Slant 6 with 200r4?

hi, iam trying to do the math here. most o/d trannys have a .70 o/d ratio. so with 4:10 rear gears, that would give you a final drive of 287:1 i thought /6 cars had a 276:1 rear gear. so if this is right, you would be getting less mpg. the only advantage would be better takeoff in first and second because of the 4:10. i would put the low ratio gear set in the tranny and call it a day. feed back please.