Impressed with my truck's mileage. - The 1947 - Present Chevrolet & GMC Truck Message Board Network

James McClure · 03-14-2011, 10:10 AM

I've posted my mileage quite a few times but I'll post it again and how I get it. Someone said something about Mythbusters and the bed cover thing. A bed cover does the same thing as leaving the gate off or open, it creates a suction at the back of the truck that costs MPG. Leave the gate on and shut and the suction is still there, BUT, it's right in the bed which fills the bed with a higher pressure and the air skips over it as you drive. It also seems that the gate replacement cargo nets help with MPG. Mythbusters also did a segment on drafting. I don't reccomend doing this though. I drove 18 wheelers for 10 years and NEVER liked anyone being real close to my back end. The ICC bumper on the back of the trailer ALWAYS wins!! (LOL) Anyway, if you can stay 100 feet behind a big rig your mileage goes up 20%. If you get within 50 feet it goes up to about 30 to 40%. In thier test they got down to 2 feet and mileage jumped almost 75%. Thats HUGE but extreamly dangerous, one "hickup" somewhere and your all done.
Now for my mileage. I have an 84 3/4 ton K20, 350 eng, 465 trans and 3.08 gears out back. I get 17MPG driving 55 to 60 on the flats. I set the eng up (timing) by ear & trial and error. Now timing is near 14 degrees. It will ping if I mash it or am pulling a grade loaded and I have to go 50% throttle or higher. I leaned the carb out (quad) with smaller jets and rods. I almost never engage the secondaries. I use the granny gear to get her moving just a bit. This does help and it saves strain and wear on the clutch. I changed it during the restoration, origional clutch over 400,000 miles. I don't use the clutch once I'm rolling. I coast long down grades and draft where ever possible. I also tow a 16 foot equipment trailer sometimes which drops mileage to about 15. For some reason, known only to God, the 15 MPG seems to hold wheather or not it's loaded, go figure. Cool weather helps as does rain. Drive your long hauls at night, sleep during the day. Less traffic, cooler temps & higher humidity. Use all synthetic lubes and keep tire pressure up to spec and be very soft on the throttle. Thats about it. Jim

tucsonjwt · 03-14-2011, 02:08 PM

I am guessing that a 3.08 rear axle makes a big difference in highway mpg. I know my 4.11 ratio is an mpg killer.

03-14-2011, 10:10 AM	#1
James McClure One shot, one kill. Join Date: Dec 2007 Location: Saratoga Springs NY Posts: 859	Re: Impressed with my truck's mileage. I've posted my mileage quite a few times but I'll post it again and how I get it. Someone said something about Mythbusters and the bed cover thing. A bed cover does the same thing as leaving the gate off or open, it creates a suction at the back of the truck that costs MPG. Leave the gate on and shut and the suction is still there, BUT, it's right in the bed which fills the bed with a higher pressure and the air skips over it as you drive. It also seems that the gate replacement cargo nets help with MPG. Mythbusters also did a segment on drafting. I don't reccomend doing this though. I drove 18 wheelers for 10 years and NEVER liked anyone being real close to my back end. The ICC bumper on the back of the trailer ALWAYS wins!! (LOL) Anyway, if you can stay 100 feet behind a big rig your mileage goes up 20%. If you get within 50 feet it goes up to about 30 to 40%. In thier test they got down to 2 feet and mileage jumped almost 75%. Thats HUGE but extreamly dangerous, one "hickup" somewhere and your all done. Now for my mileage. I have an 84 3/4 ton K20, 350 eng, 465 trans and 3.08 gears out back. I get 17MPG driving 55 to 60 on the flats. I set the eng up (timing) by ear & trial and error. Now timing is near 14 degrees. It will ping if I mash it or am pulling a grade loaded and I have to go 50% throttle or higher. I leaned the carb out (quad) with smaller jets and rods. I almost never engage the secondaries. I use the granny gear to get her moving just a bit. This does help and it saves strain and wear on the clutch. I changed it during the restoration, origional clutch over 400,000 miles. I don't use the clutch once I'm rolling. I coast long down grades and draft where ever possible. I also tow a 16 foot equipment trailer sometimes which drops mileage to about 15. For some reason, known only to God, the 15 MPG seems to hold wheather or not it's loaded, go figure. Cool weather helps as does rain. Drive your long hauls at night, sleep during the day. Less traffic, cooler temps & higher humidity. Use all synthetic lubes and keep tire pressure up to spec and be very soft on the throttle. Thats about it. Jim __________________ 1984 K20 350M engine with 465,000 miles. Well, it's finally done!! Almost 2 years of work, but it was really worth the effort. Little stuff left to do is mount winch, wet sand & buff out and build belly pan\running boards. Body work takes lots and LOTS of beer!!!! God, Guts and Guns made America and God, Guts and Guns will preserve it! The worst thing you could do is get into my sights, but that don't matter, you'll never know it, cause you'll never hear the shotJim or Paladin whichever you choose.

03-14-2011, 02:08 PM	#2
tucsonjwt Registered User Join Date: Jul 2007 Location: Tucson, AZ Posts: 3,198	Re: Impressed with my truck's mileage. I am guessing that a 3.08 rear axle makes a big difference in highway mpg. I know my 4.11 ratio is an mpg killer.



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