The /r/ sound is spelled with <r>, <rr> or <wr> in wreck. The /ʧ/ sound is spelled with <ch> or <tch>.
This is a contrast between a voiced alveolar retroflex continuant and a voiceless alveolar affricate. It only ocurs initially or medially. Some of the medial pairs, such as marring/marching, only arise because of a linking -r introduced by the -ing inflection. It is not a problem.
Interesting pairs include:
redder Cheddar
terrier tetchier
The mean density value is 1.2%. The lists make 77 semantic distinctions, a loading of 52%.
Carrie catchy carrier catchier carries catches cowries couches error etcher errors etchers ferries fetches harries hatches hurries hutches marries matches marring marching parries patches parry patchy purring perching race chase raced chased races chases racing chasing racer chaser racers chasers raft chaffed rain chain rained chained raining chaining rains chains rein chain reined chained reining chaining reins chains reign chain reigned chained reigning chaining reigns chains raj charge rajah charger rajahs chargers ramp champ ramped champed ramping champing ramps champs range change ranged changed ranges changes ranging changing rap chap rapped chapped rapping chapping raps chaps rare chair raring chairing rat chat rats chats ratted chatted ratting chatting ratter chatter ratters chatters ratty chatty rattier chattier rattiest chattiest rattle chattel rattles chattels raw chaw raw chore reaper cheaper rear cheer reared cheered rearing cheering rears cheers redder Cheddar reef chief reefs chiefs reek cheek reeked cheeked reeks cheeks rest chest rests chests Rex Czechs Rex cheques Rex checks Rhine chine Rhyl chill rhyme chime rhymed chimed rhymes chimes rhyming chiming ride chide rides chides riding chiding riled child rink chink rinks chinks Rita cheetah rive chive rives chives roar chaw roars chaws roared chawed roar chore roars chores rock choc rocked chocked rocking chocking rocks chocks rood chewed rose chose rouse chows row chow rows chows ruck chuck rucked chucked rucking chucking rucks chucks rude chewed rue chew rues chews ruing chewing rued chewed rug chug rugs chugs rum chum rummy chummy rump chump rumps chumps ruse chews Ryde chide scorer scorcher scorers scorchers scoring scorching starry starchy starrier starchier starriest starchiest starring starching terrier tetchier Terry tetchy Tories torches wreck Czech wrecks Czechs wreck cheque wrecks cheques wreck check wrecks checks wrecked checked wrecking checking wrecker checker wreckers checkers writ chit writs chits
John Higgins, Shaftesbury, December 2010.