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Discussion Starter #1
First of all I am a newbie to this forum. I just ordered the fuel filter kit. Its like Xmas waiting for it to arrive. Ive been reading all the install tips and polishing my tools :)

What I want to ask is this. Do bigger diameter tires gear the truck up so it gets better mpg on the freeway. I would also like to go a little faster. I get passed by big rigs on long trips. I play it safe and keep it 1K rpm's under red line which is 72 miles an hour. My truck is new and has 8k miles on it. I bought a 03 duramax/alison 5 spd 4x4. I got the zero financing and extended warranty from gmc.

I must say I love this truck. It always gets looks of envy from everyone who owns a gas pickup. I put a snugtop on it for winter camping. I have a 800w inverter, coffee pot, heater, flourescent light, laptop and a 4 inch memory foam sleeping pad (heaven).

 

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Problemchild,

Welcome aboard. Whether larger tires will benefit mileage depends on a number of things. The best mileage will be obtained at a speed where the power the engine is developing matches the truck's need for it. In some cases a taller gear (lower numerical ration) will allow the engine to turn at a more efficient rpm for the desired speed and better mileage will result. Larger tires might provide that benefit, though at some, perhaps minor, expense of low end grunt for towing heavy loads. It depends on your actual usage.
If you are concerned about the big trucks passing you the cure is as close as your right foot. You do not need to leave 1,000rpm cushion below the Red line. These engines will run happily at 80mph, and faster, all day long if you are inclined. The last time I drove down to Southern California, I intended to "cruise" on down at a leisurely 75, but at that speed I was a hazard to navigation, so I upped it to 80 and annoyed fewer people. I got 19mpg for my efforts. Go ahead and press the pedal a little harder. You'll love the truck even more.
TC Edited by: Tsckey
 

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I found that with my truck a taller tired did help the mpg. I drive 47 miles each way to work when I am in the states and most of it is freeway. The speed limit on most of my trip is 75mph so, I usually cruise closer to 85mph. I put a set of Cooper 255/85/R16's on my truck and found that it improved my milage about 1 to 1 1/2 mpg. I also tow 8000lbs of rockbuggy and trailer around frequently and did notice a little bottom end grunt loss. I run the 6 speed so, what that meant to me was that I had to use 1st gear more often.
One of the real reasons I went to the taller tire was pulling through the mountains. I was pulling Vail pass at 65 mph in 5th gear but, that was close to redline. I hit readline on stock tires in 5th at about 72mph. With the new tires, I can pull the pass at 75mph and not worry about redline. There seems to be no end to this trucks power. Edited by: camp
 

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265 BFG's gave me better fuel mileage and not much loss of power when towing. 285's gave me worse fuel mileage and a PITA to tow with. Juice may be on the way so hopefully it will help with the 285's.Edited by: neverenuf
 

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With the engine set at factory settings, 305's reduced my MPG by 2-3 MPG. However, MPG's have gone back up with the Juice and/or Predator.


neverenuf: the Juice will make you forget that you have 285's.
 

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I don't get it! I can run 70 all day long at 2K RPM, are you keeping it in 5th or driving in 4th?? Dumb question I know, but what else can it be??


Or maybe you have the 4:11 rear gear. Mine is 3:73 or something close to that.


OH Well......
 

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Chevysrus said:
I don't get it! I can run 70 all day long at 2K RPM, are you keeping it in 5th or driving in 4th?? Dumb question I know, but what else can it be??


Or maybe you have the 4:11 rear gear. Mine is 3:73 or something close to that.


OH Well......

He has the handshaker.
 

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Don't know if it's a good comparison from stick vs. Alli but, I (6spd) don't see a difference in mileage with my 265 BFG's. It might be 1/2mpg better, but pretty close. I do however feel better knowing all the while I'm turning 3.9% less RPM's at any given speed. And since looking good directly relates to feeling good, I know my truck feels better after getting rid of the factory skateboard wheels.



