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We are very close to purchasing our first HDT. Looking for a Volvo 730 with an I-Shift and between 450-500hp. Came across one today with Super Single rears. Salesman said it got better mileage. What do the people in the real world say? Also, anyone willing to help with the gear ratio choice? I have heard all the comments regarding personal preference and depends how much weight you tow or your driving habits. Those of us who must purchase on theory, DO NOT have a personal preference. Our new trailer will weigh @ 34K lbs, I normally drive around 65 mph when conditions are optimal. This particular truck has a 2.64 rear end with the XE package but we are also looking at one with a 3.08 (ecoTorque package). Choosing a 730 over a 780 mainly due to having to use it as a daily driver for a few years until someone builds something under 104" in length. Smart car is not an option. I understand there will be more bugs on the trailer until I install a fairing. Are there other considerations like reduced fuel mileage (aerodynamics) that should be considered.

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Super singles are ok if you plan to leave it tandem. I understand they are not recommended if it is a single rear axle. They are suppose to get better mileage. We think they look cool with nice polished aluminum wheels!

 

Now the draw backs we have heard: Maybe be hard to find a replacement if you get a flat in a remote area, you can not use a steer axle tire on the rear rims, traction on wet grass and or snow/ice could be an issue.

 

Regarding the rear end ratio, if you were ordering a new truck then it makes sense to order one with a ratio that works for you as recommended by the engineers helping you spec it out but on an used truck, that would be the least of our worries. The affect on the mileage and your cost savings will depend on how much you plan to drive each year. Let's say you get 8 MPG with the worse of the two ratios and you drive 10000 miles in a year at $4/gallon. That would cost you $5000 in fuel. If you average 10 MPG with a different ratio it will cost you $4000 or a savings of $1000 which is a savings of 10 cents per mile. The problem is one ratio may work good on flat land, but then be bad in the mountains. If you do more mountain driving it could actually hurt your average mileage for the whole year. Also winds make a big difference in mileage. So what we are saying is there really is no right or wrong answer. It is all a personal preference. We were looking it was hard enough to just find a truck that matched all of our other criteria so we had to compromise.

 

For the height issue between the 730 & 780, again that is a personal preference. We like the 730 but some people like the 780 for the extra available storage in the top bunk. Again, I doubt you will see much issue with mileage

 

Dave

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The 3.08 will give you slightly better power to the rear wheels and will tow the 34,000 lbs better. My kenworth is 3.55. The 2.64 will possibly never see high gear. Take them both for a ride and see what the rpms are at 65 and 70 mph. I have a roll back deck and never have bugs on my RV front cap. Don't let super singles discard you. Check the date code and realize they will need to be replaced at about 7 years so you may need to negotiate that into the price or have them replaced as part of the deal.

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Besides confirming the rear ratio, the iShift comes in multiple configurations. Overdrive & Direct Drive is a couple. Most will be OD, Confirm what your getting. I have the Overdrive with a 2.64 and it purrs 65-70 in 12 gear, 55-60 mph is at the lower end of the sweet spot of the power range.

 

Super Singles, very cool, but finding one on the road may be difficult. My understating is they mainly came to be for weight savings, saving 400 lbs with dual axles. I'll let the experts chime in on the technicial fact stuff.

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This retired trucker didn't care very much for the singles. Didn't handle as well on snow/ice and did not notice any better fuel mileage. Going into Atlanta just before dark one day had one blow out. When it happened I was able to take an exit ramp that had a shoulder wide enough to park. I was hauling refrigerated liquid oxygen and had to be careful where I parked. I thought that this couldn't have happened at a better place than Atlanta because there should be plenty of these tires in the city. WRONG! I called our road service company and they were unable to find anyone that had a single. My terminal manager had to go to our company shop in Chattanooga and get one and bring it to Atlanta for a road service man to install for me. I was after midnight getting back on the road.

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Super singles are ok if you plan to leave it tandem. I understand they are not recommended if it is a single rear axle. They are suppose to get better mileage. We think they look cool with nice polished aluminum wheels!

 

Now the draw backs we have heard: Maybe be hard to find a replacement if you get a flat in a remote area, you can not use a steer axle tire on the rear rims, traction on wet grass and or snow/ice could be an issue.

 

 

Cool is always a nice feature but I lean more towards 'form follows function'. My research has led me to the some of the same conclusions you stated. Tires are not interchangeable front to back and getting a flat means you are stuck on the side of the road where you are until rescued. Sorry if this is a stupid question but unless you carry a spare then won't the same be true? I have not noticed a spare tire location on singled trucks (underneath maybe?). It also stands to reason that it might not be desirable on snow and ice due to more contact area but I have never owned or driven a 60K lb vehicle (previous setup was closer to 21K). I can't wait!! We are soooo done with the never ending construction projects. Much more excited about the truck project but trying hard not to make any long lasting mistakes.

