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Old 09-14-2016, 09:58 AM   #31
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I absolutely agree. The factory "recommended" pressure is just a starting point. You need to know how your actual vehicles weight is distributed and with all the mods made on a typical skoolie, the real weight bias can be very different from the placard.
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Old 09-14-2016, 10:30 AM   #32
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when i ran the tires at what is stamped on the 'Carpenter' Tag i had a soft , mushy ride and a lot of rear-end sway in the bus as well as lower MPG...

and YES i set the pressures with the tires cold.. 60 degrees f, thats when I went back to the tire store and they said 'on a school bus id run 100 all the way around'.. so I do and the bus drives wonderfully.. perhaps a harsher ride but I dont have that sway and I dont go broke buying diesel

-Christopher
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Old 09-14-2016, 10:40 AM   #33
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As a side note...just about every trucker I have chatted about tires said they always run their pressure a little bit over what the weight and book numbers call for. They do it for both MPG and swear they get more miles out of the tires (?).
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Old 09-14-2016, 11:30 AM   #34
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as long as I wont blow up in a ball of fire for running my tires at 100 then im going to keeop doing so because running them at the 80 / 85 stamped on the bus label makes it a real Chore to drive... at 100 its all smiles for me..

the sidewall says max 120 PSI cold..

-Christopher
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Old 09-14-2016, 01:58 PM   #35
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Originally Posted by cadillackid View Post
as long as I wont blow up in a ball of fire for running my tires at 100 then im going to keeop doing so because running them at the 80 / 85 stamped on the bus label makes it a real Chore to drive... at 100 its all smiles for me..

the sidewall says max 120 PSI cold..

-Christopher
MIne are all at 110psi.
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Old 09-14-2016, 09:03 PM   #36
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when i ran the tires at what is stamped on the 'Carpenter' Tag i had a soft , mushy ride and a lot of rear-end sway in the bus as well as lower MPG...
Yeah, there's definitely some room to play with for personal preference. The trick is to not run 'em low, but over probably won't kill ya

Here's a nice read about tire pressure courtesy of truckinginfo.com: The Magic Number - Article - TruckingInfo.com. It showcases several opinions about tire pressure.

To get back to the original question about fuel economy, I wouldn't attribute any huge drop in MPG to that low of tire pressure. 60psi is too low and 70psi is possibly low, but if the rears were properly inflated you would likely be looking at a reduction in fuel economy of less than 3% not a whopping 30%.

Perhaps its worth focusing some attention on the turbo. Check the boots for cracks and the exhaust up-pipe for black soot (which would indicate an exhaust leak). Might be worth doing a boost pressure test. I think these engines want to be around 15-20psi of boost.
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Old 09-14-2016, 09:35 PM   #37
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Our BB's handling and comfort is optimized at 105psi steers and 85psi drives cold.

The wt. dist is 50-50 at 9700# per axle w/265/70/19.5 tires.

Our boost at max pull is just above 18psi per scan gauge D on a 5.9 Cummins.

11 mpg avg over 6000 mi.
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Old 09-15-2016, 09:03 AM   #38
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fuel pump timing can be a huge factor in power and fuel economy... it is the ignition timing of a diesel.. there are lots of articles and yest data out there on various engines as to what brings the best of both worlds...

I dont k ow on a diesel with a PCM how you check or set the actual timing.. on a mechanical its easy to do .. just remove the cover plate and look, and adjustment is usually very easy too..

I definitely didnt have 30% MPG loss when my tires were run lower so I wouldnt expect the OP to either.. now running my bus fast I definitely had huge MPG loss...
running a diesel at its max rated RPM and the extra wind resistance makes it thirsty for sure..

as for my own bus I havent experimented with running my rear pressures lower than my fronts.. I do know performing some practice near-panic stops on wet pavement that its Pretty easy to lock the rears up on my bus while still having plenty more braking in the front... im guessing the weight transfer rear to front must be quite high on a CE style bus during a stop... esp since its an empty short bus... perhaps a little lower tire pressure in the rear would alleviate that a little..

Turbo leaks will surely kill MPG like mentioned.. not only look at the pressure side of the intake but also make sure the exhaust manifold is not leaking or has loose bolts.. since exhaust spins the turbo any exhaist leaks pre-turbo will reduce the amount of boost it produces..

-Christopher
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Old 09-15-2016, 05:47 PM   #39
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The recommended inflation should be stamped on the tire. My old ones ran at 125psi and the new ones (as I recall) are about 90-100.
I've read twice now that on RV & semi truck tires the psi stamped on the tire is a minimum pressure, This was mentioned twice now in motorhome magazine.
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Old 09-15-2016, 05:51 PM   #40
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mine actually has the word Max as 125 PSI COLD.. its assumed the pressure will be measured between 50-70 degrees F (tire temperature)is what I was told..

-Christopher
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