hello
i have a 86 f250 2wd with a 300/6 with 3.55s the best mpg i get is 10 .
i can get a three speed brownie for 400.00 including shafts and shifters .
i want better mpg is it worth the effort to put it in just to get 1-2 better mpg
thank you
lrd56

 

Hello

I am assuming that 3 spd. box is a Spicer 5531 or 5831. Not all these boxes have overdrive, it depends on what letter series they are. I have ratio charts for both series of boxes, but can't find them right now (bugger!) however I will continue the search. Parts (other that seals and bearings) are not available anymore. Some underdrive gears work as overdrive gears and so on, so keep your eyes peeled for spare boxes for future consideration. 10 m.p.g. is not great, how much does it weigh? Stock, my light duty '81 F-250 light duty w/ 4spd. and 3.55's gets 20 m.p.g. and the dual exhaust manifolds from an E.F.I. engine will only improve that. As far as putting the brownie in, it can only help, although depending on the overdrive ratio, 3.55 may not be the ultimate when you view the whole combination. Those gear boxes are strong, so if it's in good shape, and you keep the oil good, it should last indefinately.

Good luck
MetalZ

 

10mpg!!? When's the last time your rebuilt the carb and replaced filters on that thing?

 

the carb is a rebuilt .the filters are new.this brownie has .73 over.the truckweighs5050.the 300 block is a 78 irragation block the head has been rebuilt.the motor has been converted back to points and carb .i got the truck like this.i live in a small town in new mexico and used parts for 300/6s dont exist here 300/6s are mostly used for pump motors.my 71 f250 with a 390 with a holley 650 and 3.73s gets 14 in town and 16 highway.maybe i should park the 86 .
thank you
lrd56