Tech Q&A: Adam Blattenberg

Dodge Fans

Dear Jim,


I enjoy your magazine, and I read every issue cover to cover. I currently own a 2005 Dodge 2500 Heavy-Duty that I bought new. The truck currently has just over 100,000 miles and is used primarily as a commuter for my business. I haven’t done any towing yet, but I do want to keep the door open to the possibility. Retirement is coming.

I love this truck and everything about it except for the 4-speed automatic transmission. I would like to install the newer 6-speed automatic that arrived in the 2007 model year, or learn about some of the other 6-speed automatics that would hold up to the rigors of towing.

Because I don’t currently use the truck to tow, I was considering re-gearing the truck from 4.10s to 3.42s for better fuel economy. Would that make much of a difference? The truck is currently bone stock, but I am willing to install some modest modifications. Again, I am primarily interested in fuel economy, but I would prefer not to install taller gearing if that takes towing off the table.

Lastly, I’ve heard that a Flex-a-Lite electric fan could improve fuel economy. What should I know about the fans to keep me out of trouble? I value your input.

Thank you,

Bruce Wender

Via the Internet


Thanks for reading. By replacing your 4.10 ratio with 3.42 gears, fuel economy will definitely increase, but your truck will loose some towing ability. The 3:42 ratio is a bit tall for towing heavy but will perform very well for everyday driving; also, you could keep your original 4-speed automatic if fuel economy is your goal.

Running 265/75R16 tires, 4.10 gears and a .75 overdrive ratio will produce approximately 2286 rpm at 70 mph. Installing 3.42 gearing would reduce the engine speed to 1907 rpm at 70 mph. Generally speaking, the closer to the engine’s torque peak it is run, the better economy it will see. And I would expect a 2-4 mpg increase when switching from 4.10s to 3.42s depending on how you drive. However, if you tow heavy, you’re better off with 4.10s, both from a power standpoint and a transmission longevity standpoint. Dodge preferred to install 4.10 gearing behind their 48RE automatic for durability reasons.

Years ago, I installed an overdrive automatic in a diesel pickup I owned at the time that had been running a non-overdrive 3-speed automatic. This coincided with the lifting of the 55-mph national speed limit. Having the overdrive made driving that truck a real pleasure. I could not hear the engine at freeway speeds, and fuel economy improved by nearly 5 mpg.

The principal advantage of the newer Aisen 6-speed is its double-overdrive, which lowers the engine speed while cruising at highway speeds, yet maintains the lower (numerically higher, like the 4.10s) differential gearing that is necessary for towing heavy. offers a variety of conversion kits for Dodge owners, including an installation package that allows installing an Allison 6-speed automatic behind the Dodge Cummins. They may also offer a similar package for the Aisen automatic.

Either 6-speed automatic would probably be an expensive and technically challenging upgrade – being an entirely custom installation, and you wouldn’t likely recover the conversion cost by a small increase in fuel economy. Most people alter the gearing (via tire size and/or axle ratio) and then find a tuner that includes both an economy and a power tune, so the engine output could be optimized for how the truck is being used. That’s what I would recommend—at least as a first step.

When compared to the engine-driven fan, electric fans improve fuel economy when they are not running. When on, they will consume some engine power due to more load on the alternator (a second alternator would be a wise upgrade when running electric fans). According to Flex-a-Lite, replacing the original engine-driven fan with an electric fan is not recommended for those trucks that run at 75% or more of their Gross Combined Vehicle Weight Rating. Electric fans are appreciated by truck owners living up north, who like the faster engine warm-ups. If towing is in your future, your original engine-driven fan will pull more air and provide better engine cooling. Derale makes a line of improved viscous fan-clutches if you are looking to replace your original. Good luck.

6.5L Overheating

Howdy Jim,

I know GM’s 6.5L diesel is now out of date, but my 1996 Suburban has been good to me and my family. We love the room and ride quality. I bought a mid-size travel trailer a couple of years back to allow my family a few weeks of vacation each summer, and I planned to tow that trailer with our trusted Suburban.

However, we soon experienced a lack of power and sometimes high engine temperatures that sometimes reached 220-230 degrees while pulling some of the longer/steeper hills. What temperature is considered too hot? What can we do to improve performance and lower the engine temperatures? I installed a new heavy-duty clutch fan on it and that helped some but was not a solution.

I was also wondering if I should be concerned about transmission temperatures while towing. The Suburban has 150,000 miles on it now, and a new fuel injection pump was installed a couple of years ago. It still runs and looks great because I try to take good care of it. I love my diesel Suburban, and any help you might offer would be great—also, your magazine is great.

Thank you,

Howard Blanche

Newark, NJ


We love diesels here too, and thanks for your comments about our magazine. What follows has appeared here before, but it remains excellent advice for 6.5 owners. We know that performance, engine cooling, economy and drive-ability are all important to all diesel owners, including us.

Power and cooling are two watchwords on the minds of many 6.5L diesel owners who use their rig to tow. There is hope. The following list of suggestions should be performed in the order in which they appear, and which begin with least-cost to more expensive.

