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Unread 08-06-2010, 04:31 PM   #46
wrestlerobx
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I drive an automatic and when i am driving at highway speeds (65-75) I turn OD off unless it is purely flat terrain and i get better gas mileage by about 1 sometimes 1.5 miles to the gallon. calculated by hand. I attribute it to the constant RPM and not having to shift and hunt for the right gear. Im not 100% sure why but i know it works for me

automatic 4 door, 3.73 gears, 2.5" lift and 34" tires

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Unread 08-06-2010, 04:36 PM   #47
RMiksits
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I'm must be doing it wrong. I'm getting 13 with my stock rubi......
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Unread 08-06-2010, 05:08 PM   #48
Grizzly1
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maybe your boots are heavy
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Unread 08-06-2010, 05:11 PM   #49
jeepinvader
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It's a Jeep. It's gonna suck gas, and haul...... well, it'll just suck gas.
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Unread 08-06-2010, 05:29 PM   #50
wrestlerobx
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its true it will suck gas and I havent owned a vehicle that got over 16 regularly so i suppose ignorance is bliss huh. lol
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Unread 08-06-2010, 06:34 PM   #51
Poor Boy
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Quote:
Originally Posted by wrestlerobx View Post
I drive an automatic and when i am driving at highway speeds (65-75) I turn OD off unless it is purely flat terrain and i get better gas mileage by about 1 sometimes 1.5 miles to the gallon. calculated by hand. I attribute it to the constant RPM and not having to shift and hunt for the right gear. Im not 100% sure why but i know it works for me

automatic 4 door, 3.73 gears, 2.5" lift and 34" tires
you also need deeper gears to move those tires. because i'm not going to re-invent the wheel on this:

Quote:
Whenever larger-than-stock tires are installed on a truck, it will have a direct effect on the truck's performance. Why? It has to do with the effective gear ratio. Your truck comes from the factory with the optimum axle gear ratio to work with the truck's engine, transmission, and stock tire combination as well as provide a good balance between acceleration and fuel economy. When taller tires are installed on a truck but the axle ratios stay the same; the effective gear ratio is reduced. This means the engine is forced to operate below its power band, and performance and fuel economy suffer as a result. In order to restore the effective gear ratio (and the truck's performance), you'll need to have the axle gears swapped to lower (numerically higher) ratios.
Fortunately, finding the correct gear ratio to run with the tires that you plan to install is simple using a mathematical formula. There are actually several different formulas available to determine the correct ratio with varying degrees of difficulty. The simplest and most reliable method we've found is to determine what engine RPM the truck runs with the stock gears and tires at a given speed, and then closely match this rpm with the larger tires. In order to use the following formula, you'll need to know the stock axle gear ratio as well as the stock tire size. It also helps to know what RPM the truck runs at a given speed with the stock setup, but it's possible to find this information with the formula. Keep in mind all RPM readings should be taken with the transmission in a 1:1 ratio (Fourth with most manuals or Third with an automatic).


The Formula
2242 = 65 x 3.08 x 336 / 30

To use an example, let's say we want to know what gears should be installed when putting 36" tires on a transmission, 3.08 gears, and 30" tires in stock form. Since the truck doesn't have a tach, we first need to find out what RPM the engine is spinning at a given speed, say 65 mph. With this information, the formula looks like this:

rpm = mph x gear ratio x 336 / tire diameter

At 65 mph, the truck's engine is spinning at 2242 rpm, which is right in the optimum powerband for most V-8 engines. Now all that's left to do is plug in the new tire size and a couple different ratios available for the truck to see which one closely matches the RPM.

2263 = 65 x 3.73 x 336 / 36

As you can see from the above formula, 3.73 gears with 36" tires is optimum for restoring stock performance with this truck. Keep in mind that not all ratios are available for every axle, so do some research to make sure there's a ratio close to what you need available for your truck's axles.

One last note regarding trucks equipped with overdrive. The Overdrive gear in a transmission allows the drivetrain to run below a 1:1 ratio, which brings down engine RPM for good fuel economy when running down the highway. Most Overdrive gears run somewhere around 0.73:1. As we said before, the best option for people who use the truck primarily on the street is to match the formula given above. However, Overdrive offers a unique opportunity for dedicated off-roaders. Since overdrive decreases the reduction at highway speeds, it's possible to "cheat" and select gears that are a little lower (numerically higher) for better slow gearing off-road without sacrificing highway drivability. This advantage becomes apparent when you do the math by multiplying a prospective gear ratio by the Overdrive ratio of your transmission. To use an example, 5.13 gears in a truck without an overdrive and moderate-sized tires would offer awesome slow-speed crawling ability for off-road situations but would leave the engine screaming at highway speeds. However, if the same 5.13s are installed in a truck equipped with an overdrive of 0.73:1 and we do the math (5.13 x 0.73 = 3.744), we find that the Overdrive gear gives the effect of a very streetable 3.73 ratio for highway cruising. In other words, the truck has the same good crawling speed of 5.13 gears (because Overdrive is never used off-road) combined with a very street-friendly effective ratio of 3.73 gears. Of course the engine will still rev higher on the highway compared to having the proper ratios found by following the steps we laid out earlier. Overdrive does offer dedicated four-wheelers the option of installing low, off-road friendly gears and still be able to drive around on the street with some fuel economy left intact. To plug overdrive into the gearing formula above, simply multiply the Overdrive ratio by the axle gear ratio.
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Unread 08-06-2010, 09:18 PM   #52
RMiksits
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Quote:
Originally Posted by Grizzly1 View Post
maybe your boots are heavy
My commute is 17 miles with 15 on the freeway. I only go 70-80

