Would you change rear gear?

[Sawlog]

I just put my new motor in my 69 c10. A also put a 2000 stall in.
My plan was to change the rear 3.07 gear to 3.42 with posi next.
With the new motor and stall, I can now honestly smoke my one wheel peal from a 20 mile an hour roll all the way into 2nd gear. My old set up would fall on its face.
So.. now I'm not sure I need to change gears. It doesn't sound like I need much help on the low end like I thought I would. What do you think?

[Steeveedee]

You might change your mind with more traction. See if you can borrow wider tires to experiment with. It does look like you have enough torque, though. Are you towing anything with it?

[Sawlog]

No towing, I have 31 10.50s on the rear

[ElKotze]

I wouldn't change it. I have 3.07's in my '68 and I love it, great on the highway if you have enough motor to push it.

[jocko]

Cruising the highways comfortably (rpm-wise) and being burnout king are pretty much mutually exclusive endeavors without an overdrive trans. Right now, you are probably enjoying the reasonable highway rpm more than you realize. 3.42 won't be too bad on the highway, but will be higher rpm than 3.07, a great highway ratio.

The question is: what is your goal (burnouts) and is reaching it worth the sacrifices at the other end of the rpm band? 3.07 to 3.42 isn't a very drastic change and neither is a very deep 1st gear for burnouts, to be honest. Tire smoking is usually reserved for 3.73 and higher numerical ratios (lower gearing) depending on your engine, of course.

So, what is your goal? If burnouts only and you don't care about highway rpm, then I'd jump straight to a 3.73. What is your budget? Is there funding for an O/D trans AND a gear swap? If so, a TH700R4 or TH2004R, either with 3.73, will make your day from a dead stop AND on the highway. I'm going to assume you don't want a different trans if you just installed a new torque converter (what IS your trans, and engine, by the way?) - so if highway rpm is important to you - I'd leave it as is. If you want more umph, then 3.42 or even 3.73, depending on how much importance you place on highway rpm.

EDIT: one more point - I'd agree with Steeveedee, you're just breaking loose the one tire and then it's spinning to make your smoke - if you add a posi, it will likely take more gearing to break it loose in the first place.

[Sawlog]

My motor is a 357, tranny th400. Most of my driving is on the highway or interstate. So I do enjoy my lower rpm. I also don't want a log at take off either. My new motor dynod at 388 pounds of tourqe at 3700 rpm. What I difference this motor has made! I might have found a happy meduim with low end power and good highway rpm. Just don't want to mess it up.

[Mike C]

I would gear it.

Be careful with extended one wheel peals. The spider gears and side gear spin EXTREMELY fast when smokin' one tire and they will fail...

[RichardJ]

>>Tire smoking is usually reserved for 3.73 and higher numerical ratios (lower gearing) <<

That's actually backwards. Once a tire breaks loose and looses traction, it takes less power to keep it spinning. With "3.73 and higher numerical ratios" the tire is actually spinning slower so it takes less vehicle speed to match tire speed and at that point the tire matches ground speed and stops spinning.

I agree with Mike C about the spider gears. With the 3.07s the sixteen year olds will applaud now and the neighbors will applaud when you trash the rear end.

[Mike C]

This is absolutely the truth. It is easier to break the tires loose with a higher numeric gear, but it is so much more easy to modulate the wheel spin. That is amplified with a traction device as well.

With 2.56, 2.73. or 3.07 gears once they start spinning they just seem to spin forever.

I just went from 3.54 open to 4.57 with a PowerTrax no slip in the back of my CJ-5 (running 33" BFG mud terrains) and it is really interesting to see how the gear made the powerband much more linear. With the 3.54 it would start off slow and really accelerate as it "came on the cam". With the 4.5's, it just goes really linearly until it quits pulling.

This is the most gear change I have done in recent history. I went from 3.07 in my Camaro to 4.56 and you talk about changing the personality of a motor.

