[Nonfractal]

Yes...if someone could decode tyre sizes and insert sizes, and which part of the wheel their diameters reference, that would assist me greatly!

Slot-it Part numbers represent a combination of DWG (sizing) and compound. 
Eg PPT1219F22. (Policar mold 1219 compound f22)

Sizing is OuterDiameter x TyreWidth in millimetres.
The DWG in parenthesis below is the the tyre mold used in manufacture and fixes the unfitted tyre size.
NThe DWG does not represent the material used. I'll list that further below.

14.2 X 8 (1096)
14.6X 8.4 (1214)
16.1 X 8 (1088) 
17.2 X 9.5 (1159) 
17.8 X 9.5 (1228)
19.8 X 9.8 (1140) 
16.5 X 10 (1067) 
17 X 10 (1170) 
17.8 X 10.5 (1167)
18 X 10 (1207) 
18.5 X 10 (1045) 
18.8 X 10 (1027)
19.5 X 10 (1171 & 1121) 
20 X 10.5 (1120) 
20.5 x 11.7 (Policar 1219 slick)
19 X 10.5 (1152) 
19.9 X 10.5 (1046) 
20 X 11.3 (1172) 
19.8 X 12.2 (1028) 
20.4 X 12.4 (1068 from the packaging but 1608 on the sidewall.

Sizing inner diameter:
Typically manufacturers list their wheels by the largest diameter (the bead) which is not the same as the rim diameter (visible once the tyre is fitted)
Slot.it do not list inner diameters
The non "F1" tyres seem to fit wheels on 15mm (bead measurement) 13.5 mm (rim) without stretching.
These tyres will easily stretch to fit a "non air hub" rim of 15.5mm but deform badly if fitted to an air hub of similar size, sinking into the air well and ruining the contact patch. The tyres can also be stretched further with gentle warming.

The "F1" tyres have the same inner rim diameter (12mm) but are a lot less stretchable and will only fit hubs listed as "F1" type.
The "F1" tyres will fit "Formula1" hubs from slot-it, policar, NSR and and All-Slot-Car.

Unfitted tyre Diameters taken from actual tyre digital calliper measurements:
1068/1608 has an inner rim diameter of 12mm
1170-E1 has an inner rim diameter of 12mm
1207-f22 has an inner rim diameter of 12mm

Compound type
All slot.IT compounds are "rubber" unless stated, eg "S1" and "sponge". 
Slot-IT does not make Urethane tyres.
The single letter refers to the compound used

Compound hardness
In some cases, The number is the durometer measurement, in others , lower number is harder, bigger numbers are softer.
Slot.IT have moved from the "1 to 4 " numbers to durometer readings, then back again but it's not possible to know why this would be.

Compound, hardness and descriptions taken from various sources
- P1 hard for smooth plastic tracks - Discontinued
- P2 Medium for smooth plastic tracks - Discontinued
- P3 Soft, for use on smooth plastic track
- P4 Anti-wear for use on abrasive plastic track (Ninco)
- P5
- P6
- S1 (Silicone) smooth dust free track
- Z series tyres (Low Grip), for front wheels
- C1 
- F15 
- F22 DiSCA homologated tyre for endurance events.
- F35
- E1 "harder but grip very well on Ninco (track)" (Maurizio Ferrrari 2012)
- N18 DiSCA homologated tyre for endurance events.
- N22 "A racing compound which from our testing works extremely well at least on Ninco, wood, and that can be glued and trued equally as well" MF 2012)

"For the F and N series, the number is the shore hardness" (MF 2012) 
All of the different tyres will fit on any of the hubs which Slot.it produce, except the "f1" tyres which only fit the f1wheels.

Alan