Small block gas mileage DD magnum/LA ?

I have a 97 ram with 5.2 and 87 Ramcharger with a fresh 88 roller cam 318. All stock rebuild except for a cast spreadbore and 650 Holley and HEI conversion. The ramcharger gets better fuel mileage than the 97 which has EFI and OD trans. Same gear ratios, tire size and 4wd. I dunno, but even before installing the rebuilt roller motor, the LA still got better mileage.