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New here guys. I'm picking up my '07 Duramax tomorrow. It's a GMC quad cab reg bed. I'm thinking about asking them to swap my tires from my trade in and put them on the new truck on the factory wheels but I want to know how much gas mileage I will lose doing this. Please let me know asap. Thanks!
 

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I can not answer that question but I can say swap your tires now. I did not and now have to wait for the 245's to wear out before Momma will allow new tires........

Easier to spend it all at one time, at my house anyway.

Congrats on the new sled, you will love the Dmax,

SS
 

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More than likely going to 285's you will need to turn the torsion bars up so you will have to get the front end aligned. Also you might have to do some trimming to get them to fit and not rub. The other thing is that your speedo will be off after that as well. I think in th elon grun you will lose most likely about two miles per gallon depending on how you drive. hope that helps.. Good luck
 

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Hard to say exactly how much mpg you'll loose. I lost about 1-2 mpg since I installed 285s...from 265s. BUT, I also added about 600lbs (plow frame and balast), removed valance and cranked truck up 3" (increased wind resistance). Also, its been alot colder now...and winter fuel supposed to decease MPG. So...I'd say at worst...2mpg loss.

Be sure you remember that 245-285 is almost 8% different, so either recallibrate or rememeber to add 8% back onto your MPG calculation.
 

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Discussion Starter #6
Thanks guys. I'm about to leave to go pick it up. As far as the speedo being off, you can have that reprogrammed right?
 

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yeah you can but like the guy said without a little lift you will rub
 

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welcome and congrats on the new ride. IMO, quit worrying about mileage in an 8000lb truck. you'll hardly see a difference anyway. I have 285s on mine ('02 lb7) and the speedo is dead on according to my GPS. close enough for me

have fun with the new truck
 

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:grd: -:t SWAP YOUR TIRES!!! Stockers are worthless!!
 

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You don't lose milage going from a smaller diameter to a larger one. It's effectively the same as going from a higher rear diff ratio to a lower one (a 3.73 rear gets better milage than a 4.10 rear).

The amount of milage you gain will depend on the size of increase, and will be a percentage figure, not a number of MPG. (A MPG figure can be calculated from your current MPG, but you can't say accross the board going from one size to another will add or lose a set number of MPG).

For example, a 245/70R16 tire has a diameter of 29.503937007874015 inches. If you upgraded to a 285/75R16 tire, that diameter will be 32.830708661417326 inches. [(32.8*100)/29.5]-100 will give you your percentage difference in diameter (circumference is a direct dependant of diameter). In this case, the increase is 11.2%. You'd get 11.2% better gas milage. To use this equation, you have to know the old and new diameter. [New Diameter] x 100 devided by [Old Diameter] all minus 100 will give you the size differential percentage.. The percentage difference in size will also be the percentage change in your milage. A good tire diameter calculator can be found here: http://www.americanracing.com/techcenter/DiameterCalculator/DiameterCalculator.htm

Another way of looking at it is this; a 245/70R16 tire has a diameter of 29.503937007874015 inches. If you upgraded to a 285/75R16 tire, that diameter will be 32.830708661417326 inches. A 29.5" diameter tire has a circumference of 29.5x[3.142 (pi)], or 92.7". A 32.8" diameter tire has a circumference of 32.8x[3.142 (pi)], or 103.1". In other words, a 285/70 tire will move you 10.4" more forwards than a 245/70 tire will, upon each revolution.
 

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it alllll depends on your right foot....
 

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beeler311;1602060; said:
New here guys. I'm picking up my '07 Duramax tomorrow. It's a GMC quad cab reg bed.
No its not. Only D*dge builds a quad cab. If your back doors open with the front ones closed it is a crew cab. If they don't it is an extended cab. Otherwise...... :welcome: to the diesel place.
 

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Schulte's math is right, but one variable left out, is the fact that with a larger tire your engine/tranny will have to work more to get the rolling mass to move. (you've effectly raised your axle ratio, going from a 3.73 to around a 3.28 or so) So where you might lose MPG when you are taking off/how you take off, you may gain MPG once everything is moving at a higher speed due to less work being done to maintain that speed. There are tons a variables, that's why everyone will report a different change. The only true way to raise your tire size and not change MPG is to change the gears. If your tire increase changed 12%, your gear needs to be changed 12%, basically go from a stock 3.73 to a 4.10 roughly. Chris
 

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285's

When you do figure out your mileage, make sure you add about 5% on your miles driven. I followed my truck with 285's the other day with cruise @100 km and my radar showed it at exactly 105.. So that means speedo is out 5%..
 

