Gear Ratio! Is this normal?

[paulrowe]

I have a 1979 K10 with a 350 and TH350. It has a np203 transfer case. The truck has always been a bit of a dog starting off and lately it's been bothering me. I want to tow a #5000 trailer with it this summer so I started looking into it tonight. I jacked the rear and spun the tire with one tire blocked and it appears I have a 2.23 gearing in this truck. Is this normal? Would the factory make a truck with this Gear? It originally came with a 400 motor but that's long gone if that makes any difference. I'm assuming I'd be best putting a 3.73 or 4.10 gears in this thing to tow? Also I still have the factory exhaust manifolds which I'm guessing headers would help a lot. Thanks for any help and advise you guys can provide.

 

[1987 GMC Jimmy]

I have a 1979 K10 with a 350 and TH350. It has a np203 transfer case. The truck has always been a bit of a dog starting off and lately it's been bothering me. I want to tow a #5000 trailer with it this summer so I started looking into it tonight. I jacked the rear and spun the tire with one tire blocked and it appears I have a 2.23 gearing in this truck. Is this normal? Would the factory make a truck with this Gear? It originally came with a 400 motor but that's long gone if that makes any difference. I'm assuming I'd be best putting a 3.73 or 4.10 gears in this thing to tow? Also I still have the factory exhaust manifolds which I'm guessing headers would help a lot. Thanks for any help and advise you guys can provide.

No, they wouldn't make a truck with that ratio. The smallest would be 2.73, and that's no good. Are you sure it's not a 2.73? Anyways, I'd suggest 3.73's and nothing less than 3.42's. And yeah, long tube headers would liberate some power. Getting rid of the catalytic converter or getting a high flow model would help, too, if city and/or provincial emissions regulations don't prevent you from doing so. You can add power all day in many ways. It just depends on how much you want to do.

 

[Keith Seymore]

Check your SPID label.

If the truck was built with the base ratio there will either be no axle noted, or it will say "GQ1" - Standard axle.

If an optional axle was installed it will say the actual RPO and noun name (ie, "GU6" - 3.42, or GT4 = 3.73).

Base ratio for your truck from the factory would be 3.07:1, although it looks like a 2.76 was available optionally.

https://www.gmheritagecenter.com/do...its/Chevrolet-Trucks/1979-Chevrolet-Truck.pdf

All this assumes it hasn't been changed in the last 40 years.

K

 

[Craig 85]

My '79 was the same set up as your, but a California truck. It had 3.08's.

 

[paulrowe]

Thanks for the replies guys. I'm going to the wreckers this weekend and I'm going to remove a 14bolt SF rear end and put that in. I'll change my front gears on my corporate. Looking at 4:10s right now. I'm going to switch to a RV type cam and go headers with an H pipe. I'm thinking this should wake her up quite a bit. I'm sure a 400 or a BBC would be a better towing rig but I gotta work with what I have currently.

 

[68post]

That will wake it up in major way. Think small when you pick a cam.

 

[Craig 85]

I did axle swap on my '79. It was cheaper to switch to 3/4 running gear with 4.10 gears than re-gear the 1/2 stuff. I picked up front and rear axles for $400. The rear was a SF with a limited slip. The only issue was I went from 2000 to 3000 RPM at 65 MPH.

The only thing you may need to do after the swap is add a second proportioning valve for the rear brakes to dial them back. The 13" brakes really grab when you have to stop fast.

 

[75gmck25]

As already mentioned, if you are okay with 4.10 gears, that was a very common ratio in the K20 and K25 trucks with NP203. Just pull both axles and the driveshafts from a donor truck.

With the 4.10 ratio it would help if you also upgrade to 33" tires, and they should fit without a lift kit. With a 4'' lift you can usually move to 35" tires.

Bruce

 

[paulrowe]

Ok thanks guys good advise. There's an old wrecking yard around the corner from me. It's been closed for years but they have a lot of squares still in there. The neighbor knows the guy and can get me in there. I'm going to head over and see what I can come up with. Will a 14sf from a 2wd also bolt in? Hoping to find some 4wd stuff but it's a lot harder to come by around here.

 

[paulrowe]

Oh I forgot to mention I already have 33's on my truck.

 

[Craig 85]

A 2WD rear end should bolt in, the only thing I'm not sure if the parking brake cables are the same length. I had to buy new ones for my swap as someone had used a torch on them to remove the axle. You will need a hybrid u-joint at the rear axle. I also had to shorten my drive shaft due to the differential being larger in size. My rear end was out of an '82. Here the u-joints and measurements I used.

Spicer 1310-1350 Part# 5-460X or O’Reilly’s – Precision U-joint Part# 348
3.219 / 1.062 to 3.625 / 1.188

 

[HotRodPC]

I too would consider trying to find a set of axles from a K20, not only matching gears, but the front too will also already be 8 lug to match the back. Or were you intending to have your rear 8 lug axles re drilled or replaced with 6 lug axles?

 

[shiftpro]

That will wake it up in major way. Think small when you pick a cam.

Good advice here.

 

[shiftpro]

Thanks for the replies guys. I'm going to the wreckers this weekend and I'm going to remove a 14bolt SF rear end and put that in. I'll change my front gears on my corporate. Looking at 4:10s right now. I'm going to switch to a RV type cam and go headers with an H pipe. I'm thinking this should wake her up quite a bit. I'm sure a 400 or a BBC would be a better towing rig but I gotta work with what I have currently.

If you can swing an 'x' pipe crossover it will be better. But, takes a good muffler guy to fit it in. 'H' is second best. It will help with reducing sound, have to give it that. And a hellofa lot easier to fit it in. Right behind TC and under the drive shaft.

 

[HotRodPC]

After reading the original post again, I am wondering if he don't already have 3.73 or 4.10's. He says he blocked 1 wheel ..... If this is an open diff, you take turns of the pinion yoke and multiply by 2. So lets say he got just over 2 turns in an open diff. Multiply that by 2, he'd have 4.10's. Where did he get his 2.23 ratio from? If he turned 2 1/4 turns, then he has 4.56's. I'm just saying, there might be something wrong with the way he calculated this ratio. Maybe he did do the x 2 and got just over 1.5 turns which would be 3.08's.