I would gladly sacrifice an under-bed spare tire to have more fuel capacity. I wish somebody would work on that. Would be sweet if it could be set up to transfer fuel to the main tank with an electric pump via an Aux switch in the cab. That way the fuel gauge would remain accurate, and you wouldn't have to stop and get out to use the extra fuel.40 gal aux is huge imo; I would be very happy with another 18 gal fit into the spare tire space, even 15 gal if that was all that would fit without exhaust mods.
just found this one...I sen TITAN Gladiator Sidekick Tank (5060015) (titanfueltanks.com) t Titan Fuel tanks an email on if they had fitted the 40 gal to a Gladiator. They sent me a note back today they they are fit testing one this week. So we will find out how it fits, and if it dose.
This is the tank they are installing.
https://titanfueltanks.com/products...ekker-40-gallon-auxiliary-fuel-system-5410040