Thread: Perenial tyre size and economy questions

My experience from going to larger tyres:

29" (235) standard tyres to 33" (255) Cooper STs worsened economy around town by 1l/100km but improved it by 1l/100km on the hwy.

33" (255) Cooper ST to 33" (255) Cooper STT muds then worsened economy by 1l/100km everywhere.

33" (255) Cooper ST to 33" (285) GY MTR didn't make much difference

33" (285) GY MTR to 33" (10.5") Simex JT2 cost another 1l/100km everywhere.

I now use:
33" (285) Pirelli Scorpian ATR on the road
33" (285) GY MTR when touring / holiday 4WDing etc
34/35" (12") Simex JT2 when playing on tracks or in mud.

My opinion is that the biggest factor with consumption is rolling resistance generally, but gearing affects the stability of the consumption.

Originally Posted by gavinwibrow

When I fitted 265/75/16 tyres to my D2a TD5 auto (2" oversize and also a 35 mm lift), I was aware that fuel economy would suffer, but wanted diff clearance.
I went up by about 1L/100km plus, irrespective of driving type eg urban, country, and even in WA's flat areas could not improve on that on country highways, driving like a nanna.

So, what are the main contributors?
Obviously extra effort to turn larger diameter (and wider) tyres, possibly extra wind resistance from tyre width and being lifted?

I would have thought the slightly lower revs at 100 and 110 kmph would have compensated, but no, plus I now have to get to 89 kmph before I can get 4th lockup.

Comments, theories and even facts welcomed

Did you realise that when you put the bigger tyres on that your speedo was out so the odo is out as well.
All you calculations on a standard speedo will be up **** creek untill you correct your speedo reading

Forgot to mention that I fitted a speedo adjuster at the time, plus have a Nanocom Evo in permanent instrument mode with adjusted tyre ratio, and gps isn't bad either, so all kms etc are corrected/actual.

Rolling resistance, IMHO.

Going back in time to last Century, I noticed that Jaguar tended to go to taller tyres, whereas other makers favoured wider/lower aspect ratio.

I wonder if they knew something...

Originally Posted by superquag

Rolling resistance, IMHO.

Going back in time to last Century, I noticed that Jaguar tended to go to taller tyres, whereas other makers favoured wider/lower aspect ratio.

I wonder if they knew something...

I suspect not. No high performance car like Jaguar runs taller narrower tyres, they all have shifted to wide low profiles tyres. What is interesting though, is that the magical profile from some testing I saw was actually 50% and after that performance dropped off again. Not sure if that is still the case.

As for rolling resistance, I firmly believe based on my experiences that the biggest factor for this is the tread pattern.

for rolling resistance just look at what the bike riders use
the less rubber on the road the less resistance...

go and talk to a competent MG owner that does club trials,, ask to see his special fuel economy tyres, up the back of his shed under a tarp


I believe that covers all three requirements

As the other answers have indicated, it is not that simple! Let's look at the multiple factors involved.

Tyre overall diameter - changes gearing, slight effect on frontal area. But modern engines are not very sensitive to small gearing changes as far as economy goes.

Tyre width - no difference straight ahead, but drag on corners, slight increase in frontal area.

Tread type - significant influence on rolling resistance.

Rubber composition - significant influence on rolling resistance

Carcass construction - significant influence on rolling resistance

Tyre pressure - major influence on rolling resistance.

Carry on!

John