Box Truck purchase. Any real fuel savings on FWD

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So looking to purchase SRW(single rear wheel) chevy with 12 foot sartin box
I have 2 of them already
Florida AC on 100%, bit of weight, average 11.5 MPG

Will new style Transit or dodge/fiat? cab chassis with 12 foot box get much better MPG?

ANother to consider is any there reliability

Thanks for any feedback