was, I had to remind him that he was only running on about TWO liters and my '71 Road Runner was being pushed along with over SEVEN liters. The difference was pretty eye opening to him once he realized the comparison. After he drove it once he ALMOST converted over to the old school V-8 RWD camp!
I’ve always hated Imperial measure, especially when I was just starting out and I’d reach for the 1/2" wrench and find that it was too small, and then I’d be under the Plymouth doing MATH to figure out that 5/8" was really 10/16ths and that I needed the 9/16ths wrench because the 8/16ths wrench was too small. When I work on the Toyota and try a 9mm socket and it’s too small, I instantly reach for the 10mm socket. Easy!
I also bought a metric tape measure when I was in Europe expressly for homeowner chores like hanging pictures in the center of a wall… Ever try to figure out what HALF of 118 and 3/32" is in your head? With the metric tape measure I can easily figure out half of 300 cm in my head! Depends on the situation, I suppose!


@johnvukovichjr - Regarding your nephew and his FRS engine, that 2 liter makes 200hp. Your Road Runner (assuming stock, correct me if I’m wrong) makes around 400hp. If that FRS engine scaled up to similar displacement it would be 650-700hp. We have figured out volumetric efficiency these days, engines simply don’t need the massive displacement they used to.

I wholeheartedly agree with your socket statement though. Certainly easier when it’s not fractions!