PORTLAND, Ore. — Daimler Trucks North America (DTNA) is questioning recent test results released by Navistar, which showed its International ProStar+ and MaxxForce 13 engine consumed less total fluid, including diesel exhaust fluid, than competitive models.

Specifically, Navistar claimed its truck and engine combo consumed 0.9% less total fluid than the Freightliner Cascadia with Detroit Diesel DD15 engine using selective catalytic reduction (SCR) when tested by Transportation Research Center. It also consumed less total fluid than a Kenworth T660 with a 15-litre ISX, according to the test results.

However, the fact Navistar pitted its 13-litre engine against competitive 15-litre products makes the comparison unreliable, Daimler said in statement.

“We run stringent fuel economy tests at DTNA which are both accurate and substantiated,” the company said. “We test back-to-back componentry which is comparable from both a truck and an engine perspective. Ratings, displacements, truck configuration and more are matched to achieve valid results. The combination chosen by our competitor does not comply with these basic premises for proper engineering work and thus doesn’t provide a trustworthy result.”

Freightliner officials also said the 440-mile test run was not long enough to deliver reliable results, since the short distance doesn’t accurately reflect diesel particulate filter (DFP) regeneration cycles.

The company also said it’s looking forward to conducting its own tests.

“We’re eagerly anticipating acquiring an EPA2010-certified series production 12.4L MaxxForce engine in order to run our own comparison study,” the company said. “It is neither appropriate nor credible to compare the 12.4L MaxxForce ‘mystery’ engine with proven technology available in the market.”

Daimler noted it has received more than 25,000 orders for EPA2010-compliant trucks.

And as a parting shot, the company added “We are unaware of any announcements made by Navistar on their sales track record in this category to-date.”