It looks like the calculation conversion is different/opposite for costs and mileage log.
For instance my main currency is CHF and I added the HRK with the exchange rate as: 0.14 (meaning 1HRK = 0.14CHF)
So if I add a cost of 150HRK for car wash it is correctly translated to 21CHF (done by 150*0.14)
However if now I do a fuel log of 700HRK I would get a price of 1071CHF (done by 150/0.14) which is wrong.
Conversely if I correct the exchange rate by adding HRK and putting 6.8 (meaning 1CHF = 7.8HRK)
I get the opposite problem (fuel is correct but car wash is wrong)
I'm not sure if this bug depends on a particular currency but I would assume it's just a wrog formula in Costs log or Fuel log)
Customer support service by UserEcho