Best Tire size for a 2002 Freightliner Classic?

24.5's would be a little disadvantage with heavy weight or in hill / mountain country. That Detroit is happy to run 1500 to 1600 rpms, and torque falls off badly below about 1,325. If you're running light loads on the flats, I'd keep the 24.5's for sure. If heavier, you'll have to make the call. Either way it's more of a driveability issue than a fuel economy issue. My Detroit did better at 1500 RPM on fuel than 1400.

 

You’ll get better MPGs if you slow down. According to Cummins MPG Guide/Study, one of their list of “rock-solid rules” is:

Above 55 mph, each 1 mph increase in vehicle speed decreases fuel economy by 0.1 mpg.

I am not recommending driving at 55. I, personally, drive at 62.

 

7 mpg is good, but 7.5 is better. If you can make on time delivery with a slower speed and deliver the same loads per week, why not? Some people can’t stand driving slower, but the truth is, it’s all in their head. It really is.