Member Since: 28 Feb 2019
Location: Bucharest
Posts: 3
Size, weight, profile - which affects fuel economy the most?
My Disco would like, by default, 255/60/R18 tyres.
That's a diameter of 76.32 cm or about 30 inches.
Tyres of this size tend to weigh about 17 kg / 37.5 pounds (at least the stuff I've had on the car so far).
I replaced the tyres with 265/70/R17 muddies.
That's 80.28 cm / 31.6 inches and about 24 kg / 53 pounds.
Fuel consumption went from an average of 10 l / 100 km (standard sized winter or AT tyres) to about 13.5 l / 100km (the big, heavy, muddies).
I was thinking of dialling back a bit and replace the muddies with some BFG KO2s, 245/70/R17 (77.5 cm / 30.5 inches and 21.27 kg / 46.9 pounds). A less naughty profile, closer to standard size but...still significantly heavy. Mind you, that extra mass does not simply add up to the car mass...it has to be rotated, as well (additional energy that one would have to spend even if rotating the wheel while being stationary).
So...does anyone have some additional insight into this? Would the fuel economy get back closer to 10 l / 100 km or remain closer to the current 13.5?
Somehow I feel that "midpoint" would not be it (more likely to be biased to either end).
28th Feb 2019 12:22 pm
JonM
Member Since: 24 May 2012
Location: North Yorkshire
Posts: 1247
Tread pattern, width, rubber compound and pressure will all affect rolling resistance.
Weight will only make a difference during acceleration. Once at a steady speed, weight is irrelevant.
Diameter will make a significant difference to the l/100 km if you are using measurements from the car to calculate it. You've increased your tyre circumference by 5.2%. This means that your car will be measuring the difference between two points as 5.2% less even though you've used the same fuel to get there. So you'd see a 5.2% increase in indicated fuel consumption just from the size change alone (but the real fuel economy if you measured the real distance on the ground hasn't changed).MY2022 D5 HSE D300 - with extra nice bits added
MY2019 D5 HSE 3.0 SDV6 - sold to a dealer for a crazy price! - So good I bought another!
MY2012 D4 HSE 3.0 SDV6 - retired to a new home
Reduce rotational mass and you reduce the energy required to gain and lose speed.
Reduce width and you reduce friction and the interface between tyre and tarmac (assuming a similar tread profile and compound for both tyres)
Larger wheels with low profile tyres weigh more than smaller wheels with deeper profile tyres.
Follow the above, and it follows that higher profile, narrower tyres on smaller wheels will give better economy, as demonstrated by the choice of wheels offered on "eco" variants of other manufacturers' models.
There are obvious compromises in comfort and roadholding to consider, but if fuel economy is your goal, narrower, taller tyres are what you need.I know it's not considered "kind" to say no these days, but no. Just no, ok? And if it's not ok, still no.
28th Feb 2019 12:54 pm
DSL Keeper of the wheelie bin
Member Since: 11 May 2006
Location: Off again! :-)
Posts: 73081
Tyre choice and, more importantly, tyre temp. If nothing is binding or not working properly, all else is fluff.
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