Originally Posted by dazdconfsd
In that bus it was ~65 was most of the time because the fuel savings wasn't worth the time savings. Did go up a bit if I kept it more 55mph, which makes sense with the wind resistance.
My parents went across country with my current one last year and it has cruise along with being diesel, but my dad said when it was ~45 on back roads he was getting a couple mpg more.
General rule, every one mile per hour over 55 is going to affect fuel economy by approximately 2%, especially with a flying brick, even with a diesel. Example, at 67 mph governed speeds pulling 45,000 lbs of freight, I got around 6.4 mpg with a 15-liter Detroit. Same truck, same load, same conditions, mpg increased to 6.9 by dropping speed to 63.
6.4 ÷ 6.9 = 0.928, or, 6.4 × 1.08 = 6.9.
4 mph difference in speed = 8 % improvement in economy, thus proving the 1 mph = 2% rule.
Keep in mind, this is in reference to 80,000-lb trailer trucks with 15-liter engines, not a 12,000 pound school bus with a 6-liter engine. But I have noticed similar or better results even in my personal vehicle. At 68 mph, my 2.5-liter Camry averages 38.4 mpg. At 63, it averages 43.3. Roughly 12% better mpg, for 5 mph difference.
38.4 ÷ 43.3 = 0.886, or 38.4 x 1.12 = 43.1 (43.08 rounded)
The same change makes 4% more of an improvement in a smaller vehicle with a smaller gasoline engine. I imagine this change would work out to about 3-4% increase in the right conditions, as school buses are characteristically geared lower to help the reduced power pull increased weight.
So a bus that gets 8-9 mpg at 65 mph, would likely get 9-10 mpg at 60 mph. I calculate this based on the 429 Ford gasser I previously owned getting about 6.5 at its governed speed of 68. A similar diesel rig would likely have gotten about 7.5 - 8.0 at the same governed speed. Your results may vary. But remember, the vast majority of skoolies were never meant for highway use.
3-4% may not seem like enough of a benefit for most people. But if you're driving across country, it adds up. For example:
Today's average US price for diesel was $2.41 / gallon.
2000 mile round trip at 8.5 mpg = 235.3 gallons.
At 9.5 mpg, the same 2000 miles uses 210.5 gallons.
That's 24.8 gallons less.
24.8 x 2.41 = $59.77 savings.
This is, of course, assuming that my 3-4% is uncannily accurate, which it may not be. It may well get 9% better, or 15% better. Smaller engines tend to use less fuel than larger ones highway cruising, while larger ones outshine smaller ones on back roads and hills.
While $60 may not seem like much for 2000 miles, this will likely translate to savings down the road through fewer and less expensive repairs from not stressing the engine as much. More speed translates to more heat generated, especially with turbo diesels, and automatic transmissions do NOT like heat. It kills them quicker. And for those with rear-engine pushers, keeping your rig running cool is already a challenge.
One of the major factors here is that running the speed limit with a larger, heavier vehicle such as a bus or a truck, you are more likely to have to make frequent adjustments to speed for traffic conditions and errant drivers to maintain a safe following distance (15 seconds for 18-wheelers, and I would say a solid 10-13 seconds for a school bus, maybe closer to the 15-seconds, as weight helps braking to a degree, and they aren't as heavy as an 18-wheeler).
But overall, I found that driving a few mph slower made a world of difference, because I could just set the cruise and let it eat. And steady cruising speed is critical to fuel economy, likely the major factor. But a lower cruising RPM is a good start. This, and driving conditions, driving habits, load, hills, even weather can affect fuel economy.
Also, diesels will generally do better than gassers on back roads and such because they have more torque at lower RPM. This means they don't have to work as hard as a comprable gas engine, therefore they are more efficient. You'll notice that a gas engine will run up a hill a lot easier with a comparable and doable load than a diesel will, but you'll notice also that they have to rev themselves silly to do it, burning more fuel.
This is primarily because diesels are big on torque in comparison to a gas engine being big on horsepower at a given RPM with comparable displacement and load. But while a gas engine has to rev up to make that horsepower, a diesel doesn't have to make the same RPM to make peak torque. Remember, torque is what does you're pulling, horsepower is simply how fast it falls. And these two factors, in comparison, factor greatly into fuel economy, as well as how you drive it.