Can this be right???

[rex22]

I was told that I would get better mpg's with bigger tires (315's) on the highway? I have an 07 megacab that is bone stock with a bullydog. Is that true???

[mzcummins]

You can get better mpg if you go taller but wide tires will hurt you.

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[energinerbuzzy]

No thats not right. There are many variables to this such as what gears you are running, ect. But in almost all cases, you will lose milage with taller tires. I don't care how skinny they are the milage will drop. When you are at lower RPM's driving around town, stopping and starting, you engine is lugging much more than it did with stock size wheels. There is more mass to get started turning now and your gear ratio will change due to the larger diameter tires.

There are a few exceptions to this such as, if you are running a really low gear like 4:55 or 4:11 already. You are probably getting horrible highway milage already and taller tire would improve your over-all milage. Or if you run nothing but Highway miles over long distances, even with 3:73 gears you may see an improvement in fuel milage.
But for almost all other situations, you will almost surely lose milage with a taller tire.

I went from stock wheels to 35" x 12.5" tires and I dropped 2.5 MPG I put on a 6" lift and upraded to 35" x 13.5" tires and lost another 2 MPG for a total loss of 4.5 MPG loss over stock.

Ofcourse this is assuming you are talking going to a 33" or 35" tire. You haven't mentioned what size you are wanting to go with . Telling me "315's" means nothing, all that is, is the width of the tread in millimeters. 315/?/? and then we will know what size tire we are dealing with here.

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2007 Ram 2500 SLT QC 5.9L , 35" Pro Comp Xtreme and 18" American Racing "Clash", 6" Superlift, Bushwacker pocket flares, J-Mark Wheel to wheel steps, Smarty level #7, Cat MIA, Aero 4040XL, Sendera Runner Baja bumper.

[lvin4jc]

315's are generally around 35"

Taller but not wider tires will increase mileage on the highway because it changes your final gear ratio and drops your RPM's.

__________________

2001 4X4 Auto SLT Laramie - DANA 60/DANA 70, 3.55's - Isspro Gauges, FP, Pyro, Boost, Trans. Temp. - HBS 2" Leveling Kit - Straight Pipe - Amsoil Nanofiber 4510 CAI - Quadzilla XZT+ - Debadged - 285/75/16 Firestone Destination M/T's

230,000 miles as of 08/09 - - All Cummins all the time!

[awenthol]

Originally Posted by rex22

I was told that I would get better mpg's with bigger tires (315's) on the highway?

Originally Posted by energinerbuzzy

When you are at lower RPM's driving around town, stopping and starting, you engine is lugging much more than it did with stock size wheels.

See previous quote.

Originally Posted by energinerbuzzy

There is more mass to get started turning now and your gear ratio will change due to the larger diameter tires.

It's the intertial moment..for the most part, the mass increase will be negligible.:thumbsup

[energinerbuzzy]

Originally Posted by lvin4jc

315's are generally around 35"

Taller but not wider tires will increase mileage on the highway because it changes your final gear ratio and drops your RPM's.

Key word there is on the HIGHWAY
Around town you will drop milage. I drive a 50/50 mix and have dropped a lot in MPG since going with 35". Yes I have also made a few long trips and saw milage increase but not by a measurable amount.
The theory of changing the final drive and increased milage on the highway would assume that he is driving 100% Highway miles in 4th gear (if auto) with the T/C locked up the whole time.

if he drives like 80% of the rest of us, he will have a lot of city miles in there too and will see a loss in his milage.

I'm not arguing here, just being realistic and answering his question.

__________________
2007 Ram 2500 SLT QC 5.9L , 35" Pro Comp Xtreme and 18" American Racing "Clash", 6" Superlift, Bushwacker pocket flares, J-Mark Wheel to wheel steps, Smarty level #7, Cat MIA, Aero 4040XL, Sendera Runner Baja bumper.

[lvin4jc]

Originally Posted by energinerbuzzy

Key word there is on the HIGHWAY
Around town you will drop milage. I drive a 50/50 mix and have dropped a lot in MPG since going with 35". Yes I have also made a few long trips and saw milage increase but not by a measurable amount.
The theory of changing the final drive and increased milage on the highway would assume that he is driving 100% Highway miles in 4th gear (if auto) with the T/C locked up the whole time.

if he drives like 80% of the rest of us, he will have a lot of city miles in there too and will see a loss in his milage.

I'm not arguing here, just being realistic and answering his question.

I understand that. That's why I said on the highway. In the personal experience you related before, you went with wider tires every time you upped the diameter. That is the main reason you saw such a drop in mileage, you're adding even more weight and pushing more tread surface down the road increasing the friction and decreasing efficency.

__________________

2001 4X4 Auto SLT Laramie - DANA 60/DANA 70, 3.55's - Isspro Gauges, FP, Pyro, Boost, Trans. Temp. - HBS 2" Leveling Kit - Straight Pipe - Amsoil Nanofiber 4510 CAI - Quadzilla XZT+ - Debadged - 285/75/16 Firestone Destination M/T's

230,000 miles as of 08/09 - - All Cummins all the time!

[Smokinrebel]

One other thing to remember is that your spedometer will be off too...so you might not know exactly how many miles your running

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98 24V Injectors, Quad Adrenalin, 4in Straight Pipe, K&N Intake, Silencer Ring Removed, Dead Pusher Pump, Georend Triple Disk Convertor and few other odds and ends in tranny All topped with a Loose Nut behind the wheel

[lvin4jc]

Originally Posted by Smokinrebel

One other thing to remember is that your spedometer will be off too...so you might not know exactly how many miles your running

:thumbsup Good point, you're actually going farther than the odo. will read

__________________

2001 4X4 Auto SLT Laramie - DANA 60/DANA 70, 3.55's - Isspro Gauges, FP, Pyro, Boost, Trans. Temp. - HBS 2" Leveling Kit - Straight Pipe - Amsoil Nanofiber 4510 CAI - Quadzilla XZT+ - Debadged - 285/75/16 Firestone Destination M/T's

230,000 miles as of 08/09 - - All Cummins all the time!

[Smokinrebel]

yepper...mine is off by 3-5 mph with just 33's. I know this because the cops have told me a few times...hahaha. "But officer i had no idea how fast i was going, the tires must throw off the spedometer"

__________________
98 24V Injectors, Quad Adrenalin, 4in Straight Pipe, K&N Intake, Silencer Ring Removed, Dead Pusher Pump, Georend Triple Disk Convertor and few other odds and ends in tranny All topped with a Loose Nut behind the wheel

[theskippuh]

Lets not forget that a taller tire lifts the truck which increases airflow under the truck thereby increasing your drag coefficient and decreasing mileage. IMHO, you will always lose MPG with larger tires either taller or wider or both.

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06'/White/2500/SLT/5.9L/48RE/3.73/4x4/SRW/ Bone Stock
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[rex22]

I'm not positive but I think I have 3.73 gears with AOD. Have stock 265's and want to get a 295,305 or 315's. I'm putting a leveling kit on an want a little beefer look but don't want to totally kill my mpg's. What size tire would be best given this info.

The reason I asked about highway mpg's is that is the only time I really drive it. I have a company vehicle that is my everyday driver. I only take my truck to the deer lease and back.

Thanks fellas