38'' Big daddies - The 1947 - Present Chevrolet & GMC Truck Message Board Network

wiggins · 03-29-2011, 03:37 PM

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?

Altec · 03-29-2011, 03:43 PM

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.

**Zoomad75** · 03-29-2011, 11:58 PM

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.

wiggins · 03-30-2011, 10:13 AM

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!

vtblazer · 04-01-2011, 07:03 AM

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.

liftlawssuck · 04-07-2011, 03:21 PM

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.

wiggins · 04-07-2011, 03:58 PM

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.

propanemudtruck · 04-07-2011, 04:47 PM

Quote:

Originally Posted by wiggins

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.

Hoods69BadBowTie · 04-07-2011, 10:30 PM

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!
Posted via Mobile Device

wiggins · 04-07-2011, 10:48 PM

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

Quote:

Originally Posted by propanemudtruck

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.

propanemudtruck · 04-08-2011, 05:30 AM

No problem. That calculation was a lot easier than keeping 1/2 ton axles in 1 piece with 38's!

wiggins · 04-08-2011, 10:26 AM

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

03-29-2011, 03:37 PM	#1
wiggins Registered User Join Date: Mar 2010 Location: Deep East TEXAS Posts: 453	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? __________________ Preserving American History one truck at a time.

03-29-2011, 03:43 PM	#2
Altec Getcha Pull Join Date: Nov 2010 Location: MD Posts: 691	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. __________________ '81 K10 454/SM465 '86 S10 Blazer '95 H-D Softail '96 Integra

03-29-2011, 11:58 PM	#3
Zoomad75 K5Camper Join Date: Mar 2004 Location: Pueblo, CO Posts: 1,513	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. __________________ Rob Z. 1975 K5 350/465/205/D44/12b 4" lift on 35's- RIP 1991 K5 8.1L/NV4500/241/D44/14b FWC Camper

03-30-2011, 10:13 AM	#4
wiggins Registered User Join Date: Mar 2010 Location: Deep East TEXAS Posts: 453	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! __________________ Preserving American History one truck at a time.

04-01-2011, 07:03 AM	#5
vtblazer Registered User Join Date: Oct 2003 Location: Central Vermont Posts: 8,537	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.

04-07-2011, 03:21 PM	#6
liftlawssuck Registered User Join Date: Mar 2011 Location: MA Posts: 353	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.

04-07-2011, 03:58 PM	#7
wiggins Registered User Join Date: Mar 2010 Location: Deep East TEXAS Posts: 453	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. __________________ Preserving American History one truck at a time.

04-07-2011, 10:30 PM	#9
Hoods69BadBowTie Chevys Kick A$$ Join Date: Jan 2005 Location: Zootown, MT Posts: 12,699	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! Posted via Mobile Device __________________ HOODS is what I answer too -'79, '77, '88 Pickup Sold, '85 Camaro, '83 T/A, '81,'83,'90 K5 Blazer All Sold -'79 3/4 ton "Big Yellow Bananna" Lifted 4spd. 39.5 TSL Swampers, The money Pit -'86 K5 Blazer Silverado 6.2 4" lift 35"s -'95 Ext. Cab Shorty 4" Tuff Country rolling 35" M/T's -'83 Monte Carlo T-Tops. 126,500 Original Miles -LATER I would rather push a Chevy then drive a ford!!

04-08-2011, 05:30 AM	#11
propanemudtruck Local Redneck Authority Join Date: Mar 2009 Location: Grand Rapids, MI Posts: 584	Re: 38'' Big daddies No problem. That calculation was a lot easier than keeping 1/2 ton axles in 1 piece with 38's! __________________ -Kevin four on the floor feel the beat in your soul movin to the rhythym pumpin til ya cant take no more dance if you want to do what you came to you cant ignore the feeling of that four on the floor

04-08-2011, 10:26 AM	#12
wiggins Registered User Join Date: Mar 2010 Location: Deep East TEXAS Posts: 453	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 __________________ Preserving American History one truck at a time. Last edited by wiggins; 04-08-2011 at 10:26 AM.

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