All dmax equipped trucks were 3.73. Do they now have a different option?
 

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What Mackey said. 265's didn't change my mileage a bit.

Fundamentally, it takes a fixed amount of power to move a given vehicle a given speed. Power is torque times rpm, so when you gear taller and reduce the engine rpm, you have to increase torque an equal amount to get back to the same level of power. You increase torque by pressing the throttle farther. So even though you've lowered rpm, burning fuel less often, you're forced to burn a little more fuel with each revolution, there's no free lunch.

That said, the engine will be most efficient at a certain rpm, by that I mean it'll produce the power needed to support that speed using the least amount of fuel. More rpm tends to have more frictional losses and therefore it can be less efficient, but too low of an rpm can cause you to lose certain effects that help efficiency, not the least of which is boost. Another thing to be aware of is that the rpm of max efficiency will vary depending on load. The gearing that gives you the best mileage at 75mph empty isn't going to be optimum for 75mph loaded.

What Mackey said, 265's gave me the same mileage.

I ran 285's for awhile but my mileage has improved since then for other reasons so I hate to draw a conclusion. Didn't like the 285's, couldn't get even tire wear no matter what I did with the pressures.
 

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If you'd go to a 235/85-16 (tall and Skinny) it would be better than the 265, 255, 285, and 305's that others talk about. They're too wide for good milage. Tall and skinny is where its at for best milage. Also a street tread gets better milage than an all terrain or mud tire.

I personally believe that the Dmax/ally combo with 3.73's is in need, and will benefit from the 235/85's I speak of.
 

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OH! Didn't think about the stick, but in 6th shouldn't it be running the same RPM or better than the Allison in 5th?


Also I think there are 4:11 gears out there with certain combinations, maybe the 4-wheel drive. I know I saw it on the stickers of some trucks when I was looking for mine and I wanted the 3:73 specifically. It might have been the gassers all came with 4:11 gears. I am sure something had it, because I remember passing up on a couple of trucks that had the 4:11 instead of the 3:73's.


I am referring to the 2001 models, have not looked at the '02's and '03's hard enough to notice any changes.


Tall and Skinny Huh, kinda the way I like my women....LOL
 

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I JUST WANNA KNOW WHERE THIS 19MPG AND 20MPG STUFF COMES FROM??? !!!! I GOT 285'S AND HAVE NEVER EVER GOT OVER 16 BABYING IT AROUND EVERYWHERE..USUALLY 14.5..SOMETHING ELSE MUST BE WRONG WITH MINE THEN !
 

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I used the formula:


(<Miles traveled> / <gallons>) + 4


You ask why add the 4? Makes me feel better
 

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Discussion Starter #14
I used to get 16.6 town and 20.4 freeway from my second tank of fuel on. Then I bought a shell and put fuel additive in now I get 14/18. I dont know if it was the shell weight and or fuel additive?
 

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Chevysrus said:
OH! Didn't think about the stick, but in 6th shouldn't it be running the same RPM or better than the Allison in 5th?


Also I think there are 4:11 gears out there with certain combinations, maybe the 4-wheel drive. I know I saw it on the stickers of some trucks when I was looking for mine and I wanted the 3:73 specifically. It might have been the gassers all came with 4:11 gears. I am sure something had it, because I remember passing up on a couple of trucks that had the 4:11 instead of the 3:73's.


I am referring to the 2001 models, have not looked at the '02's and '03's hard enough to notice any changes.


Tall and Skinny Huh, kinda the way I like my women....LOL

I think stick vs alli, one od is .72:1 and the other is .73:1 IIRC. I believe 2004 is the first model year that the 4.10 gear is offered with the dmax. Ditto on the women.
er, I mean woman(married)
 

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I've gotten a best of 24.3MPG with the stock rubber, all highway, very little idle time and stop and go, in the mountains, averaging 55mph and never exceeding 65mph.


