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Discussion Starter · #1 ·
Tuff country is sending me their 3" lift for the Cummins 3500, should get it by the end of next week. Gonna run Swamper LTB 34.5x10.5 tires. I am hoping to gain some mpg with the increase in tire size, but staying the same in tire width as my current tires.
 

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I know I don't have to tell you this but,,, Keep us posted! lol
 

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The more you do to the motor, eg. air intake, exhaust, computer, the better the fuel milage gets.

I had 35's on my 92 and didn't notice a huge change.
 

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Discussion Starter · #6 ·
Diesels are a whole other ball game. My cummins stock has 460 ftlbs of torque, your TD has maybe 200. The cummins has so much power that it benifits from a bigger tire, just as your TD would if you were running 4.56 gears and 30" tires. Your mileage would suck with that, but if you put on some 36's or 38's your mileage would go up. Same deal with the cummins.

Another odd thing with the cummins, if you add bigger injectors you get better fuel efficiency as well. Normally with a gas motor if you go with bigger injectors you lose mileage since you are dumping more fuel in as the same rate as before.

Diesels are just awsome, plain and simple :)
 

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let me get this straight:
lifting a gas powered truck to fit those 38s under there and adding 4.50 gears does help gas milage? Uhm, I thought due to aerodynamics and stuff milage would decrease rapidly...or did I misread something here? ???
 

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Discussion Starter · #8 ·
No, adding bigger then stock tires to a diesel helps fuel efficiency. My example for a gas motor was with 4.56 gears and 30" tires compared to 4.56 gears and 36 or 38" tires. The 36 or 38" tires would get better mileage then the 30" tires with 4.56 gears. I was trying to explain why the diesel can gain mpg due to it's massive amount of torque.
 

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I could give a lengthy mathematical explaination of why a Cummins can get better fuel economy with larger tires, but, I'll keep it simple.

Think of torque as you would think of leverage. The more leverage you have, the more you can do and the less effort you need to exert.

Did you ever ride a 10 speed bicycle? You know that when you try to start pedaling from a dead stop in 10th gear, you had to exert alot of effort, to start rolling, but once you got up to speed you were going very fast.

To reduce the effort in turning the cranks on the bike, you shift into a lower gear. Gears are torque multipliers. When you shift a bicycle into a lower gear, your legs didn't suddenly get stronger. You took your existing strength and added some mechanical advantage, in this case you used a lower gear to multiply the existing torque your legs can make. The drawback is you cannot go as fast, as you could in 10th gear but you use less effort to start rolling.

But what if you were able to lengthen the cranks? Now you got more leverage to push against. With the added crank leverage, you can now exert less effort and use a higher gear.

Increasing tire diameter is alot like having a taller gear if we did nothing to the axle gears. As the example above shows, you now have to exert more effort to turn those tires, that is why it is recommended you lower the axle ratio when you increase tire diameter. The diesel however makes more torque, much like having the additional mechanical advantage of longer cranks.

Ed
 

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Okay, I understand now. Thanks Sam and Ed. {cool} While I'll be getting a Diesel sometime in the next couple years, I'm currently stuck with the TD gasser. To keep my efficiency up, would you recommend more hp and torque for the engine before going with taller tires, so that I maintain the leverage? Not too interested in really tall tires, just an inch or two above my little 31x10.5s. Thanks guys! ;)
 

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so if all you do is IN-town driving, would your milage still go up? Or is it just cruising where you would notice it?
 

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Raminator said:
Okay, I understand now. Thanks Sam and Ed. {cool} While I'll be getting a Diesel sometime in the next couple years, I'm currently stuck with the TD gasser. To keep my efficiency up, would you recommend more hp and torque for the engine before going with taller tires, so that I maintain the leverage? Not too interested in really tall tires, just an inch or two above my little 31x10.5s. Thanks guys! ;)
The thing your truck really needs is another gear. Trust me, I know. Your truck was built back in the days of the 55 MPH speed limit, and back then, 3 gears and 3:55 or 3:73 or even 4:10 axle ratio was fine. Now, to run 70-75, you're winding out.

Bigger tires will help with that, but you'll pay the price in low speed driveability.

I now de-hijack this thread... ;)
 

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just curious on the wheel issues that come up in putting that wide of a tire on a dually. Need spacers between the wheels or custom offsets to keep the sidewalls from rubbing?

And on the MPG thing, couldn't it also be thought of as just getting the right overall gearing to put the engine in it's highest output RPM range when at highway speeds. The gassers spin faster than a diesel so the optimum overall gearing just moves around.
 

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Part of it too is that these motors are choked from the factory. The more you help it breathe...the more efficient it gets
 
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