I'm looking to buy a 2005-2006 CTD truck currently. One concern is that I will be driving faster on the highway than the 48re and 3.73 gearing will be happy with for reasonable mileage and longevity. Many posts here refer to driving at 60 mph to save fuel but I don't want to die on the roads here. 75 mph is a suggested minimum speed, I usually drove 80 in my gas truck that I just sold. I typically towed my 9300 lb trailer at 70 mph.

What are the best options for decreasing the highway RPM's without killing the truck's ability to tow? The Gear Vendors unit looks great, but $3500 isn't for me. I'm thinking 3.42/3.55 rear end gears or taller tires. Gearing shouldn't be cost prohibitive on a 2wd truck, taller tires are usually wider and have more rolling resistance. This truck is a daily driver/commuter that will see the front end of my trailer once a month (twice if I'm lucky). I usually drive 15K miles a year. I realize by the time I pay for the mod my MPG savings will be moot, but I have to believe the lower RPM will be better for the engine long term.

Thanks,

Mitch

 

When I'm on the interstate I'll do 80 all day long. I wouldn't hesitate to drive at that speed except for the cost of fuel now. 80mph is good for around 2200 rpm in my truck. The difference between 70mph and 80mph in my truck is about 1-1.5mpg, but at almost $5/gal that starts to add up.

 

i can do 80mph at 2k rpm i have 48re, 3.73 gears and 35 in tires

 

Gearing for that speed will really hurt your towing. Since you will have only 4 gear steps, you will find yourself in a large zone of lugging the engine or spinning real fast. You should look for the gearing calculator to see the steps and speed.

 

My engine spins around 2200 rpm around 80mph too. No problems except the money flowing out the exhaust.

 

So, I'm going out on a limb here with this remark but what the hell... If diesel goes up to 7$ - 8$ a gallon, you still think you'll be able to drive 80 with everyone else that's left on the road driving the most economical speed of say, 50-60 mph? You just might be camping in that RV out in the backyard.
LoL

 

I think going to a 315 tire or maybe a a 285/75R17 would be good way to try to help mileage and keep the RPM down.

A 315 has more rolling resistance but looking at Toyo's website, there 285/75 weighs the same as a 315/70R17 BFG.

 

I have 285/70-17 BFG All Terrain tires, 2006 QC 5.9L Cummins, 48RE and 3.73 rear gear. It runs 2000 rpm at 71-72 mph. I wouldn't worry about longevity at 80 mph. These things will run forever with proper maintenance. It is going to be quite a jump in fuel costs between 65 mph and 80 mph though especially at $5 per gallon.

I'm wondering when Feds will take over speed limits from states again and set it back to 55 mph to conserve. I'd gladly go 10 - 15 mph slower if it will conserve, drop demand, and bring prices back down. Heck, I'm driving that much slower already. I remember complaining about $2.57 I paid for diesel last summer. It's amazing we're paying double the price in one year. Will it be $10 per gallon next year and all be wishing for $5 diesel?

I think the US should go to a 4 day work week too, 10 hours per day. Would cut down that much more on oil.

 

Do not worry about engine life due to rpm.The Cummins can sit on redline all day long.........2200rpm is nothing

Bob

 

I pull my 10k lb toy hauler at 75-80mph turning 2100-2200 rpms (285s, 3.73, 47RE). It's a perfect combo, even when I had the stock 265s it was about the same.

When towing heavy, the "hole" in the power curve comes around 50-55mph. At that speed, 3rd is to high of rpm for good economy, and OD is too low of rpm for good power.

 

Go for tires. slight difference in rolling resistence(if any) can not be measured. Run full pressure on tires to regain some mmilage.