Hello everyone.. I have just completed a rather extensive rebuild of a single cab '79 C30 dually with a 454 & T-400. It is a Camper Special and I have quickly discovered that the diff ratio is ridiculously high with the motor turning over 3,000 RPM at 60 MPH..!! I don't think I have to tell anyone what sort of fuel consumption that is causing....!! Can anyone advise what ratio that sounds like..?? And are there other ratios available that will reduce the RPM at cruising speed? If so what ratios are available... As the vehicle is not being used as a camper nor will it be pulling any big weights I'm not concerned about loss of top end torque etc. Any assistance will be greatly appreciated as fuel down here is VERY expensive..!! ( Currently running at over $4.00 a gallon....!!!! ) Cheers, Bob Australia
Sounds like you have either 4.11's or 4.56's. If you can get 3.42's or 3.73's that can help out some, might drop a couple hundred rpms at 60 mph.
i would put 3.73's casue you don't wanna get them to low casue i think the 454 needs to turn about 2500rpm at cruise. i have a 454 in a 69 4x4 and it has some pretty high gears in it and it cruises about 60 at 2000 and i think it needs to be up just a little bit more. we have a 83 1 ton crew cab dually with a 454 and a 4 speed and it seems to get its best mileage at around 55mph. i don't knwo what rpm it is turning but my guess its close to 2500rpm. cowboy
Bob Your C30 has a Dana Series 60 rear end and I think the ratios are either 4.10 or 4.88 and you would need to know the circumference of the rear tires and the rpm @ 60mph clocked to calculate your actual ratio. If I can be more help let me know
IN THE OLDER CHEVYS THEY C30 CAME WITH 3 GEAR RATIOS 373 410 AND 456 AND THAT SOUND TO ME LIKE 456 GEARS.
Sounds like you have a 4.56 ratio diff. My C20 with 245/70/16 turns 3000 rpm at 65 and i do believe to have a 4.11 in my 14-bolt 10.5 ring diff.
Jack up the rear end and turn the rear wheel twice (hold the other rear wheel if you dont have posi) and count the number of turns the drive shaft makes that is your differental ratio. If it turns 4 and 1/2 times you got 4.56 if it turns closer to 4 times you got 4.11.
If your pulling very heavy load you will make the motor work harder buy taken away the gear, how much do you drive it . You could all so up tire size a little witch will knok off some RPM to. As said bye "nerfherder78" I would not put lower then 3.73 the truck will find the load harder to pull, you will feel very slugish to what it was. If your tire on the small size it will make your gear higher to. Oh if the trans is not in good shape to slipping to much that will make the motor try to make up for the trans not working right by higher RPM.