Yup boots are heavy.
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Unread 08-06-2010, 11:33 PM   #53
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Poor Boy, you've got it down. Almost everyone else is still stuck in the "lower RPM's = better gas mileage" box. That may work for aerodynamic cars with small, light tires. It doesn't ALWAYS apply, especially for a heavy vehicle with a coefficient of friction of .5x and a 200hp V6 attempting to turn 100 lbs at each corner.

Copied and pasted from a thread I posted in earlier today:

There is more to fuel consumption than RPM's only....if that was the case, people would gear the axles higher instead of lower. You need to know where the engine operates most efficiently. The 3.8 likes to be in the 2.5-3k range at highway speeds. You have a heavy vehicle shaped like a brick. It doesn't flow through the air nicely and doesn't have small tires. The larger tires require more leverage to turn efficiently. Combine all of that with the lethargic 42rle auto and you need some low gears to put you back in the engine's sweet spot. You're getting 12mpg because your engine is having to work so hard. If you were ~1,000 RPM's higher, you would likely be in the 16-18mpg range at the same speed.

An analogy I like to use--you go for a ride on your 10 speed bike. From a dead stop, it's much easier to get moving when in first gear vs. 7th. Why? LEVERAGE. If you take off in first, your RPM's are significantly higher than if you took off in 7th but I guarantee you'd have to put more effort and energy into starting from 7th than in first. The same goes for biking into a headwind (similar to the JK on the highway). If you're cruising along in 10th into a headwind, you'll be exerting a lot of energy to keep moving because you don't have much leverage over the tires. Your RPM's will be less than if you were in 5th gear. Downshift to 5th and now you're pedalling faster but exerting less energy. That is analogous to better fuel consumption.

Make sense? With the 42rle and .69 OD (very steep), you need a minimum of 5.13 gears with 35's to come close to making the engine happy. You'll be turning higher RPM's at every speed but the engine won't be lugging. Your fuel milage will definitely go up.


There is a good reason I get better mileage now (17mpg average) with my "ancient" 4.0 than I did with 3.07 gears and 31" tires. Numbers don't lie gents.
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Unread 09-27-2010, 07:05 PM   #54
1Ordo_Malleus
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Hmm.. I'm only getting about 9.5 mpg.. I get JUST barely 180 miles on a full 19gal tank... 3" lift, 35's and it looks like 4.10 gears. But I do have a snorkel... I know I'm getting a bit more in actuality than 180, but it certainly isn't more than 200... Is there anything I can do??
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Unread 09-27-2010, 07:35 PM   #55
rirrgang
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Good day, bad day, unlimited, auto, 18's, 4:10's. 17-18 in town, 19-21 freeway. Syn oil.
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Unread 10-10-2010, 07:52 AM   #56
Ajaxxx
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Below is my Iowa to Black Hills Jamboree trip fuel economy breakdown.

Driving style: Gentle, using cruise control as much as possible.

Weather - Cool, rainy, strong side and head winds in South Dakota on first half of trip.

Speeds: Ran as close to speed limit as possible, except in South Dakota where we ran 70mph.

Fuel: Ran 87-89 octane with most tanks at 10% ethanol.

Cargo - Two passengers plus about 150 lbs in cargo.

Tires - 29psi

Rig options - see signature for other accessories

Elevation change - Iowa is about 1000 feet, went to Custer and Deadwood where elevation is about 4000-6000.

Starting odometer - 2056
Ending odometer - 3795
Total miles - 1739


Superchips - Used 87 octane tune for all tanks except as noted.

Tank 1 - 14.36mpg
Tank 2 - 14.38mpg
Tank 3 - 13.04mpg
Tank 4 - 13.73mpg
Tank 5 - (trail riding all day) - 8.44mpg
Tank 6 - (trail riding all day) - 5.76mpg (used crawl tune)
Tank 7 - 14.15mpg
Tank 8 - 11.36 mpg
Tank 9 - 13.46mpg
Tank 10 - 15.03mpg (used mileage tune)

Overall for trip - 12.98mpg

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Unread 10-10-2010, 08:05 AM   #57
0sevenjk
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07xu stock. 17mpg. 100% city driving.
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Unread 10-10-2010, 09:48 AM   #58
ak67sd
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Stock '08 w/3.73 gears... I get 14-15 in the city (lots of stop and go, and all under 40mph) and 19-20 on the highway...

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3.8L V6 , 35" tires , JK Wrangler , mileage , miles per gallon , mpg , wrangler
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