Originally Posted by cadillackid
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.