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there should be plenty of these tires in the city.

This is a concern of mine. Did this happen recently or when they first came out? My understanding is that Michelin and others are doing a huge market push to put more of these tires on the road. We will not be driving for a living of course, but forced 'downtime' is never a fun time.

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Over on the Truckers Forum, Most Super Singles are only run on loads that are weight restricted, as in hauling Gasoline to retail where another gallon hauled is profit. They seem to agree that on anything other than a dry road they are slippery, and most would never run them as a rear single axle.

As far as gear ratio, the small difference in ratios is not worth worrying about. When the trucks were originally spec'd, the optimum transmission ratios and gearing was selected for mileage based on where the truck was operating. Unless you are moving 100,000 a year, ratio isn't going to make a difference. Our truck has a 2.84 rear, with a 10-speed trans. Generally, the more gears you have the higher the ratio can be without hurting mileage. The 2.64 coupled with a 13sp autoshift should in theory, give good low end power, and decent mileage.

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Personally, if I bought a truck with super singles I would negotiate removal/replacement to standard rubber as part of the deal. To virgin tires. For all the reasons mentioned.

 

Michelin now says you can run super singles on a single rear axle. But I would not. Mainly for traction reasons.....but also because if you lose one you are stopped right there. Traction in iffy conditions is already a major issue with these trucks....you do not want to aggravate the situation.

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A little different direction here. I would not run singles for the simple reason that I'm hauling my home and all my belongings back there. A blow out could roll them all in a ditch and that would be a very bad day for this full timer. As for being on ice and snow. If that happens I have screwed up badly. I left Michigan for Texas 7 years ago because I never want to experience snow again.

 

Brad

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Regarding the rear end ratio, if you were ordering a new truck then it makes sense to order one with a ratio that works for you as recommended by the engineers helping you spec it out but on an used truck, that would be the least of our worries. The affect on the mileage and your cost savings will depend on how much you plan to drive each year. Let's say you get 8 MPG with the worse of the two ratios and you drive 10000 miles in a year at $4/gallon. That would cost you $5000 in fuel. If you average 10 MPG with a different ratio it will cost you $4000 or a savings of $1000 which is a savings of 10 cents per mile. The problem is one ratio may work good on flat land, but then be bad in the mountains. If you do more mountain driving it could actually hurt your average mileage for the whole year. Also winds make a big difference in mileage. So what we are saying is there really is no right or wrong answer. It is all a personal preference. We were looking it was hard enough to just find a truck that matched all of our other criteria so we had to compromise.

I'm struggling to comprehend how one ratio would be good in the flats and bad in the mountains. That's what the transmission is for: to adapt ideal engine RPM to current road speed.If it's an iShift, every gear step is almost exactly the same (one of the advantages of an iShift over the traditional 9/13/18s out there), about 27%. If you cruise in 11th and find a hill that needs a bit more power, it'll drop to 10th; a different truck with the same (everything else) but a different axle ratio that cruises in 12th at the same speed would merely drop to 11th (aka drop one gear, the same action as before) and climb the hill the same.

 

There's a tiny percentage of energy loss possible depending on the actual gear arrangement within the iShift (i.e. if you have an OD box vs. a DD box), but most likely so minute that you wouldn't be able to measure it well.

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I'm struggling to comprehend how one ratio would be good in the flats and bad in the mountains. That's what the transmission is for: to adapt ideal engine RPM to current road speed.If it's an iShift, every gear step is almost exactly the same (one of the advantages of an iShift over the traditional 9/13/18s out there), about 27%. If you cruise in 11th and find a hill that needs a bit more power, it'll drop to 10th; a different truck with the same (everything else) but a different axle ratio that cruises in 12th at the same speed would merely drop to 11th (aka drop one gear, the same action as before) and climb the hill the same.

 

There's a tiny percentage of energy loss possible depending on the actual gear arrangement within the iShift (i.e. if you have an OD box vs. a DD box), but most likely so minute that you wouldn't be able to measure it well.

 

The difference in the gear step of 27% is not likely to be the same as the gear ratio difference. This means your final ratio depending on what gear you are in will be different. One truck may be at the low end of the rpm range for that final ratio and another at the high end. As such they will burn fuel differently. Also throw in ideal rpm for peak horsepower and if you are not in that ideal rpm it could also affect fuel efficiency. In some cases one truck might not need to downshift at all but the other might need to if the truck senses you are not in the optimal gear range for the conditions.

