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Old 07-25-2009, 06:38 PM   #1 (permalink)
eco....something or other
Join Date: Jul 2009
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wood hauler - '91 Ford F-250
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just tried out my new vac-gauge

at idle it reads 19-20 depending on alt load. At highway speeds it reads 12 inches in fourth gear and 17 inches in third gear. I would like to get some opinion of which gear i should cruise in because third will probably be easier for the engine, but fourth has lower revs. i just don't know if the lower revs are making up for the lack of vacuum. and i also realize that at lower revs the engine does not pull a high vacuum because it is not using that much air.
i know i will probably end up testing to find out anyway but i am wondering if anyone has been over this dilemma before.

oh and is 4 inches going uphill in fourth better than 7-8 in third?
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1991 F-250:
4.9L, Mazda 5 speed, 4.10 10.25" rear

Last edited by IsaacCarlson; 07-25-2009 at 07:26 PM..
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Old 07-26-2009, 10:54 AM   #2 (permalink)
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The Odyssey Van - '11 Honda Odyssey EX-L
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When I had my toyota tacoma I used a scan gauge that would read out real time air flow through the MAF meter to get an idea on my fuel being used. Take your air flow in lb/min and divide by 14.7 which will give you the lb/min of fuel being injected. I used 14.7 as this is the air/fuel ratio my o2 sensor tried to keep the mixture. I did this at 60mph so it was 1 mile per minute to make calculation easy. At 0 vac in o/d it was using roughly 5lb/min air so it was injecting .34lb/min or mile fuel. Gas is around 6lb/gallon from what I remember so I divided 6 by .34 and came up with 17.65 which will be my mpg at 60mph and 0 vac. You can do this at any speed, gear, rpm, vac, boost, conditions, ect as long as you can hold the vac/boost steady long enough to get a reading. Hope this helps.
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Old 07-26-2009, 02:09 PM   #3 (permalink)
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IsaacCarlson -

Cool beans. It's a 2 for 1 because the gauge will also help you "gauge" the health of the engine :



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