[skybolt]

The following is based on:

const char overFlowStr[] PROGMEM = "999999";

  reserveQuantity = doCalculate(instantIdx, tFuelEcon); //instant fuel economy
  if (doCalculate(instantIdx, tFuelEcon) > 999999) {
    text::stringOut(devLogSerial, PSTR("999999\",\n")); //infinte fuel economy, good for overflow and reasonable for anything above 999,999 (current overflow is 4,294,967.295 or (2^32)-1), check decimals is 999999 999.999 vs 999999.000? 
  } else {
  doOutputNumberJSON(reserveQuantity, 3, PSTR("\",\n")); // instantaneous fuel economy    
  }