The 'sweet spot'....

[Mr_Mark1]

I've seen several posters indicate that they have found the 'sweet spot' with their engines.

How do you know?

My thoughts are that the sweet spot is traveling in the highest gear at the lowest speed?

How do you determine your 'sweet spot' for your engine?

Thanks,
MM.

[JetAonly]

Mr.Mark wrote:
Ok, I was out in the motorhome today but was only able to drive 55 mph. It was a short trip and traffic was heavy and it was very windy not to our advantage.

So, at 55 mph I'm running 1,200 RPM's in 6th gear.

MM.

That sounds about right. I asked an OTR driver what RPM he used to shift, he said that the newer motors are designed to pul down to 1200 rpm before a shift. That was the way the company wanted him to operate it, too.

[Gjac]

As others have said the highest gear at the lowest rpm on a flat will give you the best MPG sweet spot. When you factor in head winds, tail winds, hills the speed and rpms vary to maintain the highest gear. I find that by driving with a vacuum gauge this will compensate for these conditions. The highest vacuum reading will give the best MPG for each condition. Also I know when the transmission is about to downshift so to maintain the highest gear I stay over that vacuum reading.

[Mr_Mark1]

Ok, I was out in the motorhome today but was only able to drive 55 mph. It was a short trip and traffic was heavy and it was very windy not to our advantage.

So, at 55 mph I'm running 1,200 RPM's in 6th gear.

MM.

[JetAonly]

wolfe10 wrote:
Reality is the "best MPG speed" varies with coach, gearing and terrain.

On flat ground, the lowest RPM you can hold high gear is the best "reasonable" speed. That will be right at peak torque RPM for your diesel.

When I say "reasonable" in most coaches peak torque RPM in 5th gear (MUCH less aerodynamic drag at lower speed) will give better MPG, but is excruciatingly slow!

But, that speed (peak torque RPM, 6th gear) in rolling hills will cause a downshift on each hill-- so a little higher speed that will allow you to stay in high gear is better.

I agree. 1450 is max torque on my ISC. 67mph is 1750 in 6th. A down shift leaves the RPM at 2050 right at peak HP.

[time_to_go_now]

My 6.7L Cummins is 62 mph. It is just slightly faster than the final shift into high gear. Loping along at 1,600 RPM's. I love to set the cruise control right there and go for miles and miles and miles. Does not matter if I am towing anything or not. Just the mileage will vary.

[gemert]

DSDP Don wrote:
I know technically, the sweet spot my be described in torque, horsepower and mpg numbers, but I've always felt there was a speed where a particular coach settled in and seemed to want to run.

On my 1990 Class C, it was 63 mpg. On my 05 Diplomat, it was 58. On my current coach, we often run about 60, because others we travel with like that speed, but 63 mph is where it settles in. I think it's different for every coach.

Mine seems to love 75! Handles well and really nice ride. My sphincter on the other hand gets tired of puckering!!

[DSDP_Don]

I know technically, the sweet spot my be described in torque, horsepower and mpg numbers, but I've always felt there was a speed where a particular coach settled in and seemed to want to run.

On my 1990 Class C, it was 63 mpg. On my 05 Diplomat, it was 58. On my current coach, we often run about 60, because others we travel with like that speed, but 63 mph is where it settles in. I think it's different for every coach.

[tinkerer]

Mr.Mark wrote:

tinkerer wrote:
A lot depends on head winds, but with my C7 Cat it seems that 72 mph is it's sweet spot as it will cruise over the hills with not downshifting especially when I run in the economy mode on the transmission,That is around 2000 rpms.

Tinkerer, are towing when you are going 72 mph?

MM.
24 ft car trailer with a Honda CRV and Polaris trailer. If the ground is more level I like to cruise at 65. If I am bucking a strong headwind it might be 55. I like to see my turbo boost 25 or less when crusing.

[Mr_Mark1]

Crespro wrote:

wolfe10 wrote:
Reality is the "best MPG speed" varies with coach, gearing and terrain.

On flat ground, the lowest RPM you can hold high gear is the best "reasonable" speed. That will be right at peak torque RPM for your diesel.