 

Dave

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I'm struggling to comprehend how one ratio would be good in the flats and bad in the mountains. That's what the transmission is for: to adapt ideal engine RPM to current road speed.If it's an iShift, every gear step is almost exactly the same (one of the advantages of an iShift over the traditional 9/13/18s out there), about 27%. If you cruise in 11th and find a hill that needs a bit more power, it'll drop to 10th; a different truck with the same (everything else) but a different axle ratio that cruises in 12th at the same speed would merely drop to 11th (aka drop one gear, the same action as before) and climb the hill the same.

 

There's a tiny percentage of energy loss possible depending on the actual gear arrangement within the iShift (i.e. if you have an OD box vs. a DD box), but most likely so minute that you wouldn't be able to measure it well.

Ya nailed it. Differences are likely to be very small. Driving habits make a far bigger difference than gear ratios.

 

Besides, we're talking pennies of fuel in a big dollar game. RVing, at this level, ain't cheap.

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The difference in the gear step of 27% is not likely to be the same as the gear ratio difference. This means your final ratio depending on what gear you are in will be different. One truck may be at the low end of the rpm range for that final ratio and another at the high end. As such they will burn fuel differently. Also throw in ideal rpm for peak horsepower and if you are not in that ideal rpm it could also affect fuel efficiency. In some cases one truck might not need to downshift at all but the other might need to if the truck senses you are not in the optimal gear range for the conditions.

 

Dave

OK, so if you cross the border from a state that has a 55MPH limit to a state with a 60MPH limit, you'd better pull to the side of the road and re-gear your axles. Heaven forbid the limit be 90KPH...

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All trucks have an range of RPM that you can run in each gear. If you are doing 55mph and the speed limit changes to 90kph (which is 56mph) you will not need to change gears or gear ratios. Now if your truck does not have another gear and you are at the high end of your RPM range at 55mph then if the speed limit changes to 60 mph you may want to continue driving at 55mph (nothing says you have to drive at the speed limit!) or risk doing engine damage. This is not likely to happen on an OTR truck as they are usually spec to run highway speeds but if it was a vocational truck like a logging or off road type they may be geared for using more torque in liue of speed.

 

Our truck which has a 10 speed Ultrashift, does not like 55mph as it wants to drop into 9th if I go slightly below that. I have to exceed the speed limit by a couple of mph to keep it from down shifting. We like 63-65mph and will drive that speed even if speed limit is 70mph, but we have reasons other than truck related to do that.

 

As others have said the fuel mileage difference is minor so find the truck you like and do not worth too much about the rear end ratio.

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We are very close to purchasing our first HDT. Looking for a Volvo 730 with an I-Shift and between 450-500hp. Came across one today with Super Single rears. Salesman said it got better mileage. What do the people in the real world say? Also, anyone willing to help with the gear ratio choice? I have heard all the comments regarding personal preference and depends how much weight you tow or your driving habits. Those of us who must purchase on theory, DO NOT have a personal preference. Our new trailer will weigh @ 34K lbs, I normally drive around 65 mph when conditions are optimal. This particular truck has a 2.64 rear end with the XE package but we are also looking at one with a 3.08 (ecoTorque package). Choosing a 730 over a 780 mainly due to having to use it as a daily driver for a few years until someone builds something under 104" in length. Smart car is not an option. I understand there will be more bugs on the trailer until I install a fairing. Are there other considerations like reduced fuel mileage (aerodynamics) that should be considered.

 

It sounds like this truck is at a dealership in the Nashville area. Back in June I drove a 730 with super singles on it. If I remember right, they were virgin Bridgestone tires.

 

Good luck with your hunt for a truck.

Al

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It is an incredible asset to have so much information at my asking. We know an HDT is the right way to go and to have so much help with the (somewhat frightening) learning curve is a blessing we appreciate more than you can know. Super singles are off the table now. stressing less about the gear ratio and focusing more on the removal of all our 'coveted stuff' we have been collecting for 31 years. Hope to have the house on the market in the next few days and join you all on the road soon! Thank you, thank you, thank you!

 

PS: I will change the picture as soon as we find a new truck.

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This is a concern of mine. Did this happen recently or when they first came out? My understanding is that Michelin and others are doing a huge market push to put more of these tires on the road. We will not be driving for a living of course, but forced 'downtime' is never a fun time.

 

Can't remember the exact year but it was quite some time after the singles hit the road (Years, not weeks/months). I'm guessing about 2005. Should not have been a problem in a city the size of Atlanta.

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