MPG and big tires

6

6.6

New member
Dec 18, 2011
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I recently purchased an '06 2500HD with 60k on it. The truck has oversized tires - 305/65/17 and stock was 245/75/16 according to the door sticker. It also has a 5" lift on it. Also, the exhaust coming out of the cat is 3.5", and from the cat back they ran a 3" pipe. Not sure if this could cause a problem but if Chevy used 3.5" from the factory, I can't see how reducing it could be healthy.
I have talked to several people with the same truck who get an average of 16-18 on the highway. I can't get past 14mpg highway (using a GPS to calculate mileage because the odometer and speedometer is inaccurate because of the tires). Around town is about 12mpg, even driving conservatively.
Are the lift and oversized tires really eating my mpg that much, or could the reduced exhaust size be part of the problem? I can't imagine 14mpg is normal.
 
01LB7

01LB7

New member
Feb 18, 2010
206
0
0
Missouri
I vote both. Usually a larger exhaust is helpful in gaining mpgs and the larger tires and lift makes it less aerodynamic creating more wind resistance.
 
Kat

Kat

Wicked Witch of the West
Aug 2, 2006
17,899
13
38
60
Norco, CA
Lift and tires are killing your mileage.

If I had it to do over again. I would not of went with 20's when I replaced the stolen rims and tires on Wendy. Not only did they hurt the mileage... The handling sucks.
 
smarquis

smarquis

Triple Turbo LBZ
Jul 15, 2010
188
0
0
Las Vegas, Nevada.
Kat said:
Lift and tires are killing your mileage.

If I had it to do over again. I would not of went with 20's when I replaced the stolen rims and tires on Wendy. Not only did they hurt the mileage... The handling sucks.
Click to expand...

Did your wheels make your handling suck? I've had 35's on mine, but I am wondering what the truck could do if I went down to 33s or something. I always thought that aluminum wheels were lighter than tires, so it would help performance to obtain the biggest diamerter tirme you can.

Thanks,
 
TeaBagger2006

TeaBagger2006

Im a Garrett Nut Swinger
May 11, 2008
3,123
15
38
Bis ND
Well bigger tires don't really help and a smaller than stock exhaust really doesn't help either. Replace your exhaust with a 4" turbo back exhaust and get a tuner to set your speedo right ..... And mo' powa too:D

for what it's worth I get 14 around town and about 17 on the highway with my set up which is cosiderably bigger than yours. To get those numbers there can be no fun involved :(
 
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Kat

Kat

Wicked Witch of the West
Aug 2, 2006
17,899
13
38
60
Norco, CA
smarquis said:
Did your wheels make your handling suck? I've had 35's on mine, but I am wondering what the truck could do if I went down to 33s or something. I always thought that aluminum wheels were lighter than tires, so it would help performance to obtain the biggest diamerter tirme you can.

Thanks,
Click to expand...

I had 17's and got approx 20 mpg on the fwy and it drove as good as my caddi :woott: and it dropped to approx 16 with the 20's and now drives like a truck :(
 
6

6.6

New member
Dec 18, 2011
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I figured all these factors would play a part in bad mpg, I just didn't realize how much. The dealership told me that the Duramax required some amount of exhaust back pressure and that's why they set up the exhaust like it is. I don't know much about diesels but that doesn't sound right to me. If I put a bigger exhaust on, would it cause any problems?
 
duramaxdiesel

duramaxdiesel

Dmax Nut
Oct 23, 2008
1,378
0
36
47
Montreal, Canada
I know this might sound funny to you guys but my mileage actually increased when I went from my 285's to my 37's. All I do is highway though. My guess is that at the same speed my RPM's dropped with the 37's installed. This is just what I observed on my truck. Results may vary for sure.

Nick
 
dwax

dwax

New member
Jul 21, 2009
127
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0
6.6 said:
I recently purchased an '06 2500HD with 60k on it. The truck has oversized tires - 305/65/17 and stock was 245/75/16 according to the door sticker. It also has a 5" lift on it. Also, the exhaust coming out of the cat is 3.5", and from the cat back they ran a 3" pipe. Not sure if this could cause a problem but if Chevy used 3.5" from the factory, I can't see how reducing it could be healthy.
I have talked to several people with the same truck who get an average of 16-18 on the highway. I can't get past 14mpg highway (using a GPS to calculate mileage because the odometer and speedometer is inaccurate because of the tires). Around town is about 12mpg, even driving conservatively.
Are the lift and oversized tires really eating my mpg that much, or could the reduced exhaust size be part of the problem? I can't imagine 14mpg is normal.
Click to expand...

I think the best way for you to improve your mileage is by going with a 4" straight exhaust, and having a reputable tuner give you a good efi live tune. I had your setup and tire size at one time and got 20 hwy with just a hand held PPE.
 
GeneralTJI

GeneralTJI

Turbo Todd
Jun 1, 2010
1,272
0
36
Colorado
You can pick up some mileage with full exhaust and a good tune!!... but it will never be great with the lift/tires.
 
RENODMAX

RENODMAX

Dead Wrong
Mar 4, 2008
3,602
0
0
I run 285/70-17s on my truck and going to a 315/70-17 I lose 1-2 mpg. I have always averaged 19mpg mixed with my truck but tuning is the only thing that is going to get you there.
 
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