Cruising Speed

[Walt Graham]

One thing I have noticed in some of the posts is that while a majority appear to stay below 75 mph a fair number seem to exceed the 75 mph max speed shown on the tire sidewall. It's obvious that they don't understand the 75 mph max and the reason for it because if they did understand after talking to any tire manufacture they would slow down. The common enemy of any tire is the resulting heat caused by speed that in fact shortens tire life thus allowing the tire to fail. Now this failure maynot happen the next day or month after a day spent doing 80 to 85 mph but the tire life has been shortened thus allowing a weakness in the tire structure be it sidewall or tread, To understand this you need to understand how a tire is manufactured and cured . Applying this secondary heat cycle by exceeding 75 mph is just pushing your luck when you take the attitude of what the heck 85 to 90 mph gets me there sooner of course not taking into account the time to change the blow-out or that your MH is now on its side in the median !!!!!!!!!

[Walt Graham]

Just as a sidebar - How about the guy with the 5th wheel or TT that has tires rated for 65 mph on the sidewall that's doing 80 mph or better then throws a fit when a blowout rips and damages the side of the RV.... all this and that moron just passed me a mile ago !!!!!

[BamaBob]

Originally Posted by Walt Graham

One thing I have noticed in some of the posts is that while a majority appear to stay below 75 mph a fair number seem to exceed the 75 mph max speed shown on the tire sidewall.

I am curious - what type of a RV do you have? Our Michelin tires (which are really OTR Truck Tires) have no speed limit on the side wall.

Having said that, I never exceed 65MPH - my Diesel milage goes into the toilet when I do. If I keep it at 63MPH on flat/Hilly terrain I can usually see 10MPG towing our Jeep.

[richard5933]

Originally Posted by BamaBob

I am curious - what type of a RV do you have? Our Michelin tires (which are really OTR Truck Tires) have no speed limit on the side wall.

Having said that, I never exceed 65MPH - my Diesel milage goes into the toilet when I do. If I keep it at 63MPH on flat/Hilly terrain I can usually see 10MPG towing our Jeep.

Most tires on motor homes will be 'L' rated, which is 75 mph. There may or may not be a marking on the side of the tire somewhere with this 'L' on it. The best way to find your specific tire's speed rating is to simply go to the manufacturer's website and look for the tire specs on your tires.

There are some motor homes out there with 65 mph max tires, but that's not the most common.

ST trailer tires are, by industry standards, rated for 65 mph. That's it, just 65 mph. There are some tires with higher limits, I'm sure, since nothing is an absolute.

Since a 75 mph speed rating means that 75 mph is the upper limit, I try never to push that limit unless circumstances dictate, and then only for a short while.

IMO, it's always good to leave a little head room on any system that has limits. Run at the limits all the time and things can go wrong in a hurry.

Also, just because the tires are speed rated to 75mph, it doesn't for a second mean that the rest of the rig (brakes, steering, coach suspension, trailer, tow hitch, etc) is safe to operate at 75mph.

Ever wonder why all the U-Haul trailers have a seemingly low speed limit painted on their sides? Because that is their recommendation for a safe operating speed. We all need to determine the safe operating speed for our rigs taking into account all the various factors.

[Rob Mitch]

I like 62 to 68 for fuel milage in my Monaco. Get about 8 mpg . Maybe the PO meant KPH ?

[Skip426]

Originally Posted by Rob Mitch

I like 62 to 68 for fuel milage in my Monaco. Get about 8 mpg . Maybe the PO meant KPH ?

I do hope that members are reading my post #15 before responding .

The PO and the current owner are in Ontario , Canada , not Ontario California .

The mileage calculation; by the previous owner; may have been done to the larger 160 floz. Imperial gallon ; so to factor to the 128 floz American gallon , the MPG claim must be reduced by 20% . So 12 MPImperialGallon , becomes 9.6 MPAmericanGallon.

Many members ; including myself; have replied with MPAG numbers in the 9.5>10 range , and most will admit conditions have to be just right to achieve the good numbers.

Now if every one is ready we can start doing the conversion from the current Canadian metric system of "liters of fuel used in 100 kilometers " to something drivers over the age of 30 can relate to.

[MrGoodwrench]

Originally Posted by promaster18

Around 24 -28 mpg at around 65 -75 mph on our v6 gasser

Excellent mpg and good retirement cruising speed and we can park anywhere

“24-28 MPG at 65-75 MPH”??

You must be in some sort of minivan, and not trying to propel some sort of huge box with associated weight/wind resistance down the highway. Most people driving the Sprinters, Transits, and Ram conversions are lucky to squeak by 14-15 MPG...

I barely get that kind of fuel economy out of an empty 3900 lb 2 wheel drive crossover SUV.

[jdw1]

What rpm are you folks with the DP at 65 mph?

Thanks

[Dana Anderso]

Check out lehtos law on utube. Most times it’s a scam.

