Thread: Tyre treads and sizes, gearing and fuel economy.

In my opinion, the biggest dent from putting on larger tyres, is the accuracy of your speedo and odometer. This makes knowing what your actual fuel economy is more problematic. If your odometer is out by 10% then so is your fuel economy, plus you need to be careful near radar speed traps. It makes comparisons concerning greater mass and more drag difficult.

You also have to remember that you can't go too far oversize with tyres without engineering approval - but I like your thinking, albeit an expensive way to save fuel (I know, there are other issues involved - GRIN).

Once you get away from town, and start travelling over 60kph, the increased height and tyre width will also increase the aerodynamic drag, with the amount of this increase very difficult to predict. But I would expect that at highway speeds this increase would be much greater than any saving from lowered rolling resistance, as aerodynamic drag increases as the square of the velocity, whereas rolling resistance actually decreases a little with speed as centrifugal force has the same effect as an increase in tyre pressure.

John

Jeep claim that the GC's air suspension lowering by 13mm above 100 kph decreases fuel consumption and some D3/D4 owners have commented that lowering their vehicle by 20mm to its high speed height also decreases fuel consumption.

Originally Posted by Graeme

Jeep claim that the GC's air suspension lowering by 13mm above 100 kph decreases fuel consumption and some D3/D4 owners have commented that lowering their vehicle by 20mm to its high speed height also decreases fuel consumption.

Yes, for most vehicles, and especially four wheel drives, aerodynamic drag will be very largely due to turbulent airflow between the vehicle and the road, and will increase rapidly with increases in the gap between the underbody and road. This is why racing cars almost always have very low clearance air dams under the front.

A very few cars have done what Citroen attempted (with some success) on the D-Series, to streamline the underside of the vehicle to reduce turbulence in this layer. And this would seem impossible for a four wheel drive with live axles and plenty of clearance round the wheels.

John

Interesting.

I know adding a short but reasonably thick fiberglass mobile phone antenna to a Subaru Outback I had years ago added about 0.5L per 100km to it's fuel consumption.

I wonder how much of mud tyre fuel consumption is due to the squirmy blocks of tread creating drag on the road and how much is due to the wind drag of the blocks with large gaps moving forward on the top of the tyre as it rolls forward.

Happy Day.

my 88 rangie gets smack on 20 mpg on the highway on the pirelli scorpion atr's i usually have fitted

my wife's disco has much chunkier atr's same size etc etc whose name escapes me at present.. maxxis i am fairly sure come to think of it

i tried swapping them over and did a run up to cotton tree

mileage on rangie went down to 17mpg

pirelli's went straight back on

her 300tdi can have them

JD is on the money with drag ratios.
Off topic a little but relevant.
Wheel spats on the old Cessna I used to fly would increase airspeed by 20kts for the same power setting as without spats.
However at lower speeds the difference was negligible.

Back to your thoughts, it is my opinion that the losses due to aerodynamic drag on a 4x4 with Barwork, accessories etc must be accepted and only when attempting speeds greater than 110 will it feature as a major contributor to fuel consumption increases.
The primary fuel consumption increases would be attributed to the combined weight of all additions the motor has to push rather than the drag produced.
Larger Wheel tyre combo and std ratios (corrected for speedo errors) will make a noticeable difference to fuel consumption.
However, a larger combo with gearing to return to similar or actual std config RPM will often mitigate increases in fuel usage.
I must return to to JD's post however and my opening remarks, there is a magic number usually around 100-110 ( particularly for a D2).
where all mods and accessories exacerbate induced drag and will impact economy