3.73 differentials

[nicshorse]

i am very new to all of this and have used the search function and can't find any thing about this:
i just bought this bone stock K5 from my dad and he told me that pushing the blazer much passed 55 would hurt the diff's.he told me that since it was built in '77 that it was only designed as a 55 mph vehicle. all i need to know is if this is true or would i blow up the engine before i blow up the rear end. here is what i know about the truck:
sb 400
3.73 diff's
29" tires
tell me what you guys think.

 

[georgiared84]

That ratio is enough to drive regular highway speeds. What's your transmission?

 

[nicshorse]

i knew that was coming and i have no idea. i'll find out though.

 

[georgiared84]

If it's stock I think you've either got a th350 or sm465. (anyone know if they put th400's behind 400 small blocks) You're rear diff is a 12 bolt??? That's stronger than the rear ends they put in the eighties k5's (10 bolts) There's plenty of people here who have gears taller than 3.73 with the same transmissions that drive a tad above the speed limit on a regular basis. Those small tires will bring your rpm's up but if you're running a 400 with a decent trans and rear end there shouldn't be any problem.

 

[R72K5]

yeah its not the diffs that need to be worried about it would be the engine, 3.73 was standard equipment for decades through the 70s at the minimum, nothing out of oridinary, 3.73 rule as all around ratio, but some say its 3.40 and 3.42's that are, it just depends on tire height and tranny useage really. 3.73 with three speed and at least 29" tires is fine in my book, have had several like that daily drivers /forums/images/graemlins/laugh.gif i miss it alot..

good luck

 

[Grim-Reaper]

sb 400 doesn't like high RPMs for extended time. Your dad is right. I have a 79 with 3.73's and 31's. I dont' run it much past 65mph. Sometimes push 70. 70mph is roughly 3,000 RPM on 31's.
55mph with 27's is going to be about 2700 rpm with a Auto.
with 33's you would turn 2850 with a Auto at 70mph.
That truck will clear 31's no modification, I ran 32's with a couple taps of a hammer.
Here is a gear calculator you can play on.
http://www.ring-pinion.com/calculators.shtml
Now remember if you have a Auto add auto tranny from the 70;s add about 75 RPM. They didn't have a lock up converter back then. I also take about 1/2 an inch of what the tire size says on the side wall. Most tires rarly are exactly whay the side wall says.
But yes I agree with your dad...55 is about max with 27's

 

[Okiemuddog]

Then you van be like me, I have the opposite problem and didn't even realise it( /forums/images/graemlins/k5.gif Still has no power plant). I took a look at the calculator Grim recommended. My blazer should turn about 1900rpm at 70mph on 33's. There are some downsides to having a 700r4, oh well I guess that means I can install some really deep gears and 35's. /forums/images/graemlins/thumb.gif

 

[nicshorse]

thank you for all of the information folks. i have another question . how do you ever become any thing more than a stranger if you can get all of the answers you need by searching? i believe i may have found a new home in the search funtion.

 

[georgiared84]

You're right, the engine isn't going to like it much, but the axle won't be the weak link. That's one of the reasons I enjoy my 700r4 for highways. Lets me use low gearing and still cruise.

 

[BRN78BLAZER]

Mines got 3.73 with 31's, and the granny gear 4 speed. Quite a few times now I have driven on the highway for about 20 min running it at 160km/h or 100 MPH, which turns the engine at 3500 rpm. I figured that's still a safe rpm for the engine, and I've never had a problem with overheating or low oil pres. or anything of that manner. I was just wondering if this sounds right to you guys? Or are my gears possibly something other than 3.73. I myself was amazed at how nice it would drive at such high speeds, especially being 25 years old. Also its just a 305 2bbl.

 

[Grim-Reaper]

[ QUOTE ]
Mines got 3.73 with 31's, and the granny gear 4 speed. Quite a few times now I have driven on the highway for about 20 min running it at 160km/h or 100 MPH, which turns the engine at 3500 rpm. I figured that's still a safe rpm for the engine, and I've never had a problem with overheating or low oil pres. or anything of that manner. I was just wondering if this sounds right to you guys? Or are my gears possibly something other than 3.73. I myself was amazed at how nice it would drive at such high speeds, especially being 25 years old. Also its just a 305 2bbl.

