General 'rules of thumb'
about tire rolling resistance:
Higher air pressure reduces rolling resistance.
{some 'Hypermilers' go 20% over the maximum air pressure rating, which is set mostly by Legal Liability worries}
As the tread depth wears down,
rolling resistance goes down.
A nearly bald tread tire will have
60% to 70% of the original new thick-tread rolling resistance.
Tire rubber with tiny Silicon Carbide (SiC) particles mixed in,
instead of the cheap & traditional carbon black,
will have lower rolling resistance.
New synthetic rubber also rolls easier.
Simple 'rib' tread will have lower rolling resistance
than a blocky 'mudder' tread
but slicks have the lowest rolling resistance of all.
Narrow tread tires roll easier than wide tread for the same vehicle weight.
Larger tire diameters rolls easier than smaller diameter.
For normal 14 to 20 inch diameter wheels this effect is small,
smaller than the effects listed above,
but it was the reason for those ridiculous looking 'big front wheel/tiny rear wheel' bicycles
back around year 1900.
Notice the 'flat spot' at the bottom of a loaded tire where it meets the pavement?
The flexing of the rubber to make that flat spot is a major cause of tire rolling resistance.
To the tire, that flat spot is like trying to climb a small hill all the time, even on level roads.
Engineers think of tire rolling resistance
as like a zero-rolling-resistance tire
trying to climb a small percent grade hill.
A tire with a rolling resistance number rating of Crr = 1%,
is like a friction-less tire going up a 1% grade hill.
You can find the Crr of a real tire
by finding the downhill grade that the vehicle will just roll down without speeding up or slowing down.
You can also measure the tire rolling resistance yourself by measuring how many pounds of force it takes to keep the vehicle just barely moving once you get it started.
If a 4000 lb vehicle takes 40 pounds of push or pull to keep it moving at a steady speed,
the rolling resistance of the tires is
100% times (40/4000) = 1%
The steel wheels of railroad cars have dramatically lower rolling resistance than rubber tires.
A typical steel wheel has a rolling resistance of 0.1% to 0.2%
or ten times less than rubber.
Tire rolling resistance Crr is a useful approximation
but real tires vary with speed.
See this graph,
and note that when doing a top speed calculation you need to use a higher Crr than what works at 70 mph:
http://www.us.pirelli.com/web/technology/about-tyres/tyre-function/rolling-resistance/default.pageanother post on Moparts
on tire rolling resistance and MPG of pickup trucks:
https://board.moparts.org/ubbthreads/show...e=1#Post6493537your tax $
paid for this large report on tires and rolling resistance:
http://onlinepubs.trb.org/onlinepubs/sr/sr286.pdf"Rust never sleeps
and Friction never fails to pick your pocket"
