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Old 03-29-2011, 03:37 PM   #1
wiggins
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38'' Big daddies

My brother in law is looking at buying a 85' 1/2 ton Chevy stepside with a good running stock 350/350 combo. The truck is carrying 38'' A/t rites. The owner is not sure of the rear end size but says he get 13-14 mpg so I am thinking 373 gears. Will these gears hold up to everyday hwy driving with these tires. I am thinking maybe a 411?
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Old 03-29-2011, 03:43 PM   #2
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Re: 38'' Big daddies

65MPH=2200RPM with 3.73 gears. 4.10's would bring you to 2400RPM. I would consider 4.33's if I was to change gears.That is 2550RPM. All of this not counting a non-lockup transmission... But, I'd be inclined to ditch the balloons for something smaller if it was my street truck.
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Old 03-29-2011, 11:58 PM   #3
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Re: 38'' Big daddies

Couple of ?'s. 14 mpg with a 350/350 combo, 3.73's and 38's? Somebody might need to recheck the mileage number off of the calculator as I think the decimal point might not be far enough to the left. Reason being running 3.73's with a 350/sm465 (still 1:1 in 3rd) 3.73's and 35" tires and I'd be lucky to get 12 running downhill with a tailwind.

Driving with 38's and 3.73's means it's going to take a while to get up to speed. Fuel economy will stink because you don't have enough mechanical leverage to get those big meats rolling. Yes larger tires give an overdrive effect, but if you spend so much to get them spinning the savings from the overdrive effect is negated.

For it to drive close to stock, jumping up to 4.56's or 4.88's or higher numerically would help get the big tires going. If it's still getting 10mpg or better after changing axle ratios consider yourself lucky.
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Old 03-30-2011, 10:13 AM   #4
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Re: 38'' Big daddies

Yeah I questioned the mpg's as well and know the guy is telling him more like 12 but he thinks if the carb was "cleaned" it could get 14(hell of a salesman). I have a 1/2 ton 2wd that only gets 12 mpg with a mostly stock 350/350 combo w 373 gears. I am getting all the info from my Brother in law second hand, did I mention he is 21. I am not big into aftermarket 4x4 stuff but I knew enough to know that it all didnt add up and that was not going to be a good daily driver. Its a pretty nice looking truck, but not for what he needs. But like I said he is 21 and he says he's going to get it anyway. He just wants to look cool, I think!
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Old 04-01-2011, 07:03 AM   #5
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Re: 38'' Big daddies

I'd be more worried about stopping those meats with the under-rated braking system on the 1/2 ton axles more than the fuel mileage I was gonna get.
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Old 04-07-2011, 03:21 PM   #6
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Re: 38'' Big daddies

I have a 79 1/2 ton short bed with 1 ton axles running a 350 with sm465 and 38's. Gear ratio is 4.10. I would love to get 10 MPG out of it. No real way of knowing as my speedometer is way off. Nor do I want to know. But I do know is it ain't good. It is fun to ride 3rd gear though.
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Old 04-07-2011, 03:58 PM   #7
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Re: 38'' Big daddies

Well he bought that truck and drove it 2 hours home, says he got 12 mpg. Not sure if he realizes that his odometer is not reading correct w those tires though so who really knows what he's gettn. If the lil punk ever drives it up here to see me I'll post some pics. My wife has seen it and she says its pretty nice looking.
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Old 04-07-2011, 04:47 PM   #8
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Re: 38'' Big daddies

Quote:
Originally Posted by wiggins View Post
Well he bought that truck and drove it 2 hours home, says he got 12 mpg. Not sure if he realizes that his odometer is not reading correct w those tires though so who really knows what he's gettn. If the lil punk ever drives it up here to see me I'll post some pics. My wife has seen it and she says its pretty nice looking.


Remember with big tires, your calculation would be low. You could find the actual MPG by cross multiplication...



stock tire circumference .......... calculated MPG
___________________...... = _____________
38" tire circumference ............. actual MPG (or X)




stock circ. * "X" = 38" tire circ. * calc. MPG





............"X" ........... = (38" tire circ. * calc. MPG)
............................... ______________________
............................... stock circ.
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Last edited by propanemudtruck; 04-07-2011 at 04:49 PM.
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Old 04-07-2011, 10:30 PM   #9
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Re: 38'' Big daddies

Yea I doubt he's getting that great of mpg haha! I would defiantly look into bigger axle swap or very least ugrade the brakes!
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Old 04-07-2011, 10:48 PM   #10
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Re: 38'' Big daddies

hey thats great info, I have never seen this, thanks.


Quote:
Originally Posted by propanemudtruck View Post
Remember with big tires, your calculation would be low. You could find the actual MPG by cross multiplication...



stock tire circumference .......... calculated MPG
___________________...... = _____________
38" tire circumference ............. actual MPG (or X)




stock circ. * "X" = 38" tire circ. * calc. MPG





............"X" ........... = (38" tire circ. * calc. MPG)
............................... ______________________
............................... stock circ.
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Old 04-08-2011, 05:30 AM   #11
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Re: 38'' Big daddies

No problem. That calculation was a lot easier than keeping 1/2 ton axles in 1 piece with 38's!
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Old 04-08-2011, 10:26 AM   #12
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Re: 38'' Big daddies

Yeah I tried to tell him. I guess he will learn the hard way on this one. Thats fine though once he breaks it, I will get it from him bc he wont have the $ or the know how to fix it. lol
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