A Matter of Differentials

Careful planning of diff ratios and tyre sizing can pay dividends in fuel saving – Words by Dave Whyte.

As an employed driver, it’s easy to underestimate the number of factors involved in running a profitable and sustainable transport fleet. I learned this very rapidly when I was given the shove into being an owner driver, and so a company director. Every day since then, I have been looking for different ways to improve the reliability, efficiency and profitability of our business.

When it comes to survival, the transport business can be a hard industry. The number of operators fighting for available freight and decent rates has eroded any loyalty in the relationships between transporters and their clients, and between transporters fighting for work. For this reason, some operators take whatever work comes along, at whatever rate is offered, in the belief that if the wheels are turning, they’re earning. Sadly this can be a long way from the truth, so operators are left with a choice – continue to work on a shoestring budget, or find ways to improve their bottom line.

With this in mind, having run our DAF XF105 for a little over 18 months, it was time to see if there was any way we could improve our operation and reduce running costs. That’s not to say there was any problem with the truck’s performance – the average fuel consumption from day one has been 1.9 km/l (including PTO time for loading/unloading), and there hasn’t been any unscheduled downtime on account of the prime mover – but any operational saving we could make would only make our business stronger. I am a stickler for maintenance, and for paying a driver a reasonable rate, so the focus was on reducing the running costs.

We started by looking at tyres, and what we could do there to save a dollar. The dilemma of economics is that, where you might save on up-front cost, you lose out in long-term durability. On the other hand, the high up-front cost of longer lasting tyres might seem daunting, but over their life cycle the cost may be lower than the cheaper options.

Tyres also have an effect on fuel consumption, and this should be taken into account when considering the cost of your rubber. The tyres that came as standard on the prime mover from new were Michelins, on the steer and the drive. While we didn’t get such a great run out of the original steer tyres, the drive tyres lasted 355,000 km without any failures or irregular wear. This is due in a large part to the fact that the pressures were checked frequently, they were rotated regularly and the truck has had regular wheel alignment checks. I also believe that the AMT had a big impact on their life by reducing driveline shock, while the ABS and traction control eliminated any brake lock-up or wheel spin.

Based on the kilometres travelled, and the retail price of these drive tyres, they have proven very cost effective, at around 1.8 c/km. This made the decision on replacement tyres very easy – Michelins again.

In the interests of improving efficiency, we had the latest low rolling-resistance tyres fitted, those being the 295/80R22.5 Michelin XDA2+ Energy on the drive, replacing 11R22.5s as the previous standard issue, and 295/80R22.5 XZA2+ on the steer. These steer tyres have only recently been released to the Australian market, so we will watch with interest to see how they perform, and will be sure to let you know. We also had the wheel alignment checked at Heavy Vehicle Alignments in Brooklyn, and were told there was no adjustment needed. This is a credit the DAF’s current driver, and a testament to the fact that the guys at HVA have done a good job on this truck in the past.

Another option available to possibly reduce fuel burn was to change the final drive ratios. While this would usually be done before delivery of a new truck, we had no idea how the DAF would perform on the standard 3.9:1 diff ratios, and so trusted the engineers with their standard spec option. Some time on the road, and a little experience in other PACCAR powered trucks with taller diff ratios, lead me to think we could get better economy out of the MX13 if it was running at a lower RPM. The decision was made to change the final drive ratio from 3.9:1 to 3.73:1.

It was not as simple as taking out one set of diff centres and slotting in another set. The original centres were removed by our trusted local mechanic and returned to Meritor in Sunshine, where their experts removed the 3.9:1 gear set and replaced them with a 3.73:1 gear set. The three-day turn-around time included thorough cleaning, dismantling and then reassembling of the diff centres, and packaging them ready for pickup. Cleanliness aside, to the untrained eye it was hard to pick the difference between the 3.9s and the 3.73s, but the identification plates on each diff centre clearly displayed the new ratio. After returning the centres to our mechanic, they reassembled the driveline, and, having waited for the sealant to harden, added a fully synthetic diff oil to keep things running smoothly. The choice to go synthetic was made based on the condition of the previous mineral/synthetic blend when the diffs were drained at the start of the job. Another transport operator I know has had great results from this synthetic oil, and so I took his advice and went the same way.

All of this work has, so far, proven to be worthwhile. With only one week of fuel data to work from since we made these changes, there has been a noticeable difference in economy. While the tyres were due for replacement, changing the diff ratios was not a necessity, and was not a cheap exercise. Over the life of the truck, however, I believe there will be benefit to our bottom line, and it has provided some food for thought on the spec of any future trucks we might buy.

The big question you are asking is what difference has it made? Prior to these changes, as mentioned above, the DAF was running at 1.9 km/l (52.63 l/100 km), including PTO time. It is currently returning 1.95 km/l (51.28 l/100 km), also including PTO time, showing a benefit of 0.05 km/l (1.35 l/100 km). That’s an extra 50 metres for every litre of fuel we put through the truck, and a saving of almost 12 litres over a Melbourne-Sydney leg. This truck averages five legs a week, returning a saving of 60 litres and about $100 a week. It may not sound like much, but over a year that’s about $4,500 less on our fuel account. Over time, and with a bit of adjustment to driving style, this result should improve even further.

So, was the exercise a success? In my opinion, yes it was. Not only has the ratio change contributed to a fuel saving, but the truck now seems to pull better. Being a little lower in the rev range, in top gear, the engine is operating closer to peak torque, and so the truck loses less speed before peak power kicks in. This means that the unit maintains a higher speed over hills, and that in turn leads to reduced trip times. The difference in trip time is minimal, a little under ten minutes over the run up the Hume, but every little bit helps. The change has also brought some unexpected benefits, such as improved take-off when using the hill hold function. Whereas the DAF used to torque up and twist before releasing the brakes under hill hold operation, it now moves away very smoothly, as if on flat ground. This has got to be easier on the drivetrain and tyres, and add to the savings over the life of the vehicle.

We will follow very closely how these differences effect the operation of the DAF. Early indications are that we will benefit from the low rolling-resistance tyres and ratio change, with each bringing their own benefits. The tyres will be monitored, and it will be interesting to see if the newer Michelin tyres can match the excellent results of the previous generation. One thing is for sure, you will see the results here in coming issues as we try to achieve even better efficiency from the XF105.

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