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dundeediesel;1604438; said:
:grd: -:t SWAP YOUR TIRES!!! Stockers are worthless!!
Yea and they look like little pizzas :D
 

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silverbirch04;1605628; said:
When you do figure out your mileage, make sure you add about 5% on your miles driven. I followed my truck with 285's the other day with cruise @100 km and my radar showed it at exactly 105.. So that means speedo is out 5%..
You have radar in your truck :eek: ?
 

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Discussion Starter #17
boondokr;1605412; said:
No its not. Only D*dge builds a quad cab. If your back doors open with the front ones closed it is a crew cab. If they don't it is an extended cab. Otherwise...... :welcome: to the diesel place.
Well, whatever you want to call it, quad, crew, 4 dr, etc.
 

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Discussion Starter #18
schulte;1604480; said:
You don't lose milage going from a smaller diameter to a larger one. It's effectively the same as going from a higher rear diff ratio to a lower one (a 3.73 rear gets better milage than a 4.10 rear).

The amount of milage you gain will depend on the size of increase, and will be a percentage figure, not a number of MPG. (A MPG figure can be calculated from your current MPG, but you can't say accross the board going from one size to another will add or lose a set number of MPG).

For example, a 245/70R16 tire has a diameter of 29.503937007874015 inches. If you upgraded to a 285/75R16 tire, that diameter will be 32.830708661417326 inches. [(32.8*100)/29.5]-100 will give you your percentage difference in diameter (circumference is a direct dependant of diameter). In this case, the increase is 11.2%. You'd get 11.2% better gas milage. To use this equation, you have to know the old and new diameter. [New Diameter] x 100 devided by [Old Diameter] all minus 100 will give you the size differential percentage.. The percentage difference in size will also be the percentage change in your milage. A good tire diameter calculator can be found here: http://www.americanracing.com/techcenter/DiameterCalculator/DiameterCalculator.htm

Another way of looking at it is this; a 245/70R16 tire has a diameter of 29.503937007874015 inches. If you upgraded to a 285/75R16 tire, that diameter will be 32.830708661417326 inches. A 29.5" diameter tire has a circumference of 29.5x[3.142 (pi)], or 92.7". A 32.8" diameter tire has a circumference of 32.8x[3.142 (pi)], or 103.1". In other words, a 285/70 tire will move you 10.4" more forwards than a 245/70 tire will, upon each revolution.
Um, not exactly. A larger tire with the same ID will weigh more than a smaller tire. There is more tq required to turn that the larger tire which uses more fuel. Yes, you will move farther with a single revolution, but you will use more fuel doing so. You can't simply say that since it is larger you will gain MPG.
 

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Don't forget that the tire is wider as well. This usually effects MPG negatively.
 

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beeler311;1606003; said:
Um, not exactly. A larger tire with the same ID will weigh more than a smaller tire. There is more tq required to turn that the larger tire which uses more fuel. Yes, you will move farther with a single revolution, but you will use more fuel doing so. You can't simply say that since it is larger you will gain MPG.
This's true, the equation is not 100% accurate and does not take into account all the factors.

More torque is required to turn a larger tire, but this simply results in a change in where your truck is most efficient. Smaller diameter tires are more efficient during accelleration, and larger diameter tires are more efficient at a constant speed, because less RPM's are required to achieve the same distance traveled forwards. Becuase most of the time you spend driving you're NOT accellerating, larger tires (within reason), will give you better milage.

This's even more the case when the motor in question is a diesel; lower engine RPM's requiring higher torque are favorable over higher RPM's requiring lower torque, due to the torque curve for a diesel engine. Basically, you're usually cruising at a higher RPM than the peak torque RPM, meaning any closer you get to the peak torque RPM, the less fuel you'll use covering the same distance.

beeler311;1606003; said:
You can't simply say that since it is larger you will gain MPG.
This's true. The equation I provided is not 100% accurate, but it WILL give a rough estimation of the change in fuel economy in relation to a change in tire diameter.
 
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