The 1947 - Present Chevrolet & GMC Truck Message Board Network - View Single Post - C30 dually rear diff gearing options help please

**THI** · 01-02-2022, 01:25 PM

My 1979 C3500 Dually has the 4.10 gears (also a Camper Special) and 454 - I get about 10.5 on the highway with the TH400. My 1978 C20 (Camper Special) HAS THE 454 and 3.73 gears - I get around 11.5 highway with that combo - both empty. Load up the C3500 and it only drops to 10 - hard to say what the 1978 C20 does loaded as it is time to rebuild that 454 (it drops to around 8 when loaded had has no power!).

The 1979 C3500 is a 58,000 mile original lightly used in it's previous life. The 1978 is a 87,000 mile original that spent it's original life as a tow vehicle for a local RV dealership that worked it hard all summer and parked it during the winters.

My 1997 K2500 Suburban gets 16 mpg on the road empty and will get about 12-13 when towing a 7,000# trailer - 4L80E with 3.73 gears. If you are going use the truck regularly, I would recommend the 4L80E combo with the 3.73 gears - I am keeping my other trucks with their TH400's because they are now Limited Use Classics - the 1978 will eventually get a full restoration.

01-02-2022, 01:25 PM	#11
THI Active Member Join Date: Jan 2017 Location: Traverse City, MI Posts: 170	Re: C30 dually rear diff gearing options help please My 1979 C3500 Dually has the 4.10 gears (also a Camper Special) and 454 - I get about 10.5 on the highway with the TH400. My 1978 C20 (Camper Special) HAS THE 454 and 3.73 gears - I get around 11.5 highway with that combo - both empty. Load up the C3500 and it only drops to 10 - hard to say what the 1978 C20 does loaded as it is time to rebuild that 454 (it drops to around 8 when loaded had has no power!). The 1979 C3500 is a 58,000 mile original lightly used in it's previous life. The 1978 is a 87,000 mile original that spent it's original life as a tow vehicle for a local RV dealership that worked it hard all summer and parked it during the winters. My 1997 K2500 Suburban gets 16 mpg on the road empty and will get about 12-13 when towing a 7,000# trailer - 4L80E with 3.73 gears. If you are going use the truck regularly, I would recommend the 4L80E combo with the 3.73 gears - I am keeping my other trucks with their TH400's because they are now Limited Use Classics - the 1978 will eventually get a full restoration.