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i have a 81 F-250 4x4 subercab with a 300 (bb I6 ha ha ),t-19 w/3.54 and 285/17-16 tires.i use this truck to haul my racer and i was trying to get the best milage out of it (the former 351m only got 8 mpg )it now gets over 10 and sometimes 12 if i really watch the dash mounted vaccum gauge.my question :what do you think the lowest vaccum reading should be under a loaded condition ,pulling race car and bed loaded with support equiptment approx 5500# plus truck weight ,on a flat road at 65 mph or so .it has a 6.5 power valve in the holley 390 now .it usally runs about 7-8 inches out on the interstate at 65 but if i get it up to 75 it goes down to 5.5 or 6 inches.what do you think ? has anyone ever tested this before ? i can give details on cam ,compression,timing and other details if needed.thanks in advance ron
 

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Depends mostly on your cam and timing. More timing and you will have more vacuum under a given load as you wil lgenerally need less throttle opening to maintain speed. If you're looking for mileage just keep the vacuum above your power valve rating.

Also an option may be leaning out your primaries as much as possible and then drilling the PVCR's larger as needed to compensate and maintain power for acceleration.
 
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