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going from 3.08 gears to 5.13 gears, MPG???

85mudblazin

85mudblazin

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I will be going from the 3.08 gears in my 10bs to the 5.13's in the 14bff, what can I expect my MPG to do?? The tires will get going easier so less gas but in the upper sppeds I might loose some, What do yall think???
Thanks
-Chris
 
really I would GAIN MPG?? that would be awesome /forums/images/graemlins/thumb.gif
 
I think around town would improve lots but on the highway gearcalc says you'd go from 1349RPM on the highway....pretty low to 2240RPM in OD. Hmmm that doesn't sound so bad afterall. I'd do it.
 
those calculators suck, at about 65 I am running at about 2000RPM right know, so I am thinking around 2500?? after the swap?? That would still be good, would have more power at the upper rpms.
 
It kinda makes sence and it kinda doesn't. I will be going from 3.08 w/ 33" BFG's to 4.1 w/ 35" BFG's. I get 8-10 MPG now, will my MPG's go UP when I change? I understood the reason a lot of our trucks/K5's got the high gears in the first place was for fuel economy? How can lower gears be better? Just 'cause the motor doesnt have to work so hard?Sorry for the hijack, any enlightenment would be appreciated.
 
about 85% of my driving is just around town so im not very concerned about highway mileage. If I could pick up a few in town that would be sweet, not to mention the put you in the back of the seat feeling /forums/images/graemlins/peace.gif
 
ya round town lower gears work better...cuz the engine doesnt have to work as hard...also yer tranny will like it better...i'd imagine with the 3.08 gears and 33" tires W/ an overdrive tranny...it probably would BARELY shift into OD...now it should and cruise pretty well /forums/images/graemlins/thumb.gif
 
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It kinda makes sence and it kinda doesn't. I will be going from 3.08 w/ 33" BFG's to 4.1 w/ 35" BFG's. I get 8-10 MPG now, will my MPG's go UP when I change? I understood the reason a lot of our trucks/K5's got the high gears in the first place was for fuel economy? How can lower gears be better? Just 'cause the motor doesnt have to work so hard?Sorry for the hijack, any enlightenment would be appreciated.

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Yes.

Everyone thinks that low rpms = fuel economy.

That's pretty much all wrong when it comes to trucks.

Want MPG? Maximize vacuum so you hardly have to touch the gas to make a truck go. My 4.10 geared, non-overdrive C30 with a well-worn TBI 350 gets 14 mpg empty even at 3,000+ RPM @ 65-70 MPH.
 
I didn't notice a significant change in MPG when I went from 3.08s and 32s to 5.13s and 38s. Never ran the 3.08s with the 38s though.
 
[ QUOTE ]

Everyone thinks that low rpms = fuel economy.

That's pretty much all wrong when it comes to trucks.


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That is not totally true. On my Burb I picked up 2mpg running it at lower rpm's on the highway. Grim gave me some advice on it and I tried it. Keeping the bigblock below 3k rpm's really makes a difference. I went from just under 10mpg to 12mpg. My old Jimmy when it was stock it had 350tbi/700r4/3.42's and 31's sucked gas and after the swap to 4:56's and 35's I got a lot better milage. Never checked the actual amount but I know a tank went quite a bit farther then before the swap.
 
And all of this MPG info is based on you speedometer/odometer. So if its off you will not be able to calculate the acutal miles driven, PERIOD.

I often wonder if everyone that complains of bad milieage or someone who is gettting "great" mileage, has their speedo dead on right.

The only real way to know how fast and how far is with a GPS unit.

Just my 02,
John
 
Don't you guys have measured miles out there?
Mine goes about .99 miles on the odo for every 1 mile in the real world. That means my 16MPG in my 1990 GMC F/S Jimmy 350TBI TH700R4 3.73 31x10.5x16's, is actually a little higher. /forums/images/graemlins/thumb.gif
 
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those calculators suck, at about 65 I am running at about 2000RPM right know, so I am thinking around 2500?? after the swap?? That would still be good, would have more power at the upper rpms.

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You are running 2,000 rpm at 65 mph, in OD, with 35" tires and 3.08 gears??????........that's seems like a lot of RPM for that comibination. Maybe if it was in 3rd gear and not OD /forums/images/graemlins/dunno.gif
 
[ QUOTE ]
That is not totally true. On my Burb I picked up 2mpg running it at lower rpm's on the highway. Grim gave me some advice on it and I tried it. Keeping the bigblock below 3k rpm's really makes a difference. I went from just under 10mpg to 12mpg.

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That sounds like a different set of circumstances. You didn't mention changing gears, so the only other way to lower rpms is to slow down......slower speed equals less wind resistance which equals better mileage.

In any case it's making the particuliar engine combination run in the correct rpm band in order to get the best mileage. If the rpm's are too low than the engine is lugging and working harder to keep the truck moving (equals more fuel), too high of an rpm and you are just pumping more fuel into the engine to let if spin that high though the extra power is not needed.
 
so basicly what you want is : high rpms in town (2-3 K)? low on highway (1500-2000)?

my mpg is not that good .. but with 2:73:s and 32"s i get ok on the highway 14-15 ,but hey with tbi and everything shouldnt 17-18 be possible going slow , 50-55 mph or so?
 
Basically what you want is low load, not low RPM necessarily. You need to be geared to an RPM range that will pull the truck down the road with minimal throttle input.
 
my understanding that the more turns the engine is making the more gas its using

IE...higher the RPM, higher the gas usage

keep rpms low, keep mpg low, rpm's high, mpg high
 
It's not that simple, the workload pays a factor as well.
 
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