[jocko]

Originally Posted by RichardJ

>>Tire smoking is usually reserved for 3.73 and higher numerical ratios (lower gearing) <<

That's actually backwards. Once a tire breaks loose and looses traction, it takes less power to keep it spinning. With "3.73 and higher numerical ratios" the tire is actually spinning slower so it take less vehicle speed to match tire speed and at that point the tire matches ground speed and stops spinning.

The key phrase there is "once a tire breaks loose". Completely understand what you're saying, and yes, once it breaks looks, 3.07 will spin the tire faster than a 4.10. But if a truck can't do a burnout in the first place - it needs a deeper gear to be able to break loose in the first place (that's what I'm talkin about). We're just talking about two consecutive, but different things. I'm talking about increasing the numerical ratio to to help a trucks engine/rear combo actually be able to break the tires loose (why I don't know), and you're talking about once they have broken loose, destroying the maximum amount of tire for the least rpm. But I DID say - "tire smoking" so, I guess I stepped right into that one, ha

[Mike C]

I don't think he was finding any fault with what you had said, just trying to add to the conversation. I've never had a vehicle with too much gear until it came time to take a 6 hour road trip...

My answer was still directed at the OP to gear it. Gear makes everything better except for highway cruising. That's what converting to OD is for after you have over geared it.

I have a 427 build for my short step and am planning on running 3.73 gears with a TH200 4R. Personally, I find the 1-2 drop with a TH700R4 to be obnoxious and I am willing to pay to not have it.

The 3.73 with the the 2.74 of the TH200 4R gives a 10.22 overall first gear and the .67 OD drops the effective final drive ratio to 2.49 which is the best of both worlds.

New cars are so much more driveable than old cars due to multispeed transmissions more than anything else IMO.

[jocko]

Originally Posted by Mike C

I don't think he was finding any fault with what you had said, just trying to add to the conversation. I've never had a vehicle with too much gear until it came time to take a 6 hour road trip....

Yup, no worries from where I'm sittin, I didn't take it that way either - just pointing to the two sides of the coin. All good!

[4carbcorvair]

4.57 in my 68, 30mph I'm looking for another gear. LOL

[WorkinLonghorn]

What speed do you cruise at? Is gas mileage a priority at all? How often really do you expect to light up that expensive rubber?

If you cruise 60-65 I would think about switching to 3.73. Any higher and I think your original plan for 3.54 would work.

I notice that your torque peak is at 3700 RPM. A stock 1969 350 got 355 Ft/Lbs at 3000 (315 at 2400 net rating). I cruise at about this RPM (2400) with 3.73 rear but I rarely go above 60(for safety and fuel economy reasons). -BA

The crappiest gas mileage I ever got in a carb'd SBC was with 3.07 gears and 33" tires. (~10MPG freeway)

[KQQL IT]

Unless you're drag racing it, and it pulls the current gear. I'd leave it alone.
Last time I went from 3.08 to 3.73 and hated it until I went to the dragstrip.

[Sawlog]

Originally Posted by WorkinLonghorn

What speed do you cruise at? Is gas mileage a priority at all? How often really do you expect to light up that expensive rubber?

If you cruise 60-65 I would think about switching to 3.73. Any higher and I think your original plan for 3.54 would work.

I notice that your torque peak is at 3700 RPM. A stock 1969 350 got 355 Ft/Lbs at 3000 (315 at 2400 net rating). I cruise at about this RPM (2400) with 3.73 rear but I rarely go above 60(for safety and fuel economy reasons). -BA

The crappiest gas mileage I ever got in a carb'd SBC was with 3.07 gears and 33" tires. (~10MPG freeway)

Average speed 60 mile round trip to work, 70. Intrastate on the weekend, 75, 80.
My torque at 2300 was 354. My cruising rpm is 2600

[Sawlog]

Originally Posted by WorkinLonghorn

What speed do you cruise at? Is gas mileage a priority at all? How often really do you expect to light up that expensive rubber?

