5.14 ratio to 4.10 or 3.90 for 1 Ton Rear End - The 1947 - Present Chevrolet & GMC Truck Message Board Network

TreverSlyFox · 10-27-2003, 06:39 PM

My 71 GMC 1 Ton 7200 lb axle has a ratio of 5.14 and I get about a top speed of about 65 without really pushing it. I see that you can get a 4.10 or 3.90 ratio ring and pinion now for $350.

I've got a 4 speed tranny with granny 1st gear, if I made the swap what kind of top end speed could i expect? I'ed like to make Hi-Way speeds of 70-75 but which ratio will do it, the 4.10 or 3.90?

I imagine my speedo would be way off and have to be re-calibrated.

Anyone out there done the swap on their 1 Ton? Are you happy you did it?

Longhorn Man · 10-28-2003, 12:05 AM

4.10 gears with 9.50 / 16.5 tires will get you about 3000 RPM at 65 - 70 MPH.

Fred T · 10-28-2003, 12:34 AM

Easiest and cheapest way to make the change is to get the pumpkin (center carrier) out of a 64-72 3/4 ton with 4.10 gears. That way you don't have to mess with shimming the gears. Last I heard you could get a pumpkin for under $100, but they're getting harder to find. If you can't find one here, try Tom Myers Truck Salvage, Miami, OK (918) 542-3871. This swap is popular with those of us with trucks back to 47, we have to have the newer pumpkin, the new ring & pinion gears won't fit the older carrier.

Another option, a bit more difficult, is to get a late model full floater 14 bolt differential and swap the whole rear end. 3.73 is the most common ratio, there are some 3.42s out there. Price is in the $150 to 300 range, and requires welding on new spring perches. One thing you will need to know is the width of your differential, drum face to drum face. I measure using two short boards (1X2). You can either clamp or get a helper to hold one end while I do the other and measure between the boards.

crazy longhorn · 10-28-2003, 12:34 AM

4.10 should be a pretty good gear , with a 30" tall tire(9.50 x 16.5). If you spend a lot of time on the open hyway, you might like the 3.90 gears better tho. Good luck,crazyL

1969 GMC · 10-28-2003, 03:48 AM

i believe someone posted a while ago that the 3.90 gearsets are no longer available new. so you'd have to find one used, and since they didn't offer that ratio from the factory, it will be harder to find than, say, a set of towhooks.

10-27-2003, 06:39 PM	#1
TreverSlyFox Registered User Join Date: Sep 2003 Location: Houston, Missouri Posts: 17	5.14 ratio to 4.10 or 3.90 for 1 Ton Rear End My 71 GMC 1 Ton 7200 lb axle has a ratio of 5.14 and I get about a top speed of about 65 without really pushing it. I see that you can get a 4.10 or 3.90 ratio ring and pinion now for $350. I've got a 4 speed tranny with granny 1st gear, if I made the swap what kind of top end speed could i expect? I'ed like to make Hi-Way speeds of 70-75 but which ratio will do it, the 4.10 or 3.90? I imagine my speedo would be way off and have to be re-calibrated. Anyone out there done the swap on their 1 Ton? Are you happy you did it?

10-28-2003, 12:34 AM	#3
Fred T Cantankerous Geezer Join Date: Aug 2002 Location: Bel Aire, KS Posts: 6,264	Easiest and cheapest way to make the change is to get the pumpkin (center carrier) out of a 64-72 3/4 ton with 4.10 gears. That way you don't have to mess with shimming the gears. Last I heard you could get a pumpkin for under $100, but they're getting harder to find. If you can't find one here, try Tom Myers Truck Salvage, Miami, OK (918) 542-3871. This swap is popular with those of us with trucks back to 47, we have to have the newer pumpkin, the new ring & pinion gears won't fit the older carrier. Another option, a bit more difficult, is to get a late model full floater 14 bolt differential and swap the whole rear end. 3.73 is the most common ratio, there are some 3.42s out there. Price is in the $150 to 300 range, and requires welding on new spring perches. One thing you will need to know is the width of your differential, drum face to drum face. I measure using two short boards (1X2). You can either clamp or get a helper to hold one end while I do the other and measure between the boards. __________________ Fred There is no such thing as too much cam...just not enough engine. Last edited by Fred T; 10-28-2003 at 12:40 AM.

10-28-2003, 12:34 AM	#4
crazy longhorn Fabricate till you "puke" Join Date: Jun 2002 Location: Ill Posts: 9,402	4.10 should be a pretty good gear , with a 30" tall tire(9.50 x 16.5). If you spend a lot of time on the open hyway, you might like the 3.90 gears better tho. Good luck,crazyL __________________ 69 longhorn,4" chop,3/5 drop, 1/2 ton suspension/disc brakes,1 1/2" body drop,steel tilt clip, 5.3/Edelbrock rpm intake/600 carb, Hooker streetrod shorties,2 1/2" exhaust/ H pipe/50's Flows , 6 spd Richmond trans,12 bolt/ 3.40 gears....

10-28-2003, 03:48 AM	#5
1969 GMC Registered User Join Date: Feb 2003 Location: The Rubber City Posts: 5,498	i believe someone posted a while ago that the 3.90 gearsets are no longer available new. so you'd have to find one used, and since they didn't offer that ratio from the factory, it will be harder to find than, say, a set of towhooks. __________________ 1969 GMC K2500 1996 Honda Accord 2007 Kawasaki KLR 650

10-28-2003, 12:05 AM	#2
Longhorn Man its all about the +6 inches Join Date: Jan 2001 Location: Hilliard Ohio Posts: 2,690	4.10 gears with 9.50 / 16.5 tires will get you about 3000 RPM at 65 - 70 MPH.



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