- Were the 20K mile tests conducted during the same months, with the same weather? How many gallons of winter blend fuel were burned, vs. regular #2? Same fuel station? Same fuel quality and source throughout the entire 40K test?
- Every Duramax I've owned has recorded slightly better mileage, the older it got - as long as the injectors were in good shape. Never saw >20mpg unloaded with the 2002 until it was on the second set of injectors and nearing 200K miles on the clock. How could this observation effect your results, using your "20K without treatments, 20K with" method, as a control group - especially if you started the tests when the truck was relatively new?
- If you drove your truck 20K miles without any additives, its likely you then had deposits forming - that were lowering your mileage. Start using additives - and you will likely find your way back to your engine's MPG potential, in a few tankfuls. What if you could maintain your mileage potential by adding only one bottle every 3rd tankful? 5th tankful? Only when you begin to see a consistant, repeating decrease in mileage...?
IMO - Meaningful controls for an accurate test of fuel additive ROI are impossible with one truck - or even a fleet of 'similar' trucks.
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