How fast do you drive your Class A?

The "sweet spot " idea is over rated in my opinion. I understand "torque curve" but I don't think it's necessary for the motor to be developing maximum torque just cruising down the road. It makes more sense for racing or pulling tough grades. And I doubt best MPG will be obtained at an RPM high enough for max torque but rather at lowest RPM possible without causing downshift. You could cruise along at 65 and convince yourself that's the sweet spot but slow down to 55 or 60 and it will cruise along just as well. Since I'm not an expert I would like to hear from some automotive engineers on this topic.

55 in California, in the real world 60-63. Going up grades whatever she wants to do. I'm always towing. Over the past 88,000 miles I've averaged 7.3 mpg.

Dog Trainer wrote:
wind resistance against the big box we are driving will affect the mpg to the greatest degree. I drive about 65 most of the time and on a windy day where I have head on wind or a cornering wind I drop down a bit to save fuel.

Me too! Dang headwinds are about the worst for MPGs.....

WW

60 MPH +/- 2 MPH. I usually blend in with smooth truck drivers.

1988 to 1995 7.4 liter-454 CID V-8 290 hp (220 kW) @ 4000 RPM, 385 lb·ft (522 N·m) @ 1600 RPM

Easy to find at https://en.m.wikipedia.org/wiki/Chevrolet_C/K#454SS_.28RPO_code_.3D_B4U.29 Note max torque stated at 1600 rpm

wind resistance against the big box we are driving will affect the mpg to the greatest degree. I drive about 65 most of the time and on a windy day where I have head on wind or a cornering wind I drop down a bit to save fuel.

> It may be a bit just in my mind, but I feel much more in control at 62

Even if it is just in your mind, that sounds like as valid a reason as any to choose your speed. Driving a big rig is tough enough, if something as harmless as going a little slower reduces your feelings of stress, it would be worth it to me.

Thanks for the peak torque information. I'll try to hunt for that info for the V-8 7.4L OHV 16V engine in my Chevy P30 chassis.

60 MPH

mama.sylvia wrote:
Great comments and very helpful, thank you all!I particularly liked comparing your preferred speed with your rig for those of you who give it in a sig.

> 55 MPH feels dangerously slow on a 65 MPH highway

DH starts nagging me to turn on the emergency blinkers at that point. I ignore him. ;)

> Look at the engine specs for your rig. What is the point where you have best torque?

I am not sure what you mean, can you explain?

ETA: My underpowered DP was a Fleetwood Bounder, 34' - don't remember the year but probably 90-something. Good rig except for poking along.

Yes my rig is "vintage" (what a great alternative word to plain "old" lol) but I'm interested in what drivers of different types of rigs drive, and the responses here have been very instructive!

There's a torque curve. What this means is your engine is making the best amount of torque at a specific RPM. In this case at highway cruise ... The engine RPM that will produce the most torque ( power to the wheels making you move). At that " sweet spot" your likely to get the best MPG and less transmission searching. The engine isn't lugging or over working.


https://cumminsengines.com/isb67-medium-duty-truck-2013#overview


Go to ratings on this link. You'll see that at certain Rpms the best torque spec. Mine is right around 1750 - 1800 rpm. So that's about 65 mph. Less rpms your lugging - more - your burning more fuel and hotter temps, etc.

Chum lee wrote:

Bill.Satellite wrote:

Chum lee wrote:
Bill.Satellite wrote: "Speed does not determine your chances of being in an accident. Attention determines your chances of being in an accident."

I could not respectfully disagree with you more. If you check USA vehicular accident statistics you will find that excess speed along with mental impairment (lack of attention/judgement) ARE the primary causes of most accidents. They are also the major factors in the severity of injury/property damage. Weather conditions are a close third. Driving too fast in adverse weather conditions, well, . . . . BINGO!


Chum lee

You have made a giant leap from speed to EXCESS speed. Of course, excess speed is bad but that has nothing at all to do with driving at the speed limit.

Once again I completely respectfully disagree with you. The law says that the posted speed limit is considered to be "prima facia." Meaning that it is the speed limit when reasonably considered as safe.

That said, if your driving down the interstate on an icy bridge at 75 mph in tule fog (zero visibility) even though the sign says 75 mph you can be ticketed for excess speed for driving in conditions that are not considered safe. If you disagree, go argue with the judge.

I have no doubt that you are a heads up safe driver based on your comments. It's everyone else I have no knowledge of that concerns me.

Chum lee

Now you are just making the leap to silly. If the roads are icy and you own an RV you should not be driving at all. Additionally, I don't drive 70+ when there is zero visibility (again, I am not driving at all). If you are out there driving in icy, foggy conditions, I know why you are concerned!
Traveling the speed limit under normal conditions is not going to cause an unsafe condition. If there's a lot of traffic, the speed limit is not the speed you should travel. Conditions always dictate speed but I generally travel in good conditions with only a rain storm slowing me down. I have driven in the snow but I don't recommend it and I can promise you that I was NOT doing the speed limit.