Always concentrate on cooling first, power second. Begin by removing the top half of the radiator fan shroud, and then remove any bugs, leaves or other debris that could interfere with airflow through the ATF/engine oil coolers, the A/C condenser and the radiator. Be careful using hi-pressure washers. When used incorrectly, pressure washers can flatten the cooling fins in the various coolers and radiator, and make the problem much worse.

You mentioned installing a new viscous fan-clutch. Aside from your local GM dealer, there is a variety of fan-clutches available from a smorgasbord of 6.5L diesel vendors. Most are original equipment replacements, which unfortunately come with a fairly wide engage temperature calibration. It’s important that your fan-clutch engages before the engine temperature exceeds 210 degrees. An engaged fan-clutch is unmistakable because of the increased roar produced by the fan. If you’re not hearing the fan engage by 210, you’ll continue to have overheating problems no matter what you do or how much money you spend on aftermarket parts. A clean radiator and properly engaging fan-clutch usually solve most cooling problems. Conversely, an improperly calibrated or inoperative fan-clutch is the single biggest cause of 6.5 overheating. Discussing fan-clutch engagement with your vendor and letting him know what you expect for engage temperature can help you get the cooling performance you’re looking for. Don’t spend another dime or install any further upgrades until you’ve got the fan-clutch engaging when it should. It’s that important.

Your current fan is 20” in diameter. You can gain fan performance by upgrading to the same fan used by the 2001-2004 LB7 Duramax 6600. The LB7 uses a lightweight 9-blade 21″ diameter composite fan that is a direct bolt-on for the 1997-98 model year 6.5 fan-clutch. Most 6.5 owners don’t need to trim the fan shroud to fit the larger fan, but if you do, simply remove a portion of the inside lip of the shroud using a jig saw. The highly efficient Duramax fan (pn-15010202) is a tremendous bargain at about $40, and will increase the airflow through the radiator. Unfortunately, if you have a 1996 model year fan-clutch, the bolt pattern won’t fit the Duramax fan. Those who replace or upgrade their fan-clutch should buy one produced for the 1997-98 model year and install the Duramax fan at the same time.

Now, assuming the overheating problem has improved, you can begin preparing for a power increase. To begin, you’ll need three gauges: a 0-15 psi boost pressure gauge, an exhaust temperature gauge (EGT) and a transmission temperature gauge. These are usually installed in an A-pillar gauge pod. The boost pressure gauge helps you evaluate turbo performance, the EGT gauge helps you protect the engine from excessive exhaust temperatures, and the transmission temperature gauge helps you identify any transmission temperature problems. Those who tow with the 4L80-E automatic transmission frequently report high ATF temperatures.

Next, install a performance oriented 3-1/2” mandrel-bent exhaust system that includes a free-flowing muffler. Explain to your exhaust system vendor what you and your family expect for exhaust noise. There are many different styles and types of mufflers available for these diesels. A family-oriented Suburban usually requires a little (or a lot) quieter system than a pickup truck might.

The next step would involve a powertrain control module (PCM computer) upgrade to increase power. offers most of the items discussed here, and they can supply you with an exchange PCM. Ask for a towing or “RV” program.

The sky is the limit from here on. You can install an intercooler, aftermarket turbocharger, higher level computer programming, and internal engine modifications, which could include lower compression ratio pistons, Fluidampr harmonic damper, gear-drive timing set, and on and on. Those who go the high-dollar route usually end up with a truck or suburban that can tow with most stock diesel pickups produced in the early 2000s and do it without overheating.

Monitoring Fuel Economy

I own a 2001 3/4 ton 24-valve Dodge diesel with about 110,000 miles on it that I use to tow a horse trailer. Typical trailer weight is approximately 5,000 pounds.  The truck has the 4-speed automatic transmission with overdrive.  Some thousands of miles ago, my non-towing fuel mileage dropped from 21-22 mpg to 18, and my towing fuel mileage dropped from 12.5 to just 10.  No problems were detected by the dealer techs.  I service the fuel filter, and the engine oil/filter every 5,000 miles, and the truck runs perfectly fine otherwise.  The fuel injection pump was replaced at around 80,000 miles.  The current fuel economy really bothers me, and I was hoping you might have some suggestions to improve the situation.

Helen Sande

Via the Internet

Ms. Sande,

A fuel economy of 10-12 mpg while towing and 18-20 non-towing really isn’t terribly bad. However, if you don’t already have one, I recommend installing a fuel pressure monitor, which monitors the fuel pressure supplied by the electric fuel lift pump. The kit includes a small red dash-mounted LED light that illuminates when the fuel lift pump isn’t delivering the correct fuel pressure. The Bosch VP-44 fuel injection pump is sensitive to a loss of fuel inlet pressure. A loss of inlet pressure can affect the life of the injection pump and can produce a variety of drive-ability problems, including a loss in fuel economy. The folks at BD ( and/or U.S. Diesel Parts ( both offer a fuel pressure monitor kit that’ll do a good job for you. Good luck. DW