Gear change question

2verpsters · 02-28-2007, 12:17 PM

Haven't been on the forum for awhile since you guys helped me out with my steering issues. Truck's been good since then and I've been working on the house instead. But, I've been thinking about changing gears on my 1980 K10 for some time now. Truck has stock 2.76's in the front and rear and when the truck was new, with little tires on it, that might have been fine. Now I run 33 inch BFG Mud Terrains and even though the engine and trany are rebuilt and strong, the truck is a tad sluggish to say the least, I do get good gas mileage though. What is the simple formula to figure out RPM's at cruising speed? Trany is a stock Turbo 350, tires are 33", rear end has 2.76's, and I turn about 2350 RPM's at 70 MPH. If I change gears to either 3.42's or 3.73's, how do I figure out my RPM's at 70 MPH? Would like to keep them under 3000 if possible but I need a formula.

75K30 · 02-28-2007, 01:31 PM

I am trying to do the math here, and it does not add up. I see you running only 1529 rpm at highway speed with a TH350, and 33" tires.
Are you sure of the engine rpm at speed, and are you sure you have a 2.76:1 ring and pinion? Has the trans actually shifted into high gear? That engine rpm is closer to what one might expect in second gear at highway speeds.

2verpsters · 02-28-2007, 01:46 PM

I wish I was only turning those RPM's. Positive of the other numbers. Got an after market tach hooked to my distributor, tires are definately 33's, and I counted and divided the teeth on my ring and pinion several times because I didn't realize GM had put that high a gear in a 4x4 truck. Couldn't believe it till I checked some manuals that verified that the ratio was available. Did those trany's come with different ratio's internally? I have no idea what mine might be, just thought they were all the same. My speedo might be off a tad, but not enough to vary the numbers much. The wife's 2004 Pontiac reads 70, mine reads 68 or so. Tach is right, set for an 8 cylinder, idle speed is correct, so on and so forth. Not sure why the numbers don't add up. It is a 4X4 with the 205 transfer case, would that change anything ratio wise? Also, trany is good, been rebuilt professionally, shifts like a Corvette. Definately has all 3 gears, even has a lock up converter that should be saving me abouth 200 RPM's or so.

bradrhale · 03-01-2007, 06:33 AM

none of that makes since. with that high of gears, you should be way lower. the only other thing that might be there is that you dont have a stock converter. is there any chance that someone might have put a higher stall in it?? you say there is a lockup in it so there might be a chance that it does not have the factory stall speed of around 1400-1600. that would cause your rpms to be so high at in the highest hole. i seriously doubt that someone has changed the internal gears in the tranny.

2verpsters · 03-01-2007, 07:04 AM

Ok, you guys have got me to thinking. I'm sure the internals of the trany are stock, except for the converter. I'd have to check on that. If the internals are stock, how many RPM's would I normally loose shifting from 2nd to 3rd. A rough ball park is fine, I know it depends on a lot of factors. I'm now wondering if the shift that I thought was 2nd to 3rd is actually the converter locking up. Before you guys start thinking I'm an idiot, which sometimes I am, I don't drive this truck much. It's a winter rat, goes to the dump and the lumber store. Mostly in town. I've had it for 8 or 9 years now and put about 10000 miles on it. I change the oil once a year, not every 3000 miles. In other words, I don't really have it at 70 MPH very often. Can't imagine that I don't have 3rd gear in the trany, wouldn't it at least try to shift and slip or make noise or something if 3rd was non existent? Around town, I loose about 3-400 RPM when it shifts into what I think is 3rd, but remember, I'm only going 30-40MPH at the time. I'm going to take it out this afternoon and hold it in 2nd till I reach 65 MPH or so and manually shift it into 3rd to see what happens, but until then, hit me with some ideas.

75K30 · 03-01-2007, 07:22 AM

An aftermarket stall would have to be an 8" converter with approx 4000 rpm stall speed, and would be so inefficient the trans would hav burned up already. It sounds like a decent theory, but it would constantly fell like it was slipping if that was the case.
How is the lock-up controled? If the 350 based trans (not technically a 350, probably an mx-2) has a lock up converter,what tells it to lock up? Electronic activation is common. Sure this is a lock up?, or could it be a "switch pitch"? Remember those variable pitch converters?
Still think this thing already has 3.73 gears, or is stuck in 2nd gear.

