Speed vs mpg

I agree speed is great when needing to get around someone especially a pack of company trucks. If you do some google searches on semi truck speed and mpg’s they nearly all show that for every 1 mile an hour increase there is a .1 mpg decrease. If you pull over dimensional or non van freight it can be more. If you get 6.1 at 70 and the engine is healthy you could get 6.6 at 65.

 

My 2012 Volvo D13 10 speed seems to get the best mpg around 58 mph with a significant reduction in mpg at 65 or higher.

 

That isn't always the case. The engine has a specified operating range, and within that operating range, there is a narrow window where it operates at peak efficiency. If your transmission, axle gearing, and tire size specs put the truck at it's optimum RPM for efficiency at 70 mph, you could actually hurt your fuel mileage (or at least not realize the gains you would otherwise expect) running it slower because the engine isn't in its sweet spot. Likewise, if the truck is spec'd to run in the sweet spot at 65, you'll hurt your fuel economy running it at 70.

 

Notice I used the word could which is a possibility, but not a guarantee.

 

Yes I am very aware of many of the variables. I spec’d my rig to run in 16 for optimum fuel economy in direct drive at 60 mph. When I have the need to run faster I can stay in 16 up to 65. When I need to run faster I put her in 17 which is a single overdrive up to 75. I don’t use it but I have 18 which is a double overdrive in the event I would need to go faster. There are too many variables to list, but it is still common knowledge that nearly every engine manufacturer equates 1 mph to .1 mpg. I do agree with you that it will not always be the case.

 

You will also see in your google search that a drivers right foot can play a huge part in fuel economy. Meaning if you ease in to your starts and ease in to a stop it can help.

 

Here are a couple manufacturers opinions about the topic. Remember that the way your truck is spec’d including things such as tires, aerodynamics, rear end ratio, freight and numerous other factors can effect your actual results. Dig around on their sites and you might learn some interesting facts.

 

That's horrible for a truck that new, I can do that or a little better with a 97 w9 with a 600 horse cat.