Contemporary concerns for sustainability have prompted a move away from fossil fuels, with hydrogen being a promising alternative. In the automotive field, Type III hydrogen tanks allow for high pressures to be achieved while being lightweight and small. Their size makes them particularly sensitive to small changes in inlet diameter, which is crucial to ensuring the strict regulatory requirements for internal tank temperatures are met. However, there remains a lack of understanding of the effect of inlet diameter on the internal temperature of Type III hydrogen tanks, needed for the next generation of gaseous hydrogen regulations for land vehicles. Consequently, this paper employs computational fluid dynamics to quantify the effect of the inlet diameter for values ranging from 5 mm to 15 mm on the temperature of Type III hydrogen tanks, of internal diameter 354 mm, to comply with current automotive regulations. Here, we show that (i) an increase in inlet diameter results in a monotonic increase in internal tank temperature; (ii) a linear interpolation between the mass flow rates investigated in this study may be employed to estimate the temperature at a given inlet diameter; and (iii) pre-cooling has an impactful effect and enables control of the internal tank temperature to avoid exceeding regulatory maximum temperature, irrelevant of inlet diameter. Lastly, we provide recommendations on analysing thermal results to ensure the safety of hydrogen tanks by design, with a particular emphasis on temperature hotspots forming upstream of the inlet. These results provide novel insights into the effect of inlet diameter and pre-cooling on the temperature of hydrogen tanks for automotive applications and inform their design to meet relevant regulations inherent to their filling. Moreover, these findings are anticipated to contribute to future regulatory development and the wider adoption of hydrogen as a sustainable fuel.