In the 2026 engine ruleset there are some chassis prescriptions:
[Note: Chassis elements will be allowed in RV-PU volumes with the exception of:
• No part of the Survival Cell will be allowed behind Xpu=0
• A minimum width requirement for the Survival cell will mean that that the sides of the survival cell (radiator recesses) cannot enter RV-PU-ERS
• No bodywork visible from below will be allowed inside RV-PU-ICE (with an additional allowance for bodywork thickness)]
Presumably the latter limits tunnel height near the engine. Point 2 will limit the size of the fuel tank undercuts or 'radiator recesses.'