What's the deal with the quoted mpg figures?

Posted by IronSkywalker@reddit | askcarguys | View on Reddit | 11 comments

My car has a claimed mpg of 80mpg extra-urban. I did a 200 mile motorway drive yesterday and averaged 53mpg. Why such a huge difference? I get that those figures are done under test conditions e.g., on a rolling road with no air resistance, inclines etc. But do they also expect us to drive at 50mph on the motorway all the time to get the actual numbers?

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11 Comments

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jrileyy229@reddit

The test is in the real world, but they're done to maximize results. So in the real world you may be going 80mph... But the engineers know that 55mph is the ideal for maximizing MPG, so they'll use that.

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Concrete_Grapes@reddit

If you did 200 miles on a motorway, you were not driving extra-urban (which would be a mix of road speeds, generally medium paced, probably not over 50mph). It's not city, and it's not high speed highway, it's a mix. Think about it like a 'country drive'... that's what that 80's for i'd imagine. Our honda says it gets 37mpg highway, and, it does. However, if i drive 'the back way' on the country roads that never get above 55mph, it easily gets 44mpg. That would be the 'extraurban' type of driving. city/extraurban/highway, it's types.

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IronSkywalker@reddit (OP)

In the UK it is urban/extra urban/combined. So the assumption is it is either motorways or dual carriageways which have a national limit of 70mph, sometimes 60

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Concrete_Grapes@reddit

Ah, google says that they're testing and fuel economy rating for extra-urban is done at an average of just 39mph. Anything above 45 is going to significantly impact your mpg, so if you were cruising at 70mph, you're getting anywhere from 15-20% (depending on make) less than you would at even 50. And they're testing 11mph slower than THAT. Also, the testing is "conducted with all ancillary loads turned off, such as air conditioning compressor and fan, lights, heated rear window, etc." JUST having AC on, can lower some small engine cars mpg (and yours i think would qualify), by 25%.

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IronSkywalker@reddit (OP)

39mph?! That's some bullshit haha. Thanks for clearing that up for me

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GOOSEBOY78@reddit

no. manufacturers figures are ususally set on a lab on a test track in conditions that could never be acheived no matter how carefully you drive. also some cases marketing BS

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CarbonReflections@reddit

Those numbers are federally regulated and if found not to be possible can get the car manufacturer in a lot of trouble.

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soapy_goatherd@reddit

Just anecdotal, but my non-hybrid 2019 camry is listed at 29/41 - I do mostly highway driving, but have never come back from a trip under 30, and can push 45 without too much trouble if traffic’s light. So yeah, definitely not “marketing bs” in my case

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petergaskin814@reddit

The quoted mpg figures can only be used as a guide or comparison to other brand vehicles. Sounds like you have a PHEV if you are quoted 80mpg. Was your battery fully charged at the start of your journey? Within 60 miles, you would have been running on gas engine only. PHEVS gives the best economy in cities and while relying on Electric motor only. Their consumption tends to increase when you drive on a .motorway

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IronSkywalker@reddit (OP)

Nah it's not a phev, it's a diesel. But it is Italian so the figures could just be totally made up

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Buci__1@reddit

those are laboratory figures.