Better highway drivability - The 1947 - Present Chevrolet & GMC Truck Message Board Network

1966chevyc20 · 02-03-2017, 11:27 PM

I have a 1966 c20 with a sm420 and 4.56 rear end. It seems to be comfortable at around 50mph. I currently am swapping in a 350 for an engine. I would at least like to go 65 comfortably. So would the better option be a rear end swap or some type of overdrive tranny? If so what would be the easiest/most affordable to swap in?
Posted via Mobile Device

**LostMy65** · 02-03-2017, 11:52 PM

Easiest and least expensive.
Get a 14 bolt out of a 73+ 3/4 ton with 3.73 gears. - So very common gears.

nail pounder · 02-04-2017, 02:21 AM

You may want to do the rear end as parts can be hard to find on the HO52(?) But I have a 350/NV3500 (5spd manual) in my C20 w/ stock rear end and at 65 mph the rpm is about 2100, at 80mph it spins about 2500. If you use yours like a truck it's actually a nice set up, 1st is still real low and 5th purrs on the freeway.

1966chevyc20 · 02-04-2017, 02:49 AM

I have looked into an nv3500 swap. I just wasn't sure how well it would hold up to a mild 350 (around 350hp). Do you think it would hold up?

**Clyde65** · 02-04-2017, 09:37 AM

NV3500's are only rated 330hp max, you would need to go with an NV4500 which is much beefier. Cost wise, the rear end mentioned above would be your most cost effective option.

downeyballz · 02-04-2017, 09:51 AM

I swapped a nv3500 behind my 292 I-6. truck drives great, hydro clutch is nice and the trans is holding up fine. nv3500's are under-rated in my opinion.

if you plan on swapping the rear axle get one from a 1-ton square body (c30/k30) the leaf spring perches are in the correct location. later 1/2 & 3/4 ton trucks had wider spacing and outboard shocks.
I found this out the hard way

nail pounder · 02-04-2017, 12:42 PM

I think it all depends on what you plan do do with the truck. 3.73 rear end is a nice set up but you still only have a 1:1 ratio(sm420) in high gear. I've had no problems with my nv3500, BUT I don't hot rod it around, my tires are 31" tall BFG'S. It's not a burn out machine. In my 58 I have 350/700r4 with the stock 3.90 rear end and 28" cookie cutter tires, and it does great on the freeway and I can smoke the tires if I want to. Budget wise, price it out, both swaps take a little time to install. IMHO I think an overdrive transmission is a win win and then change the rear if you want.

**LostMy65** · 02-04-2017, 12:55 PM

http://67-72chevytrucks.com/vboard/s...hlight=14-Bolt

**LostMy65** · 02-04-2017, 01:02 PM

My thread
http://67-72chevytrucks.com/vboard/s...80#post5689580

rudyyloverboiii · 02-09-2017, 03:07 PM

So if I keep my 292 n I add an nv3500 I can keep my differential n have highway speeds?

If I keep the 292 n my sm 420 I would have to get a 73-87 3/4 ton (I have a c20) differential to get on the highway?

nail pounder · 02-09-2017, 03:25 PM

I have the stock rear end (HO52 4:56) in my 66 C20, 350/NV3500 5spd, and I run 80mph at 2500rpm. I thought about changing my rear end but I found that I didn't need to with the overdrive. Down the road if this one becomes a problem I will. If you change the rear you need to know what your getting as far as ratio. Not just any random out of a newer vehicle. Google "differential, rpm, mph, tire size calculator" and you will see what we are talking about.

02-03-2017, 11:27 PM	#1
1966chevyc20 Registered User Join Date: Jan 2017 Location: North Dakota Posts: 37	Better highway drivability I have a 1966 c20 with a sm420 and 4.56 rear end. It seems to be comfortable at around 50mph. I currently am swapping in a 350 for an engine. I would at least like to go 65 comfortably. So would the better option be a rear end swap or some type of overdrive tranny? If so what would be the easiest/most affordable to swap in? Posted via Mobile Device Last edited by 1966chevyc20; 02-03-2017 at 11:49 PM.

02-03-2017, 11:52 PM	#2
LostMy65 But Found Her 25yrs Later! Join Date: Oct 2010 Location: Oregon City, Oregon Posts: 10,468	Re: Better highway drivability Easiest and least expensive. Get a 14 bolt out of a 73+ 3/4 ton with 3.73 gears. - So very common gears. __________________ I lost my 65 - Found it 25 years later: http://67-72chevytrucks.com/vboard/s...d.php?t=426650 66 C20 Service Truck: http://67-72chevytrucks.com/vboard/s...d.php?t=428035

02-04-2017, 02:21 AM	#3
nail pounder Registered User Join Date: Aug 2014 Location: Fallbrook CA Posts: 453	Re: Better highway drivability You may want to do the rear end as parts can be hard to find on the HO52(?) But I have a 350/NV3500 (5spd manual) in my C20 w/ stock rear end and at 65 mph the rpm is about 2100, at 80mph it spins about 2500. If you use yours like a truck it's actually a nice set up, 1st is still real low and 5th purrs on the freeway.

