FORD 2001 Ranger - Manual (Page 219)

If you do not use the proper fuel filler cap, excessive pressure or
vacuum in the fuel tank may damage the fuel system or cause
the fuel cap to disengage in a collision, which may result in possible
personal injury.
Choosing the right fuel
Use only UNLEADED FUEL. The use of leaded fuel is prohibited by law
and could damage your vehicle.
If your vehicle is a flexible fuel vehicle (FFV), use only UNLEADED
FUEL and ETHANOL(E85). The use of leaded fuel is prohibited by law
and could damage your vehicle.
Do not use fuel containing methanol. It can damage critical fuel system
components.
Your vehicle was not designed to use fuel or fuel additives with metallic
compounds, including manganese-based compounds containing MMT.
Repairs to correct the effects of using a fuel for which your vehicle was
not designed may not be covered by your warranty.
Octane recommendations
Do not be concerned if your engine
sometimes knocks lightly. However,
if it knocks heavily under most
driving conditions while you are
using fuel with the recommended
octane rating, see your dealer or a qualified service technician to prevent
any engine damage.
Unleaded Gasoline engines
Your vehicle is designed to use “Regular” unleaded gasoline with an
(R+M)/2 octane rating of 87. We do not recommend the use of gasolines
labeled as “Regular” that are sold with octane ratings of 86 or lower in
high altitude areas.
FFV engine (if equipped)
Your vehicle is designed to use Fuel Ethanol(E85), “Regular” unleaded
gasoline or any percentage of the two fuels combined.
87
(R+M)/2 METHOD
Maintenance and care
219