I have a 66' D300 1 ton dually with the 225 slant, NP435 tranny, and Spicer 70 Rear with 4.88 gear. I don't drive the truck much over 40mph because it sounds like it is screaming. I know there is no sound dampening on the truck, but I am nervous to push it too much. I just ordered a tach so that I will have a better idea of how hard the motor is actually working.

I would really like to drive 55mph when I take the truck on a 15 mile trip to work occasionally. The service manual says the recommended governed speed with full load is 3500RPM. Is this the number I should be aiming for with no load as a daily driver. Also, the motor is mostly all original and has 28,000 miles on it.

Anyone have any idea on what the comfortable cruising speed, RPMs, and noise should be with this setup?

Thanks

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1966 Dodge D300 with 225 3.7L
Scott C.
A family restoration and education project.

If you're still using the 215/85-16s that you listed here, which come to about 30.4" diameter http://www.slantsix.org/forum/viewtopic.php?t=57764

@55mph you'd be 2,967rpm

Then 3,500rpm would be ~65mph

4,000 would be 74mph, etc...

Just plug in your stuff into this calculator.

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3000 RPM cruising should be fine. Yes, good to have a tach. Make sure it is set to "6 cyl" when you install it!

Lou

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Home of Slant6-powered fun machines

Thank you both for the input. So considering the RPMs at the target speed I would like to drive would be 2950RPM, should I be concerned with running that high for a specific duration. I would only be 10-15mins at that RPMs for the most part. What are the concerns of this workload or are there any?

Thanks for the help.

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1966 Dodge D300 with 225 3.7L

Scott C.

A family restoration and education project.

I run my Dart at 3000rpm for hours at a time on long trips as that equates to 60 mph. Keep an eye on oil consumption, my engine uses a lot more oil above 3000 rpm than putt-putting around at 2500rpm. I don't know if it is being sucked through PCV system or oil control rings arn't doing their job, and I never see any smoke out the back.

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67' Dart GT Convertible; the old Chrysler Corp.
82' LeBaron Convertible; the new Chrysler Corp
07' 300 C AWD; Now by Fiat, the old new Chrysler LLC