Editing Driving wheel (section)

==Diameter==
[[Image:MILW 261 drivers.jpg|thumb|300px|Half of the driving wheels of [[Milwaukee Road 261]]]]
[[Image:60163 Tornado wheel.JPG|thumb|left|One of six {{convert|6|ft|8|in|2|abbr=on}} driving wheels belonging to [[60163 Tornado|60163 ''Tornado'']]]]
Driving wheels are generally larger than [[leading wheel|leading]] or [[trailing wheel]]s. Since a conventional steam locomotive is directly driven, one of the few ways to '[[Gear ratio|gear]]' a locomotive for a particular performance goal is to size the driving wheels appropriately.<ref>{{cite book| page=[https://archive.org/details/electricrailway01greegoog/page/n594 586]| title=Electric Railway Handbook| first1=Albert S.| last1=Richey| first2=William C.| last2=Greenough| edition=first| publisher=McGraw-Hill Book Company| location=New York| year=1915| url=https://archive.org/details/electricrailway01greegoog| quote=locomotive driving wheel ratio.| via=Internet Archive}}</ref> Freight locomotives generally had driving wheels between {{convert|40|and|60|in|mm|0}} in diameter; dual-purpose locomotives generally between {{convert|60|and|70|in|mm|0}}, and passenger locomotives between {{convert|70|and|100|in|mm|0}} or so.

The driving wheels on express passenger locomotives have come down in diameter over the years, e.g. from {{convert|8|ft|1|in|mm|0|abbr=on}} on the [[GNR Stirling 4-2-2]] of 1870 to {{convert|6|ft|2|in|mm|0|abbr=on}} on the [[SR Merchant Navy Class]] of 1941. This is because improvements in valve design allowed for higher piston speeds.