When I say "reasonable" in most coaches peak torque RPM in 5th gear (MUCH less aerodynamic drag at lower speed) will give better MPG, but is excruciatingly slow!

But, that speed (peak torque RPM, 6th gear) in rolling hills will cause a downshift on each hill-- so a little higher speed that will allow you to stay in high gear is better.

Good question -- max torque first, then top gear, then sufficient kinetic energy to top the hills at your chosen minimum speed (I prefer 50 mph). On fairly flat terrain, that is 65 mph for me. If there are major hills, it is 68 to 70 mph.

Most of my driving is at 65 mph. But with 400 HP and 1200 ft-lbs of torque pulling 36,000# (coach with Enclave), I probably have slightly more power to weight ratio than many on this list.

Mr. Mark, your Volvo and Prevost are quite heavy in proportion to both HP and torque. I suspect you may be 2 mph higher than me based upon those parameters.

Crespro, I will be in the coach next week taking it for a chassis lube and a replacement fog light cover (rock hit). I'll make note of my RPM's at 55, 60, 65 and 70 mph.

We weigh 55,500 lbs., 500 hp, 1750 lb. ft. of torque and pull a 3,500 lb. CR-V.

If anyone is familiar with the Chiriaco Pass on I-10 heading east out of Palm Springs, CA, we pull that hill at 52-53 mph.

MM.

[Kayteg1]

I have ScanGauge in my pickup and car trip computer on our wagon.
I make comparison how much saved trip time cost me in fuel.
For example wagon at 65 mph makes 26 mpg. At 80 it makes 22mpg.
I calculated that on our routine 550 miles trip to Vegas going faster will give me additional 1 hr of vacations for $7 in fuel cost. Since our vacations cost us average $50 for each active hr, gaining 1 hr extra for $7 is good deal.
On my pickup going from 70 to 80 mph on the trip would cost me well over $50 in fuel cost, so I drive slower.
In motorhome you have more factors. While driving heavy set, you might notice that for example at 60 mph the transmission will have to downshift on lot of grades, what is killing economy pretty bad.
Going at 65 mph will make grades without downshifting.
So minimally lower economy on flat land can turn fuel savings on hilly highway.
Rule of thumb is that the best rpm for cruising are those between max torque and max HP

[subtroll]

At camp freightliner, I was informed that the sweet spot for my 3126 Cat was at 1750 RPm (62 Mph). Prior that I did most of my driving at 57-58 mph which is a little low for 6th gear. Now I drive at 1750 RPM in Economy mode. Over a few thousand miles I see perhaps a slight (0.1-0.2) increase in MPG.

[Crespro]

wolfe10 wrote:
Reality is the "best MPG speed" varies with coach, gearing and terrain.

On flat ground, the lowest RPM you can hold high gear is the best "reasonable" speed. That will be right at peak torque RPM for your diesel.

When I say "reasonable" in most coaches peak torque RPM in 5th gear (MUCH less aerodynamic drag at lower speed) will give better MPG, but is excruciatingly slow!

But, that speed (peak torque RPM, 6th gear) in rolling hills will cause a downshift on each hill-- so a little higher speed that will allow you to stay in high gear is better.

Good question -- max torque first, then top gear, then sufficient kinetic energy to top the hills at your chosen minimum speed (I prefer 50 mph). On fairly flat terrain, that is 65 mph for me. If there are major hills, it is 68 to 70 mph.

Most of my driving is at 65 mph. But with 400 HP and 1200 ft-lbs of torque pulling 36,000# (coach with Enclave), I probably have slightly more power to weight ratio than many on this list.

Mr. Mark, your Volvo and Prevost are quite heavy in proportion to both HP and torque. I suspect you may be 2 mph higher than me based upon those parameters.

[Mr_Mark1]

tinkerer wrote:
A lot depends on head winds, but with my C7 Cat it seems that 72 mph is it's sweet spot as it will cruise over the hills with not downshifting especially when I run in the economy mode on the transmission,That is around 2000 rpms.

Tinkerer, are towing when you are going 72 mph?

MM.

[tinkerer]

A lot depends on head winds, but with my C7 Cat it seems that 72 mph is it's sweet spot as it will cruise over the hills with not downshifting especially when I run in the economy mode on the transmission,That is around 2000 rpms.