[/ QUOTE ]
with 31's, 3.73 and a 4 speed with 1:1 forth gear you should be turning 4151 RPM. That's Asuming you have a 465 that is really concidered a 3speed with granny. Normal driving you neve use granny you start in 2nd (down and too the left on the shifter)

Now with the same combo 85mph is 3528 RPM. You sure your speedo is correct? I would believe a tach over a speedo any day. Or do you have overdrive?

Now playing around some more I pluged 3.08 gears in at 100mph you would be turning 3428

So something doesn't add up. I would be more inclined to believe you have 3.08 gears and not 3.73.

Here is the gear calculator I'm using.
http://www.ring-pinion.com/calculators.shtml

 

[Z3PR]

[ QUOTE ]

Now playing around some more I pluged 3.08 gears in at 100mph you would be turning 3428

So something doesn't add up. I would be more inclined to believe you have 3.08 gears and not 3.73.

[/ QUOTE ] Yeah, I'd think 3.08's also. I've got a 76 K5. The original owner thought it had 373's, but it turned out to be 3.08's. Seams like 3.08's are the most common gears. My K5 came with a 400cid engine, TH350 transmission/NP203 transfer case combo. Planning to go built 406, Built TH350/NP205, 3/4 Ton axles with 4.56 gears and 38" tires. (Currently have 373's in my 3/4 ton axles, but a gear swap is in my future next year.)

 

[jimmyjack]

I think you stumped some people on this query /forums/images/graemlins/laugh.gif /forums/images/graemlins/shocked.gif

 

[Bruiser]

My Blazer came factory with TH400 with 3.73 12 bolt 31" tires. I ran from Eugene, Or to N. Idaho Pulling a U-haul Trailer from 60-70 and my blazer loved and got 10 mpg with a quadra puke. If the lube in your diffs has never been changed I would at least change that and inspect the gears. I have had my up to 80 but it's kinda of scary at that speed. /forums/images/graemlins/blush.gif I wil add I drove it back and for between the closes large town and my house 59 miles driving 65-70 with no problems. It finally took my tranny line contacting my front driveshaft putting a hole in it playing in the snow to burn the th400 up (still moves just won't shift out of first /forums/images/graemlins/tongue.gif ) now I am converting to manual.

 

[firefighter184]

I'm real new to all of this too. I have an 85 Jimmy, don't know if diffs are open, think I have 3.73's, running 31's. 305CID w/ rochester4bbl. Two questions:
1) Is there a site that will help me find out specs for my K5 on-line?
2) Who wants to go wheelin May 4th near Denver CO?

 

[Grim-Reaper]

If your sure the truck is bone stock with factory gears then hit the link post at the top of the page. One of the links is how to deciper the RPO codes on the sticker in the glove box. One of those codes will have the gear ratio.
Welcome to CK5!

 

[Z3PR]

[ QUOTE ]
I'm real new to all of this too. I have an 85 Jimmy, don't know if diffs are open, think I have 3.73's, running 31's. 305CID w/ rochester4bbl. Two questions:
1) Is there a site that will help me find out specs for my K5 on-line?
2) Who wants to go wheelin May 4th?

[/ QUOTE ] Please fill out your profile. I'm sure you'll get some takers for May 4th, but we need to know where.

 

[Blazer_Boy]

Jack your back tires off the ground and put the transmission in neutral. By hand, spin one tire a full revolution while having a friend count the number of turns that the yoke makes. Be precise while doing this. Take the count of yoke turns and multiple it by two. It will usually end in with a fraction, like 1 7/8 turns. Say take that number and multiple by 2, you would get 3.75, which would be in reality a 3.73 ratio. Cool, huh? /forums/images/graemlins/cool.gif

 

[wadinator]

Sioux City huh? I"m only about 70 miles north of you. Man, there's a lot of people on this site!