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04-27-2015, 01:14 PM | #1 |
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Join Date: Oct 2013
Location: Lompoc Ca,
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Lowest most efficient low load rpm w/ 350
I'm in no hurry usually so I run @ 60 most times, lots on 2 lane roads, plenty of time & shortage of $$$.
I've got 308's and 33"ers, with a mild build 350 & SM465, rolling on flat ground most times. This calcs out to approx 1900rpm, with 35's it drops to 1774. Is this too low, or will it get less thirsty down there? I'm considering 35's for longer freeway legs if in a hurry, and more clearance under the diffs.......Thankyou...... Steve Rieck |
04-27-2015, 08:46 PM | #2 |
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Location: Visalia, Ca
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Re: Lowest most efficient low load rpm w/ 350
I would think it would start using more fuel at that point if you want 35s and you like the rpms or how it drives now I would jump up to 3.43 gears they are pretty common in late 80s blazers. I personally run 3.73 with 33s and for what I do they are not low enough 3.90 would be ideal but I will likely go with 4.11 when I jump up to 35s
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<Brandon> 1974 Chevrolet K5 Blazer 350sbc/SM465/NP205, dana44 front, corp 14bolt rear with 4.10s and a powertrax locker, custom rear bumper 400sbc in the works. 1981 GMC C1500 stepside 250i6, 3spd, 12bolt with 3.08s 2.5/4drop on 15x10 with 275/50r15 81 GMC C3500 cab-chassis dually with utility bed 350sbc, Sm465, 14bolt with 4.10s nv4500 getting ready to go in 1979 Chevrolet K5 R.I.P. http://67-72chevytrucks.com/vboard/s...d.php?t=434096 1976 Chevrolet C10 R.I.P. http://67-72chevytrucks.com/vboard/s...d.php?t=434195 |
04-27-2015, 09:04 PM | #3 |
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Re: Lowest most efficient low load rpm w/ 350
I know it's apples and watermelons, but my 06 Silverado with the 5.3 is turning about 1550 at 60 mph. I had an 01 with the same engine but 4.10 gears that turned 1850 at the same speed. I have not noticed a jump in fuel economy since getting the current truck.
My '78 with SM465, 3.73 gears and 35" tires runs right about 2400 at 60 mph. I would be very happy with your 1900 at 60 if I were looking for better compromise of fuel economy and power. But better power lays my hair back in my circumstances so my truck running as it does is OK by me. I would just be careful of getting your gearing too high though, you can end up lugging it to the point it does suck more fuel and sacrificing alot of giddy-up-go to boot. In other words, you can end up stuffing your foot deeper in the 4bbl to maintain highway speed taking your MPG's the other way.....
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04-27-2015, 10:54 PM | #4 |
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Re: Lowest most efficient low load rpm w/ 350
Yeah Im not sure what rpms I'm turning my tach has been getting more and more off but I often tow or drive on winding mountain roads with heavy uphill grades so I don't need top speed my truck rarely sees 50mph when I'm in the mountain it will do 110mph on a good stretch of road but all the vehicles I usually end up racing or ****ing with are buddy's and most of them top out at 80-90mph so I could stand to gear it a little lower it would only make me loose them quicker
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<Brandon> 1974 Chevrolet K5 Blazer 350sbc/SM465/NP205, dana44 front, corp 14bolt rear with 4.10s and a powertrax locker, custom rear bumper 400sbc in the works. 1981 GMC C1500 stepside 250i6, 3spd, 12bolt with 3.08s 2.5/4drop on 15x10 with 275/50r15 81 GMC C3500 cab-chassis dually with utility bed 350sbc, Sm465, 14bolt with 4.10s nv4500 getting ready to go in 1979 Chevrolet K5 R.I.P. http://67-72chevytrucks.com/vboard/s...d.php?t=434096 1976 Chevrolet C10 R.I.P. http://67-72chevytrucks.com/vboard/s...d.php?t=434195 |
04-28-2015, 07:22 AM | #5 |
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Re: Lowest most efficient low load rpm w/ 350
All I know is I had a '90 Blazer 350/4spd/3.08s with 33/12.50s and it rolled on down the hiway just great. Never felt the lack of overdrive. We could roll right along effortlessly with interstate traffic. It did fine at 75, although I also prefer 70 or less. The gears felt a bit tall for starting off, but there's always granny when you're starting off on a hill or with a load. I think 33s are as tall as I would go with 3.08s. Without any calculating you will have something like 2.73s which is awful lanky for a truck as it is. With 35s it will be even worse. You'll have Bonneville gears. I guess that works for flatlands ok, but the only gain would be rolling faster because at around 65-70 I think it won't be into it's sweet spot yet.
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"BUILDING A BETTER WAY TO SERVE THE USA"......67/72......"The New Breed" GMC '67 C1500 Wideside Super Custom SWB: 327/M22/3.42 posi.........."The '67" (project) GMC '72 K2500 Wideside Sierra Custom Camper: 350/TH350/4.10 Power-Lok..."The '72" (rolling) Tim "Don't call me a redneck. I'm a rough cut country gentleman" R.I.P. ~ East Side Low Life ~ El Jay ~ 72BLUZ ~ Fasteddie69 ~ Ron586 ~ 67ChevyRedneck ~ Grumpy Old Man ~ |
04-28-2015, 09:20 AM | #6 |
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Re: Lowest most efficient low load rpm w/ 350
After looking at the overall picture, I think when I get the correct cam in this thing, that 33's with the 3.08's and 1900 rpm seem pretty sweet for right now. It is already a little high geared for 4 wheelin', but that will be fixed when I put the doubler in eventually as funds permit. Thank you all for your input & help...............Steve
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70 Custom K-10 LWB w/ mild build 350 Sm465 & 205 +GM truck 12 bolt rear Dana 44 up front....3.08 ratio 33" 12.50 15 on 8" rims 46" front leafs 52" rear leafs |
04-28-2015, 10:54 AM | #7 |
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Re: Lowest most efficient low load rpm w/ 350
Hey Steve, install a vacuum gauge in the cab and drive by that. The higher the vacuum signal while cruising the more timing advance the can will offer, providing better economy.
I have a 5 speed in my truck and at 60 I usually get better economy in 4th gear because it allows the motor to stay in it's power band better and maintain a higher vacuum signal. If i grab 5th, i lower my rpm too much which makes more load on the motor, creating less vacuum signal, then the can starts dropping out vacuum timing advance, thus, there goes economy and responsiveness.
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Steve 1971 GMC Longbox. Full Roller Stroker 489 w/ FiTech EFI, Chris Straub Cam/NV4500/205/D60/14B w. Grizzly Locker and 4.11's. 2" Lift on 33's |
04-29-2015, 07:34 AM | #8 |
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Re: Lowest most efficient low load rpm w/ 350
A vacuum gauge is a great suggestion. I have one in my '72 also. That tells the true story
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"BUILDING A BETTER WAY TO SERVE THE USA"......67/72......"The New Breed" GMC '67 C1500 Wideside Super Custom SWB: 327/M22/3.42 posi.........."The '67" (project) GMC '72 K2500 Wideside Sierra Custom Camper: 350/TH350/4.10 Power-Lok..."The '72" (rolling) Tim "Don't call me a redneck. I'm a rough cut country gentleman" R.I.P. ~ East Side Low Life ~ El Jay ~ 72BLUZ ~ Fasteddie69 ~ Ron586 ~ 67ChevyRedneck ~ Grumpy Old Man ~ |
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