If you cruise 60-65 I would think about switching to 3.73. Any higher and I think your original plan for 3.54 would work.

I notice that your torque peak is at 3700 RPM. A stock 1969 350 got 355 Ft/Lbs at 3000 (315 at 2400 net rating). I cruise at about this RPM (2400) with 3.73 rear but I rarely go above 60(for safety and fuel economy reasons). -BA

The crappiest gas mileage I ever got in a carb'd SBC was with 3.07 gears and 33" tires. (~10MPG freeway)

I'm not buying that power rating on that ad. My old motor I took out had 12cc dished piston that were .045 in the hole with 76cc heads. The compression was 7.75. From what i have read and what my machinest has told me they were around 165 hp.that I would believe.

[WorkinLonghorn]

That's not an ad. That is from the Truck Data Book for GM. Anyway, the CR was 9-1 on those. Figures I listed are for the non-smog motor. NET HP with smog was 195 (gross with no accessories and optimum timing was still 255) but you could be right. My stock 350 from '69 was pretty tired so I can't say.
If you drive 80, then I suppose 3.07 is the ticket.

[68Stepbed]

Before I express my thoughts and opinions, I'd like some more info. Just copy and paste the list below and add specs.

Engine size: bore, stroke and cubic inches
Head specs: casting number, cc volume, intake runner volume
Cam specs: Lift, duration, LSA, and/or cam part number
Transmission: Also add anything "done" to it like shift kit, etc.
Rear tire size: The overall diameter of the tire plays a crucial role in final gearing.

Normally, I'd ask for gear ratio in these scenarios, but that's been discussed already.

Keep in mind a 2000 stall converter isn't a very deep stall. Stock converters usually yield about 1200-1500 stall, and "Vette" converters are usually about 1800-2000 stall.

[RetiredSquid]

Howdy folks,

Been lurking for a while as I tinker on my current project (a.k.a. money pit) 69 C10 SWB. I'm at the point in my project where I'm looking to pick rear end gears and thought I would jump in for some advice on what gears.

Truck info:
Motor: LS7
Trans: 4L85E (2.48, 1.48, 1.0, 0.75 ratios)
Probably rear tire size: 295/45R20

This will not be a road trip truck, going for acceleration and handling but I still want to be around 2000 RPM at 65 MPH or so. I'm thinking in the 3.5 to 3.7 range for the rear end.

Any recommendations?

Thanks,
Matt

[68Stepbed]

Originally Posted by RetiredSquid

Howdy folks,

Been lurking for a while as I tinker on my current project (a.k.a. money pit) 69 C10 SWB. I'm at the point in my project where I'm looking to pick rear end gears and thought I would jump in for some advice on what gears.

Truck info:
Motor: LS7
Trans: 4L85E (2.48, 1.48, 1.0, 0.75 ratios)
Probably rear tire size: 295/45R20

This will not be a road trip truck, going for acceleration and handling but I still want to be around 2000 RPM at 65 MPH or so. I'm thinking in the 3.5 to 3.7 range for the rear end.

Any recommendations?

Thanks,
Matt

3.73 or 4.11

[Sawlog]

Originally Posted by 68Stepbed

Before I express my thoughts and opinions, I'd like some more info. Just copy and paste the list below and add specs.

Engine size: bore, stroke and cubic inches
Head specs: casting number, cc volume, intake runner volume
Cam specs: Lift, duration, LSA, and/or cam part number
Transmission: Also add anything "done" to it like shift kit, etc.
Rear tire size: The overall diameter of the tire plays a crucial role in final gearing.

Normally, I'd ask for gear ratio in these scenarios, but that's been discussed already.

Keep in mind a 2000 stall converter isn't a very deep stall. Stock converters usually yield about 1200-1500 stall, and "Vette" converters are usually about 1800-2000 stall.

350, bored .040. 5.7in stroke
Dart iron eagle heads 165cc runners. 67 c.f. chambers
.221-221@.50 duration. .470 .470 lift lsa 108
Th400 tranny, 2000 stall.
31 10.50 rear tire.