[Skip426]

Originally Posted by jdw1

What rpm are you folks with the DP at 65 mph?

Thanks

RPM , vs , speed; is a function of transmission , differential gearing, and tire size . The bigger engines that develop power at lower RPM run a different gear than the smaller ( 5.9 and 6.7 Cummins in particular ) diesels that rev higher .

My Cat. with a 4.63 ratio and 38" tall 22.5" tires turns up 1850 RPM in sixth gear in the 3060 trans with sixth being second overdrive gear. To do 65 MPH.

My best fuel economy is 58>62 turning 1650> 1750.; 2500 is the engine's RPM limit .

[bucks2]

Skip426 above has a good reply to jdw1. RPM of the engine all depends on the particular MH. My C9 turns right at 1400 rpm at 65 mph in 6th gear (2nd over) It is designed to lug down to 1350 before shifting. I can't get 6th gear until 63 or 64 mph. I run 66 or 67 mph regularly just to stay in 6th while going over the little hills we have out west.

And that brings up another point that Richard5933 makes. What is the limiting point of your MH? He writes: "IMO, it's always good to leave a little head room on any system that has limits. Run at the limits all the time and things can go wrong in a hurry.

Also, just because the tires are speed rated to 75mph, it doesn't for a second mean that the rest of the rig (brakes, steering, coach suspension, trailer, tow hitch, etc) is safe to operate at 75mph. "

And I agree about running against the limits on items that aren't designed to run at the limit. Your diesel engine for example has a limit built into the injector pump to keep it from running too fast (high rpm). You can floor the throttle with no load and it won't exceed it's preset safe limit. A gasoline engine though, has no such limiter.

So I'm asking myself what is the limit for my motorhome? What speed did the designers have in mind? They designed in the stability, the strength of the axles, the horsepower, the steering ratio, the brakes, they know the aerodynamic design. They took all those factors into account and then geared my rig to top out at 108 mph. I stay well under that. Maybe my tires ARE the limiting factor.......

[richard5933]

Originally Posted by bucks2

...So I'm asking myself what is the limit for my motorhome? What speed did the designers have in mind? They designed in the stability, the strength of the axles, the horsepower, the steering ratio, the brakes, they know the aerodynamic design. They took all those factors into account and then geared my rig to top out at 108 mph. I stay well under that. Maybe my tires ARE the limiting factor.......

Aside from your tires, I'd suggest the two other limiting factors of concern would be the brakes and handling/steering.

Since we rarely drive only in a straight line, just imagine how well your coach would handle doing emergency evasive maneuvers at 108mph? Or even at 88mph? These things are not race cars, and while your coach may be able to reach 108mph that doesn't mean it's safe to do so.

Guessing that you have drum brakes on the drive axle, maybe on the front axle as well. Whether air or hydraulic, your brakes are going to struggle to bring that coach to a safe emergency stop when going 65mph. Can't imagine how much longer the stopping distance would be at 108mph, but I don't want to be the vehicle in front of you having to find out.

[tropical36]

Originally Posted by bucks2

Skip426 above has a good reply to jdw1. RPM of the engine all depends on the particular MH. My C9 turns right at 1400 rpm at 65 mph in 6th gear (2nd over) It is designed to lug down to 1350 before shifting. I can't get 6th gear until 63 or 64 mph. I run 66 or 67 mph regularly just to stay in 6th while going over the little hills we have out west.

And that brings up another point that Richard5933 makes. What is the limiting point of your MH? He writes: "IMO, it's always good to leave a little head room on any system that has limits. Run at the limits all the time and things can go wrong in a hurry.

Also, just because the tires are speed rated to 75mph, it doesn't for a second mean that the rest of the rig (brakes, steering, coach suspension, trailer, tow hitch, etc) is safe to operate at 75mph. "

And I agree about running against the limits on items that aren't designed to run at the limit. Your diesel engine for example has a limit built into the injector pump to keep it from running too fast (high rpm). You can floor the throttle with no load and it won't exceed it's preset safe limit. A gasoline engine though, has no such limiter.

So I'm asking myself what is the limit for my motorhome? What speed did the designers have in mind? They designed in the stability, the strength of the axles, the horsepower, the steering ratio, the brakes, they know the aerodynamic design. They took all those factors into account and then geared my rig to top out at 108 mph. I stay well under that. Maybe my tires ARE the limiting factor.......

With our C9, I'm probably about 1600rpm at 57mph and usually stay in 6th gear over most rises and hills. It drops into 5th at about 53mph and I'm most always in eco mode. Not that it makes much of a difference.
I weigh in at about 16 tons with GVWR being 17 tons, but I'm also towing, most all the time.

[A Traveler]

Originally Posted by jdw1

What rpm are you folks with the DP at 65 mph?

1,700 RPM.
400 hp Cummins ISL, Allison MH3000 transmission in a Freightliner chassis.