Thread: Thoughts on tyre drag.

While I'm sure all your theories will have some impact on fuel consumption, the main factor is the gearing difference.


With your average Land Rover, the engine will be working harder to maintain the speed with the higher gearing.
You might find some economy gains on nice flat freeways though.

Tread pattern influence is huge.
Go ride a bike with slick tyres and then with low pressure knobblies.

Easiest way to measure tyre drag change is to measure your coast down times and then again with Chunkier tyres.

That's interesting Dougal. Using the coast down test you could measure actual tread drag without the consideration of gearing and wind resistance would reduce as the vehicles slowed.

That would prove or disprove bigger tyres might roll easier but it would still leave the question is it the greater wind resistance or the gearing that causing the higher fuel cost.

I see how the higher gearing creating more enging load could cause more fuel consumption but I doubt it a little. The ratio change is not usually that great, maybe 10%. When we consider that most vehicles actually improve fuel consumption with higher ratios like the differance in driving in 4th or 5th gears. The differance in ratio for 4th and 5th gears are far more than the ratio change in up sizing tyres.

Some could argue the vehcile looses ratio and power for acceleration as well but that should only really effect first part of 1st gear from then on you will actually be in a lower gear for a given speed as you would hold a gear a little longer. For example if you previously changed to 2nd at 30km/h with the higher ratio provided by the bigger tyres, you'd change to 2nd at 35km/h. Meaning while the engine gets a little more load through most of the rev range, it gets it a little bit easier for the last part of the rev range.

And to top off the gear ratio thing. Most recommendations for driving economically say to short shift and rev the motor less to save fuel which indicates to me an engine will use less fuel with low revs and heavy load than high reves and low load. I know there is a bit of lose in engine friction but is this enough to acount for the extra fuel from bigger tyres.

Happy Days.

In my view, the major factors that affect rolling losses and hence fuel consumption are:-
1. the energy converted to heat in flexing the tyre - which almost entirely depends on the construction of the tyre and the rubber compound used, but is pretty much independent of tyre size.

2. Energy converted to heat and sound by the tread - which again, depends entirely on the tread pattern and composition, hence is independent of tyre size, at least within the bounds of practical sizes.

Tyre diameter affects effective gearing, but for most modern vehicles I would be surprised if it made much difference to fuel consumption, again within the limits of practical sizes. Same applies to the effects of aerodynamic drag, although this is not a simple calculation, as the turbulent airflow around the tyre will be affected by the proximity of the bodywork.

Tyre pressure is of course a major factor in fuel consumption, and another one, not often recognised, is that tyre tread width also affects consumption on anything except a straight road, as on a corner the edges of the tyre are necessarily either skidded on the road or the tread flexes (depending on the tyre properties and the surface), either process converting fuel energy to heat.

John

I'm not surprise the first 2 responses to the thread, one says drag from the tyre and the other says gearing.

Originally Posted by JDNSW

1. the energy converted to heat in flexing the tyre - which almost entirely depends on the construction of the tyre and the rubber compound used, but is pretty much independent of tyre size.

I have to disagree that it's independant of tyre size. As the diameter increases the angle the tread meets the road is reduced as such so is the flex of the tyre. So going up in size but sticking with the same model of tyre should reduce fuel consumption.

Originally Posted by JDNSW

2. Energy converted to heat and sound by the tread - which again, depends entirely on the tread pattern and composition, hence is independent of tyre size, at least within the bounds of practical sizes.

Again so if sticking with the same brand and model of tyre. Fuel consumption should stay the same at least.

Originally Posted by JDNSW

and another one, not often recognised, is that tyre tread width also affects consumption on anything except a straight road, as on a corner the edges of the tyre are necessarily either skidded on the road or the tread flexes (depending on the tyre properties and the surface), either process converting fuel energy to heat.

John

Here in the NT most roads are straight and I'd wonder how much time our vehicles actually spend turning.

I presume you are taking about slip angle?

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Everything points to bigger tyres being more fuel efficent but we all know that's just not the case so why?

Happy Days

I've been searching and reading for a while now and I am begining to be convinced that larger tyres will have less drag and going up in size has a far bigger effect on rolling resistance (reduced) than increasing tyre pressure.

Partly the reason fuel consumption goes up is down to the extra energy required to spin the heavier wheel but this is not relevant once desired speed is reached.

The main reason is wind resistance. Partly from the extra width of the tyre but grossly from lifting the vehicle up even only a small amount will greatly effect aero dynamics of the vehicle, create more wind drag and ultimatly cost more fuel.

I know from previous personal experience that lowering a vehicle, even slightly will reduce fuel consumption. I had a WRX that I lowered only 1/2" from standard and saved on average 0.5L/100km. I drove from Canberra to Bathurst plenty of times and it was immediatly obvious in my fuel consumption when I changed the suspension.

Still open to further comments and ideas.

Happy Days.

How are you calculating the distance traveled to get your figures.
Don't forget the odometer will read a different distance with the different tyres fitted