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Best ratio for RV hauler


remoandiris

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The ratios are all over the place, from mid 4.55 to the low 2.--'s. Much of it depends on the engine, transmission gearing, tire size, and the weight the truck was original built to haul. On diesel engines, there is a RPM range that the engine will be most efficient at. This varies from model to model and year to year depending on emission equipment, brand, size (displacement) tuning, etc. The trick/issue/key is to be able to pull the required weight- your RV, at your desired speed -(do you drive 55mph or 70?), in the terrain you want- mountains/flat land and get the best fuel mileage possible.

Now with all that said, there is no perfect combination. The 2.67 ratio that Greg is suggesting would give that particular set up better fuel mileage while staying in the power range that the engine likes.

Our ratio in 2.87-97? somewhere in that range. It allows us to roll along at 65-68mph and be at 1500rpm which this engine likes and gets us in the 11mpg range. Unless there is a strong headwind, then it drops to frightening numbers. :o

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As stated, the correct axle ratio depends primarily on what a given engine "likes". The same engine in two different trucks with two different ECMs (say, changes between model years, or one having special "efficiency" programming) could utilize vastly different axle ratios with all other variables (combination weight/transmission high gear ratio/tire revs per mile) the same.

 

For example, a vehicle with a "regular" model engine might need a ratio of 3.25, 3.36, or even 3.58 to keep the engine in it's "sweet spot" at 65 MPH, while the same engine in a "fuel efficiency" configuration might utilize the previously mentioned 2.67 ratio, since the second engine is programmed to produce its peak torque at 1000 or 1050 RPM, while the other engine peaks at 1200 RPM. Throwing a 2.67 gear into the truck with the "regular" version of the engine could result in a truck that can barely get out of its own way, while it performs well with the proper programming.

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I will say and it is my humble opinion, when you pay around or above 100,000 USD, i do not understand why you will try to get the best gaz / diesel millage for the low millage you will drive during the year. When I drive my truck + RV I just don't think about cost, when the damn tank is empty, get it full and that it is. It's the same as going to Walmart for parking instead of camp ground.

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I will say and it is my humble opinion, when you pay around or above 100,000 USD, i do not understand why you will try to get the best gaz / diesel millage for the low millage you will drive during the year. When I drive my truck + RV I just don't think about cost, when the damn tank is empty, get it full and that it is. It's the same as going to Walmart for parking instead of camp ground.

 

While in principle, I agree with you, but the simple truth is that many of us don't have half that amount invested in our rigs. In my case, it's very close to 1/3. Hey, I'm fiscally sensitive. My wallet's thick enough, my hand refuses to write the check. ;)

 

That said, when we travel, I don't fuss with trying to drive at the speed that will net me the most mpg, rather I drive at the speed that is safest, flowing with the other large vehicles.

 

For now, we're tandem. Changing gear ratios likely would never gain us enough to be cost effective. Perhaps someday it would, but we may change trucks before then, so why bother.

 

Paying close to $8/US gal, and getting 7 mpg in the Yukon last summer was a bit painful, but I'd do it again.

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I will say and it is my humble opinion, when you pay around or above 100,000 USD, i do not understand why you will try to get the best gaz / diesel millage for the low millage you will drive during the year. When I drive my truck + RV I just don't think about cost, when the damn tank is empty, get it full and that it is. It's the same as going to Walmart for parking instead of camp ground.

We have less than a 5th -1/5- of that price in our rig, and fuel mileage does make a difference to us. Not only in price, but also in location.

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The math is relatively simple, in the end.

 

Step 1: Find out the sweet spot for fuel efficiency for your engine. It used to be 1500, then became 1350, and seems to continue to crawl its way down. Ask here/your mechanic/your dealer with the model and year of your engine to get specifics.

 

Step 2: Find out the gear ratio of your transmission's top gear. Try googling/binging your transmission model number, plus the word "ratios", and see if that's enough to get you what you need.

 

Step 3: Find out the revolutions per mile for your drive tires. Check the tire manufacturer's website; be sure to know your tire model and size.

 

Step 4: Figure out what highway speed you want to optimize for. If you're a 62-63mph person regardless of any higher limits, there you go. If you're in a 55mph area often, maybe 55 is your number. If you're going to be in Texas and would prefer not getting blown off the road by those doing 85+, perhaps something higher than 62-63 is your answer.

 

Engine RPM divided by transmission ratio gives you driveshaft RPM. Road speed (MPH) multiplied by tire revolutions per mile divided by 60 minutes per hour gives you axle shaft RPM. Driveshaft RPM divided by axle shaft RPM gives you an ideal axle ratio; you'll need to figure out which real-world ratio option (there won't be too many to pick amongst) is the best choice for you.

 

 

If your transmission has an overdrive top gear (in other words, the top gear ratio is less than 1.00) and there's a lower gear option of 1.00 (either your second-highest gear or your third-highest gear), you could try to optimize for the direct-drive gear (also known as 1.00) in slower states/areas and for the top gear in faster states/areas, if you so desired.

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The best ratio is the one that comes with the HDT that best meets your needs for things like Cab size, transmission type, mileage, etc... For the most part, the ratio should not be a make or break for a good deal. Now if you were ordering a brand new truck then spec out the ratio that the dealer recommends for the features you have ordered.

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You can do a lot with just changing tire sizes. When I got my truck it had 24.5 Low Profile. When it was time to replace the drive tires I went with the 11R24.5 which are much taller. It increased my road speed 5mph, but it also makes it hard to drive 55mph on the Iowa secondary roads in the auto shift mode. Any small incline and the truck down shifts to 9th and with the rolling hills of Iowa on some roads it's shifting a LOT. Put it on the interstate with a 70mph speed limit and it takes a very large hill for it to down shift. It's all about finding the speed you feel comfortable driving at the most.

 

Rod

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If I'm understanding the "helpful" replies correctly, the factory does the heavy lifting when it comes to putting the best engine, trans and gearing into a given truck and I shouldn't worry too much about it when it comes to buying. Is that about right?

 

The dealer putting your truck order together is likely the one picking the ratio (using the factory's computer program) and they are going to do that using the weights and speeds you provide them. If you don't give them any input they will likely just go with whatever the default is which won't likely be what you'd be happiest with.

 

We looked at three speeds when ordering our MDT, 55 MPH towing on back roads, 65 MPH towing highway cruise speed and 75 MPH our bobtail running speed. We made sure the truck had a transmission gear that would work out well in each situation by picking a compromise gearing for the differential.

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We have not pulled our DRV yet with but we spec'd our 2016 Volvo 780 with a 2.67 ratio, 405 hp and the XE package from the factory. The maiden voyage of nearly 1200 miles bob tailing we got approximately 19 mpg. We ran between 65 - 72 and the rpm's between 1100 - 1350.

We ran mostly interstate/highways and only idled for approximately 4% of engine run time.

Volvo factory engineering determined that this would be optimal for what we projected to use our truck for but the true performance indicator will be when we hit the mountains with a full load. :o

 

We don't expect any issues based on the factory feedback.

 

David

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