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Old 03-25-2013, 11:03 PM   #1
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Tiffin Owners Club
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MPG Fuel Mileage Cummins ISL 450 vs. 425

We currently drive a 2004 Itasca Horizon 40', Cummins ISC 350, showing a lifetime 6.75 mpg. Recently at LazyDays we were looking to step up to a late-model Allegro Bus. All the coaches about had about 15,000 miles. Two 43' Busses read 10.0 mpg on the LCBU, while 2 40' Busses read 10.7 and 11.3 mpg. I would expect lower mpg on the 43' vs. the 40' given the additional weight. Are these realistic mpg for the new Cummins 450 engine? I read that with the ISL 425 I could expect maybe 7-8 mpg at most. I could never drive far enough to justify the $17k premium for the ISL 450, but the DW would sure be excited for us to average over 10 mpg, and be belching out clean air to boot!
Kent & Sue & Belle
2019 Allegro Bus 45OPP Lovin' it
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Old 03-26-2013, 03:34 AM   #2
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I *rarely* trust the on-board automatic MPG calculations. I always find them so far off from actual hand calculated MPG. You can figure out actual MPG is as little as 25 miles of driving (perfect distance for a test drive anyhow) - start with an absolutely full tank, note the current odometer mileage, drive approx 25 miles to the next fueling station. Refill to absolute full again. Note gallons needed and odometer mileage simple division.. voila, real MPG. Obviously 50-100 miles would be a better determination, but 25 miles shouldnt be out of the question for a nice test drive if they want to sell you a coach :-)
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Old 03-26-2013, 04:39 AM   #3
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Personally, I'd be surprised if you got significantly better mileage with the ISLs than you did with your ISC and suspect you'll likely get worse. I'd be very surprised if there's anyone that averages 10+ mpg with an ISL and I suspect that the LCBU was setup to show such good mileage (e.g. they got up to speed on the highway, reset it and drove on the highway for a while).

If you want to get 10+ mpg, get an ISB otherwise what you're seeing is probably as pretty much what you'll get in any coach (actually if you only get 6.75mpg, you probably either have a heavy foot, drive in mountains alot, use the generator a lot and / or tow a heavy load or are very heavy on the road - most people with ISCs get around 8-9 mpg).
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Old 03-27-2013, 10:07 AM   #4
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Hello we have the Cummins 450 ISL. fairly new not broken in yet at 9120 miles. Right now towing a 4900 lb or approx. Ford edge, coach loaded to 75 percent CCC our average is 8.1 to 8.3 depending on road environment. For interstate travel we run 1850 to 1950 RPM's. the milage has improved since purchase where it started at 7.9.
Clay & Pebble
2012 Providence 42 M. Spartan Chassis, 450 ISL
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Old 03-27-2013, 10:26 AM   #5
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My MPG-meter varies widely you can reset it and sample fuel use on a downhill the hills inside LD and make it read what you want.

My 450 ISL Spartan chassis under my Entegra gets about 7 mpg towing a 4500 lb toad. I sometimes get 6mpg. The only RV I ever saw get 10mpg were those little Mercedes based Class Cs or those 35' Monaco Vesta's. Unless the conditions are optimum I doubt a 42-45 tag axle can break the 10 mpg barrier.
Dave & Debbie
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Old 03-28-2013, 06:35 PM   #6
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We have been averging 7.6 on our 400 ISL for the last 3 years. We weigh about 31,000 lbs, and are towing a 4900 lb toad. This is considering we were doing some mountain driving mixed with the flat lands. We normally run about 62mph at about 1600 rpm's. Steve
07 Tiffin Allegro Bus 42QRP
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Old 03-30-2013, 11:35 PM   #7
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Originally Posted by steved44 View Post
We have been averging 7.6 on our 400 ISL for the last 3 years. We weigh about 31,000 lbs, and are towing a 4900 lb toad. This is considering we were doing some mountain driving mixed with the flat lands. We normally run about 62mph at about 1600 rpm's. Steve
I am curious about your post. I have an 05 allegro bus also and tow and enclave which is about the same as your Durango weight wise. I went to California from Arkansas so quite a bit of flatland driving mixed in with some mountain also. I am showing getting somewhere between 5. 7 and 6.1 miles per gallon. I set my cruise on 65 when I drive. I am sure that you either use an additive or do something different than what I do. I would love to see 7.6 mpg on my coach and I notice that the lifetime average shows 7.5. on mine but I am not seeing that.

As I mentioned in my email to you I have had some overheating issues where I had to go down to 2nd and third gear to keep the engine temp down going up some mountains in California and Nevada. Have you had to do that?
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