2verpsters · 03-01-2007, 07:40 AM

I don't know on the converter but my guess is a switch pitch since there is nothing controlling it electrically. I remember my trany guy selling me on it since it would knock down my cruising RPM's alittle and help with my gas mileage. Only like $70 extra on top of a stock converter IIRC. The trany doesn't slip, believe me. I had it built so it shifted quick and firm, not like the stock slop box that took what seemed like minutes to slide into the next gear. I'm starting to think it's stays in 2nd because there is no way I'd be that far off counting teeth on my ring and pinion. It's an open rear-end, I've had it apart twice, once for axle seals and again to replace the internals, that wonderful little threaded pin that chevy uses on the center shaft sheared. I caught it in time, but had to replace the little stuff. I'd have to be way off on my count, 2-3 teeth wouldn't do it. What would cause it to shift 1-2, but not 2-3? By the way, whats the equation you are using to figure this out. I'd like to have it for future use, once we figure this out.

75K30 · 03-01-2007, 08:42 AM

Plenty of gear ratio charts are available on-line, you will have to search for the one you like. If you have troubles finding one, PM me,and I will e-mail one that I use.
Just a little off subject, but in regards to that stupid bolt. You can purchase a center shaft (big pin that holds the spider gears) that has been clearenced to allow the removal of c-clip axles without removing the little bolt, or the big pin. You can also make your own. Just a tip for the rear axle so you never have that problem with that bolt again.

2verpsters · 03-01-2007, 08:49 AM

Thanks for the tip. Been thinking about either a posi rear end with my gear change or even a mini-spool. It would hard on it around town, but it's usually in the sloppy weather and I do tend to waste alot of gas playing in the mud and snow. The open rear end is a pain. I'll check on line and see what I find. Also will let you know what I find out when I take it for a test drive this afternoon.

2verpsters · 03-02-2007, 09:40 AM

Didn't get to test drive last nite so no new thoughts on this today. I did find a website, 4lo.com, that has all kinds of formulas available. Something is definately wrong, I should be turning about 1800 RPMs in third according to that site. With the RPM's I'm turning now, it tells me I have 3.55's in the rear. Funny thing is, thats what I wanted to install in the rear to begin with. Will be back when I have more info.

02-28-2007, 12:17 PM	#1
2verpsters Full Member Join Date: Oct 2006 Location: Baldwinsville, NY (Upstate) Posts: 23	Gear change question Haven't been on the forum for awhile since you guys helped me out with my steering issues. Truck's been good since then and I've been working on the house instead. But, I've been thinking about changing gears on my 1980 K10 for some time now. Truck has stock 2.76's in the front and rear and when the truck was new, with little tires on it, that might have been fine. Now I run 33 inch BFG Mud Terrains and even though the engine and trany are rebuilt and strong, the truck is a tad sluggish to say the least, I do get good gas mileage though. What is the simple formula to figure out RPM's at cruising speed? Trany is a stock Turbo 350, tires are 33", rear end has 2.76's, and I turn about 2350 RPM's at 70 MPH. If I change gears to either 3.42's or 3.73's, how do I figure out my RPM's at 70 MPH? Would like to keep them under 3000 if possible but I need a formula.

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02-28-2007, 01:31 PM	#2
75K30 Senior Member Join Date: Jul 2006 Posts: 2,636	I am trying to do the math here, and it does not add up. I see you running only 1529 rpm at highway speed with a TH350, and 33" tires. Are you sure of the engine rpm at speed, and are you sure you have a 2.76:1 ring and pinion? Has the trans actually shifted into high gear? That engine rpm is closer to what one might expect in second gear at highway speeds.

02-28-2007, 01:46 PM	#3
2verpsters Full Member Join Date: Oct 2006 Location: Baldwinsville, NY (Upstate) Posts: 23	I wish I was only turning those RPM's. Positive of the other numbers. Got an after market tach hooked to my distributor, tires are definately 33's, and I counted and divided the teeth on my ring and pinion several times because I didn't realize GM had put that high a gear in a 4x4 truck. Couldn't believe it till I checked some manuals that verified that the ratio was available. Did those trany's come with different ratio's internally? I have no idea what mine might be, just thought they were all the same. My speedo might be off a tad, but not enough to vary the numbers much. The wife's 2004 Pontiac reads 70, mine reads 68 or so. Tach is right, set for an 8 cylinder, idle speed is correct, so on and so forth. Not sure why the numbers don't add up. It is a 4X4 with the 205 transfer case, would that change anything ratio wise? Also, trany is good, been rebuilt professionally, shifts like a Corvette. Definately has all 3 gears, even has a lock up converter that should be saving me abouth 200 RPM's or so.

03-01-2007, 06:33 AM	#4
bradrhale Moderator Join Date: Apr 2006 Location: Texas Posts: 3,620	none of that makes since. with that high of gears, you should be way lower. the only other thing that might be there is that you dont have a stock converter. is there any chance that someone might have put a higher stall in it?? you say there is a lockup in it so there might be a chance that it does not have the factory stall speed of around 1400-1600. that would cause your rpms to be so high at in the highest hole. i seriously doubt that someone has changed the internal gears in the tranny.

