Gamespeeds are defined in "CIV5GameSpeeds.xml" and can be easily modded ... The xml-code is very much self-explanatory. Scaling of costs is defined in "GameSpeeds". Number of Turns to play is defined in "GameSpeed_Turns". (see code below)
For example "Marathon" speed allows 1.500 turns compared to 500 turns in standard speed. Costs for Research, population-growth, constructing buildings and units, etc. are tripled (= 300 % in xml) for Marathon. This seems to make no difference in game-play compared to standard speed since all actions just seem to take 3 times longer, e.g. building a warrior might cost 30 turns instead of 10 turns production. And it can become very boring in the beginning ....
Values which are not effected are yield-values (food, production, gold, science-production, culture-production), hitpoints of units and cities, combat-strength and movement-rates of units ...
Movement-rate of units is a main difference since units seem to move at triple speed in marathon compared to standard.
(Take the example above : Build a warrior, explore the map while building a second warrior and your warrior can walk/explore 30 turns in marathon compared to 10 turns in standard before 2nd warrior is ready.)
So marathon speed is best combined with a huge map or Giant World Map if your PC can handle it.
On marathon it is more difficult to correct mistakes since every construction takes 3 times longer, e.g. if you forgot to build 2 archers for defense, the enemy army has 3 times more turns to do damage (pillage) or eventually take your city. (Rush-costs are tripled, too.)
For an immersive play I prefer hybrid game-speeds (mod) where for marathon (1.500 turns) the costs for units and buildings are cheaper (150% - 200% ) but Research-costs are higher (400% - 500% ). Research-costs for the first technologies should be adjusted since it takes too many turns to get the basic techs. (Can be very boring.) In the long term your cities will be developing faster, you can build huge armies and you will have many many turns for every era / age, e.g. playing a couple of hundred turns in medevial age.
Code:
<GameSpeeds>
<Row>
<ID>0</ID>
<Type>GAMESPEED_MARATHON</Type>
<Description>TXT_KEY_GAMESPEED_MARATHON</Description>
<Help>TXT_KEY_GAMESPEED_MARATHON_HELP</Help>
<DealDuration>90</DealDuration>
<GrowthPercent>300</GrowthPercent>
<TrainPercent>300</TrainPercent>
<ConstructPercent>300</ConstructPercent>
<CreatePercent>300</CreatePercent>
<ResearchPercent>300</ResearchPercent>
<GoldPercent>300</GoldPercent>
<GoldGiftMod>67</GoldGiftMod>
<BuildPercent>300</BuildPercent>
<ImprovementPercent>300</ImprovementPercent>
<GreatPeoplePercent>300</GreatPeoplePercent>
<CulturePercent>300</CulturePercent>
<BarbPercent>400</BarbPercent>
<FeatureProductionPercent>300</FeatureProductionPercent>
<UnitDiscoverPercent>300</UnitDiscoverPercent>
<UnitHurryPercent>300</UnitHurryPercent>
<UnitTradePercent>300</UnitTradePercent>
<GoldenAgePercent>200</GoldenAgePercent>
<HurryPercent>100</HurryPercent>
<InflationPercent>10</InflationPercent>
<InflationOffset>-270</InflationOffset>
<VictoryDelayPercent>300</VictoryDelayPercent>
<IconAtlas>GAMESPEED_ATLAS</IconAtlas>
<PortraitIndex>0</PortraitIndex>
</Row>
...
</GameSpeeds>
<GameSpeed_Turns>
<Row>
<GameSpeedType>GAMESPEED_MARATHON</GameSpeedType>
<MonthIncrement>180</MonthIncrement>
<TurnsPerIncrement>100</TurnsPerIncrement>
</Row>
<Row>
<GameSpeedType>GAMESPEED_MARATHON</GameSpeedType>
<MonthIncrement>120</MonthIncrement>
<TurnsPerIncrement>300</TurnsPerIncrement>
</Row>
<Row>
<GameSpeedType>GAMESPEED_MARATHON</GameSpeedType>
<MonthIncrement>60</MonthIncrement>
<TurnsPerIncrement>170</TurnsPerIncrement>
</Row>
<Row>
<GameSpeedType>GAMESPEED_MARATHON</GameSpeedType>
<MonthIncrement>24</MonthIncrement>
<TurnsPerIncrement>201</TurnsPerIncrement>
</Row>
<Row>
<GameSpeedType>GAMESPEED_MARATHON</GameSpeedType>
<MonthIncrement>12</MonthIncrement>
<TurnsPerIncrement>129</TurnsPerIncrement>
</Row>
<Row>
<GameSpeedType>GAMESPEED_MARATHON</GameSpeedType>
<MonthIncrement>6</MonthIncrement>
<TurnsPerIncrement>180</TurnsPerIncrement>
</Row>
<Row>
<GameSpeedType>GAMESPEED_MARATHON</GameSpeedType>
<MonthIncrement>3</MonthIncrement>
<TurnsPerIncrement>264</TurnsPerIncrement>
</Row>
<Row>
<GameSpeedType>GAMESPEED_MARATHON</GameSpeedType>
<MonthIncrement>1</MonthIncrement>
<TurnsPerIncrement>156</TurnsPerIncrement>
</Row>
...
</GameSpeed_Turns>
