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burnin oil · 09-24-2015, 07:55 AM

You will want a little more RPM to keep that 3.9 happy. Basically it will be lugging around without any boost while trying to move that brick. My 95 Dodge spun 2600 RPM on the interstate and the rest of my commute was like going to a rock concert. She still knocked out mid 18s for milage in a 4x4. My dads 24v 5.9 gets just under 20mpg every tank, unless towing, running 4.10s and a 6spd in a 4x4. Don't let the RPM scare you to much. Just make sure the pumps gov springs are addressed so it doesn't go into defuel.

09-24-2015, 07:55 AM	#3
burnin oil Registered User Join Date: Jan 2013 Location: Eastern TN Posts: 1,919	Re: 3.9L Cummins/Isuzu diesel swap in 89 Suburban You will want a little more RPM to keep that 3.9 happy. Basically it will be lugging around without any boost while trying to move that brick. My 95 Dodge spun 2600 RPM on the interstate and the rest of my commute was like going to a rock concert. She still knocked out mid 18s for milage in a 4x4. My dads 24v 5.9 gets just under 20mpg every tank, unless towing, running 4.10s and a 6spd in a 4x4. Don't let the RPM scare you to much. Just make sure the pumps gov springs are addressed so it doesn't go into defuel.