03-01-2007, 07:04 AM	#5
2verpsters Full Member Join Date: Oct 2006 Location: Baldwinsville, NY (Upstate) Posts: 23	Ok, you guys have got me to thinking. I'm sure the internals of the trany are stock, except for the converter. I'd have to check on that. If the internals are stock, how many RPM's would I normally loose shifting from 2nd to 3rd. A rough ball park is fine, I know it depends on a lot of factors. I'm now wondering if the shift that I thought was 2nd to 3rd is actually the converter locking up. Before you guys start thinking I'm an idiot, which sometimes I am, I don't drive this truck much. It's a winter rat, goes to the dump and the lumber store. Mostly in town. I've had it for 8 or 9 years now and put about 10000 miles on it. I change the oil once a year, not every 3000 miles. In other words, I don't really have it at 70 MPH very often. Can't imagine that I don't have 3rd gear in the trany, wouldn't it at least try to shift and slip or make noise or something if 3rd was non existent? Around town, I loose about 3-400 RPM when it shifts into what I think is 3rd, but remember, I'm only going 30-40MPH at the time. I'm going to take it out this afternoon and hold it in 2nd till I reach 65 MPH or so and manually shift it into 3rd to see what happens, but until then, hit me with some ideas.

03-01-2007, 07:22 AM	#6
75K30 Senior Member Join Date: Jul 2006 Posts: 2,636	An aftermarket stall would have to be an 8" converter with approx 4000 rpm stall speed, and would be so inefficient the trans would hav burned up already. It sounds like a decent theory, but it would constantly fell like it was slipping if that was the case. How is the lock-up controled? If the 350 based trans (not technically a 350, probably an mx-2) has a lock up converter,what tells it to lock up? Electronic activation is common. Sure this is a lock up?, or could it be a "switch pitch"? Remember those variable pitch converters? Still think this thing already has 3.73 gears, or is stuck in 2nd gear.

03-01-2007, 07:40 AM	#7
2verpsters Full Member Join Date: Oct 2006 Location: Baldwinsville, NY (Upstate) Posts: 23	I don't know on the converter but my guess is a switch pitch since there is nothing controlling it electrically. I remember my trany guy selling me on it since it would knock down my cruising RPM's alittle and help with my gas mileage. Only like $70 extra on top of a stock converter IIRC. The trany doesn't slip, believe me. I had it built so it shifted quick and firm, not like the stock slop box that took what seemed like minutes to slide into the next gear. I'm starting to think it's stays in 2nd because there is no way I'd be that far off counting teeth on my ring and pinion. It's an open rear-end, I've had it apart twice, once for axle seals and again to replace the internals, that wonderful little threaded pin that chevy uses on the center shaft sheared. I caught it in time, but had to replace the little stuff. I'd have to be way off on my count, 2-3 teeth wouldn't do it. What would cause it to shift 1-2, but not 2-3? By the way, whats the equation you are using to figure this out. I'd like to have it for future use, once we figure this out.

03-01-2007, 08:42 AM	#8
75K30 Senior Member Join Date: Jul 2006 Posts: 2,636	Plenty of gear ratio charts are available on-line, you will have to search for the one you like. If you have troubles finding one, PM me,and I will e-mail one that I use. Just a little off subject, but in regards to that stupid bolt. You can purchase a center shaft (big pin that holds the spider gears) that has been clearenced to allow the removal of c-clip axles without removing the little bolt, or the big pin. You can also make your own. Just a tip for the rear axle so you never have that problem with that bolt again.

03-01-2007, 08:49 AM	#9
2verpsters Full Member Join Date: Oct 2006 Location: Baldwinsville, NY (Upstate) Posts: 23	Thanks for the tip. Been thinking about either a posi rear end with my gear change or even a mini-spool. It would hard on it around town, but it's usually in the sloppy weather and I do tend to waste alot of gas playing in the mud and snow. The open rear end is a pain. I'll check on line and see what I find. Also will let you know what I find out when I take it for a test drive this afternoon.

03-02-2007, 09:40 AM	#10
2verpsters Full Member Join Date: Oct 2006 Location: Baldwinsville, NY (Upstate) Posts: 23	Didn't get to test drive last nite so no new thoughts on this today. I did find a website, 4lo.com, that has all kinds of formulas available. Something is definately wrong, I should be turning about 1800 RPMs in third according to that site. With the RPM's I'm turning now, it tells me I have 3.55's in the rear. Funny thing is, thats what I wanted to install in the rear to begin with. Will be back when I have more info.