[68Stepbed]

Originally Posted by Sawlog

350, bored .040. 5.7in stroke
Dart iron eagle heads 165cc runners. 67 c.f. chambers
.221-221@.50 duration. .470 .470 lift lsa 108
Th400 tranny, 2000 stall.
31 10.50 rear tire.

OK, it sounds like you have a slightly hotter than stock engine. The heads should be good for building low end torque, but add just a tad of compression. The cam looks like it would be a good mid range power setup while still making decent torque in the 2500-3500 rpm range. Seems to be a good setup for a stout street cruiser.

Your kicker is the tires. Is your truck 2wd or 4wd? Those 31's are going to make your 3.07's feel like 2.73's. If you ever plan to run a smaller tire, the 3.07 gear would still work well, but if you plan to keep the same size tire, I would suggest a 3.42 gear to make better use of the rpm range of the engine. A LSD like an Eaton would also be beneficial to keep from one-wheel- peeling all the time.

Also, FWIW, the 3.07 gear is a 3-series gear, and 3.42 is considered a 4-series gear, but you can get a gear made for a 3-series carrier.

[halfstep]

Gear it.
My 2 cents:
- You'll get more "go" from beginning to end.
- Doesn't sound like you got this truck for family trips.
- You've already invested in reasonably high performance modifications to your engine and transmission and with that cam your 3.07's are a bit low.
- Chevy high performance vehicles of this era (67-72) ALL started with higher numerical gears. Small blocks 300hp and up, 3:73 std. , Big blocks 325 and up 3:42 or 3:55 std and our trucks are heavier are less aerodynamic.
- 3:73 gears were a common option of our trucks and I believe std. with 6 cylinder trucks.
- These trucks will never get great gas mileage no matter the gear. I have a similar set up to yours (well modified sbc, th400 with ati stall) and when I switched from 3:07 to 3:73 posi I experienced NO DIFFERENCE in gas mileage or highway drone although I was "tach'ing" more rpms. BIG DIFFERENCE on launch and anytime I hit the gas - especially on the highway.
Around 3K i am on the early side of my torque curve and the engine is very happy. I think you will be too. Unless you have fuel injection and advanced timing controls, regularly operating your carburetor engine outside of your torque curve is not efficient. I did pick up 2 mpg in tuning...after the gears!
- As far as drone, etc. if that's an issue - get better mufflers or run resonators in front of your existing mufflers, and/or insulate the cab.

[Sawlog]

Originally Posted by halfstep

Gear it.
My 2 cents:
- You'll get more "go" from beginning to end.
- Doesn't sound like you got this truck for family trips.
- You've already invested in reasonably high performance modifications to your engine and transmission and with that cam your 3.07's are a bit low.
- Chevy high performance vehicles of this era (67-72) ALL started with higher numerical gears. Small blocks 300hp and up, 3:73 std. , Big blocks 325 and up 3:42 or 3:55 std and our trucks are heavier are less aerodynamic.
- 3:73 gears were a common option of our trucks and I believe std. with 6 cylinder trucks.
- These trucks will never get great gas mileage no matter the gear. I have a similar set up to yours (well modified sbc, th400 with ati stall) and when I switched from 3:07 to 3:73 posi I experienced NO DIFFERENCE in gas mileage or highway drone although I was "tach'ing" more rpms. BIG DIFFERENCE on launch and anytime I hit the gas - especially on the highway.
Around 3K i am on the early side of my torque curve and the engine is very happy. I think you will be too. Unless you have fuel injection and advanced timing controls, regularly operating your carburetor engine outside of your torque curve is not efficient. I did pick up 2 mpg in tuning...after the gears!
- As far as drone, etc. if that's an issue - get better mufflers or run resonators in front of your existing mufflers, and/or insulate the cab.

I don't have any issues with drone. My truck is pretty quite until I open it up.thanks