Editing OO gauge (section)

== Scaling and accuracy ==
[[File:OO and HO size comparison 1.jpg|thumb|OO scale (left) and HO scale (right) models of the [[EMD Class 66]], showing the size difference.]]
[[File:Parkside Dundas Diagram 100_hopper.png|thumb|OO scale permits a level of detail including [[weathering]] and individual numbering.]]

{{RailGauge|16.5 mm}} gauge at 4&nbsp;mm:1 foot means that the scale gauge represents {{convert|4|ft|1+1/2|in|mm|0|abbr=on}}, {{convert|7|in|mm|0}} narrower than the prototype {{RailGauge|ussg}}. This difference is particularly noticeable when looking along the track. As the market for proprietary track is mostly for HO scale, sleeper size and spacing are designed for HO and are therefore underscale.

OO is also used to represent the {{RailGauge|5ft3in}} Irish gauge, where it is a scale {{convert|13+1/2|in|mm|0}} too narrow.

Though they run on the same track, OO gauge and HO gauge models of the same prototype do not sit well together since the OO models are larger than the HO equivalent.

These differences have led to the development of the [[finescale standards]] of EM gauge and [[Protofour|P4 standards]]. Nevertheless, it is possible to model using OO to standards that fall just short of finescale.

In common with most practical model railways of any scale (and not related to the OO gauge inaccuracy) the following compromises are made: Curves are often sharper than the prototype, and often not transitioned, particularly when using "set-track" systems (radius 1 = 371&nbsp;mm, 2 = 438&nbsp;mm, 3 = 505&nbsp;mm, 4 = 571.5&nbsp;mm). Overhang from long vehicles means that the normal separation between track centres are overscale to prevent collisions on curves between stock on adjacent lines, at up to 65&nbsp;mm (for set-track (reduced down to 50&nbsp;mm for Peco Streamline)). Overscale wheel width and deep wheel flanges are used on typical models (but particularly older models), and these require overscale rail profile and much larger clearances on [[Railroad switch|pointwork]] than is prototypical. Pointwork is often compressed in length to save space.