I got 22.5MPG on the same trip with the 315/70R17's on using the same driving habits.


only thing unstock about the truck at the time was a K&N filter in the stock airbox.
 

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Camstyn said:
I've gotten a best of 24.3MPG with the stock rubber, all highway, very little idle time and stop and go, in the mountains, averaging 55mph and never exceeding 65mph.


I got 22.5MPG on the same trip with the 315/70R17's on using the same driving habits.


only thing unstock about the truck at the time was a K&N filter in the stock airbox.

With the 315's were you figuring mileage manually applying a 11.33% error factor? Just curious as I too run 315/70R17's and my best has been around 18 freeway ...averaging 65.
 

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I was adding 14%, I thought that was what it was..


I just checked on a website, here's the results:






Tire Size Comparison
Specification Sidewall Radius Diameter Circumference Revs/Mile Difference
245/75-16 7.2in 15.2in 30.5in 95.7in 662 0.0%
315/70-17 8.7in 17.2in 34.4in 108.0in 587 12.8%
</PRE>Edited by: Camstyn
 

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I guess the calculators vary ...


http://www.dakota-truck.net/TIRECALC/tirecalc.html
Dimensions


<TABLE align=center>
<T>
<TR>
<TD>
<TABLE align=center border=1>
<T>
<TR>
<TH>Dimension</TH>
<TH><CODE>Tire A: 245/75 R 16
(30 x 9.65 x 16)</CODE></TH>
<TH><CODE>Tire B: 315/70 R 17
(34 x 12.40 x 17)</CODE></TH>
<TH>Difference</TH></TR>
<TR>
<TD>
<TABLE align=center>
<T>
<TR>
<TD>Width</TD></TR>
<TR>
<TD>Sidewall height</TD></TR>
<TR>
<TD>Overall height</TD></TR>
<TR>
<TD>Circumference</TD></TR></T></TABLE></TD>
<TD>
<TABLE align=center>
<T>
<TR>
<TD>9.65"</TD>
<TD>(245.00mm)</TD></TR>
<TR>
<TD>7.23"</TD>
<TD>(183.75mm)</TD></TR>
<TR>
<TD>30.47"</TD>
<TD>(773.90mm)</TD></TR>
<TR>
<TD>95.72"</TD>
<TD>(2431.28mm)</TD></TR></T></TABLE></TD>
<TD>
<TABLE align=center>
<T>
<TR>
<TD>12.40"</TD>
<TD>(315.00mm)</TD></TR>
<TR>
<TD>8.68"</TD>
<TD>(220.50mm)</TD></TR>
<TR>
<TD>34.36"</TD>
<TD>(872.80mm)</TD></TR>
<TR>
<TD>107.95"</TD>
<TD>(2741.98mm)</TD></TR></TD></TR></T></TABLE></TD>
<TD>
<TABLE align=center>
<T>
<TR>
<TD>+2.76"</TD>
<TD>(+70.00mm)</TD></TR>
<TR>
<TD>+1.45"</TD>
<TD>(+36.75mm)</TD></TR>
<TR>
<TD>+3.89"</TD>
<TD>(+98.90mm)</TD></TR>
<TR>
<TD>+12.23"</TD>
<TD>(+310.70mm)</TD></TR></T></TABLE></TD></TR></T></TABLE></TD></TR></T></TABLE>Same as with the speedometer, if your odometer has been calibrated for 245/75 R 16 (30 x 9.65 x 16) tires, replacing them with 315/70 R 17's (34 x 12.40 x 17's) would result in the odometer reading low by 11.33%.
 

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True, so when you're doing 100mph your speedo will be at ~88.7mph.


In 88.7 miles on the odo you'll have gone 100 miles. Take 100, divide it by the 88.7 and you'll get 12.8%. That's how I did the math, anywho. It seems like the correct way to do it.. If I'm in fact doing it wrong somebody please point out my error.
 
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