02-04-2017, 02:49 AM	#4
1966chevyc20 Registered User Join Date: Jan 2017 Location: North Dakota Posts: 37	Re: Better highway drivability I have looked into an nv3500 swap. I just wasn't sure how well it would hold up to a mild 350 (around 350hp). Do you think it would hold up?

02-04-2017, 09:37 AM	#5
Clyde65 1965 Chevy C10, 2005 4.8L/4l60 Join Date: Aug 2004 Location: DFW Texas Posts: 8,539	Re: Better highway drivability NV3500's are only rated 330hp max, you would need to go with an NV4500 which is much beefier. Cost wise, the rear end mentioned above would be your most cost effective option. __________________ Clyde65 Rebuild of Clyde http://67-72chevytrucks.com/vboard/s...84#post8338184 69 Aristocrat Lo Liner build http://67-72chevytrucks.com/vboard/s...84#post7561684 *support our troops!*

02-04-2017, 09:51 AM	#6
downeyballz Registered User Join Date: Jan 2016 Location: nl connecticut Posts: 22	Re: Better highway drivability I swapped a nv3500 behind my 292 I-6. truck drives great, hydro clutch is nice and the trans is holding up fine. nv3500's are under-rated in my opinion. if you plan on swapping the rear axle get one from a 1-ton square body (c30/k30) the leaf spring perches are in the correct location. later 1/2 & 3/4 ton trucks had wider spacing and outboard shocks. I found this out the hard way

02-04-2017, 12:42 PM	#7
nail pounder Registered User Join Date: Aug 2014 Location: Fallbrook CA Posts: 453	Re: Better highway drivability I think it all depends on what you plan do do with the truck. 3.73 rear end is a nice set up but you still only have a 1:1 ratio(sm420) in high gear. I've had no problems with my nv3500, BUT I don't hot rod it around, my tires are 31" tall BFG'S. It's not a burn out machine. In my 58 I have 350/700r4 with the stock 3.90 rear end and 28" cookie cutter tires, and it does great on the freeway and I can smoke the tires if I want to. Budget wise, price it out, both swaps take a little time to install. IMHO I think an overdrive transmission is a win win and then change the rear if you want.

02-04-2017, 12:55 PM	#8
LostMy65 But Found Her 25yrs Later! Join Date: Oct 2010 Location: Oregon City, Oregon Posts: 10,468	Re: Better highway drivability http://67-72chevytrucks.com/vboard/s...hlight=14-Bolt __________________ I lost my 65 - Found it 25 years later: http://67-72chevytrucks.com/vboard/s...d.php?t=426650 66 C20 Service Truck: http://67-72chevytrucks.com/vboard/s...d.php?t=428035

02-04-2017, 01:02 PM	#9
LostMy65 But Found Her 25yrs Later! Join Date: Oct 2010 Location: Oregon City, Oregon Posts: 10,468	Re: Better highway drivability My thread http://67-72chevytrucks.com/vboard/s...80#post5689580 __________________ I lost my 65 - Found it 25 years later: http://67-72chevytrucks.com/vboard/s...d.php?t=426650 66 C20 Service Truck: http://67-72chevytrucks.com/vboard/s...d.php?t=428035

02-09-2017, 03:07 PM	#10
rudyyloverboiii Registered User Join Date: Nov 2014 Location: IE California Posts: 225	Re: Better highway drivability So if I keep my 292 n I add an nv3500 I can keep my differential n have highway speeds? If I keep the 292 n my sm 420 I would have to get a 73-87 3/4 ton (I have a c20) differential to get on the highway?

02-09-2017, 03:25 PM	#11
nail pounder Registered User Join Date: Aug 2014 Location: Fallbrook CA Posts: 453	Re: Better highway drivability I have the stock rear end (HO52 4:56) in my 66 C20, 350/NV3500 5spd, and I run 80mph at 2500rpm. I thought about changing my rear end but I found that I didn't need to with the overdrive. Down the road if this one becomes a problem I will. If you change the rear you need to know what your getting as far as ratio. Not just any random out of a newer vehicle. Google "differential, rpm, mph, tire size calculator" and you will see what we are talking about.



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