Obviously gearing is going to be easier to change than a full transmission swap.

The first thing I'd do is figure out what gears are in it. There's a few ways to do this.

• Jack up one rear wheel (only one). Support on jack stands. If you do this on a lift in a shop, use somethign to keep one wheel from turning for the test. It's easier to test with one wheel. Mark the pinion (or the rear u joint) and the front of the diff housing with a line of chalk. Also mark the bottom of the tire that's free to spin with a chalk mark straight down at the very bottom of the wheel. Turn the pinion shaft and count how many complete turns it takes to rotate the one wheel that's free to turn through two complete revolutions. The gear ratio is the number of complete turns of the pinion to turn one (only one) wheel through two complete revolutions. 3.08:1 will be just past 3 complete turns. 3.73:1 will be almost 3-3/4 turns. 4.11:1 will be almost 1/8 turn past 4 complete turns.
• Remove the diff cover and find the markings on the gear(s). Usually there's numbers stamped in the outer edge of the ring gear. These can be decoded to determine the ratio. If you don't remember the last time the diff lube was changed, this is an opportunity for a lube change.
• Remove the diff cover and count the teeth on the ring gear and the teeth on the pinion gear. Gear ratio is the number of ring gear teeth divided by the number of pinion gear teeth.

Does it have a limited slip diff of any kind? That might limit how "tall" (low number) a gear set you can install. But if you aren't racing the thing, 3.08:1 or if you have a limited slip diff, maybe 3.2x:1 (whatever the minimum number is for your limited slip) will be much better on the highway than 3.73:1 or 4.11:1.

 

Obviously gearing is going to be easier to change than a full transmission swap.

The first thing I'd do is figure out what gears are in it. There's a few ways to do this.

• Jack up one rear wheel (only one). Support on jack stands. If you do this on a lift in a shop, use somethign to keep one wheel from turning for the test. It's easier to test with one wheel. Mark the pinion (or the rear u joint) and the front of the diff housing with a line of chalk. Also mark the bottom of the tire that's free to spin with a chalk mark straight down at the very bottom of the wheel. Turn the pinion shaft and count how many complete turns it takes to rotate the one wheel that's free to turn through two complete revolutions. The gear ratio is the number of complete turns of the pinion to turn one (only one) wheel through two complete revolutions. 3.08:1 will be just past 3 complete turns. 3.73:1 will be almost 3-3/4 turns. 4.11:1 will be almost 1/8 turn past 4 complete turns.
• Remove the diff cover and find the markings on the gear(s). Usually there's numbers stamped in the outer edge of the ring gear. These can be decoded to determine the ratio. If you don't remember the last time the diff lube was changed, this is an opportunity for a lube change.
• Remove the diff cover and count the teeth on the ring gear and the teeth on the pinion gear. Gear ratio is the number of ring gear teeth divided by the number of pinion gear teeth.

Does it have a limited slip diff of any kind? That might limit how "tall" (low number) a gear set you can install. But if you aren't racing the thing, 3.08:1 or if you have a limited slip diff, maybe 3.2x:1 (whatever the minimum number is for your limited slip) will be much better on the highway than 3.73:1 or 4.11:1.

Good to know! Thank you!

 

How "built" is the 350? A stock 200-4r (even a Z-code) might not like the cubes if you abuse it. A 200-4r can be built to take some torque, but open the big wallet.

The upside is that a 200-r4 is as close to a bolt-in as you can get. Same length, same output shaft, no brainer. Make sure you get the entire linkage including the column parts, otherwise a full stroke of the shifter will only get you to about neutral or Drive. Different throw ratios on the shifters between 350 and the metrics

 

Yea if I went with the 200 it would have to be built. The engine is bored .30 over, aluminum heads, headers and pretty big thumper cam. It's got some torque.

 

Mclark: Welcome to Hotrodders. I edited the thread title. We do not use HELP in thread titles here. The members here offer advice on all subjects with cars

 

You got the big thumper cam, eh!
that cam needs the gears you have in it now.
‘It’s operating range is likely from 2500-6000 rpm.
‘It can be a cantankerous beast at low rpm.
You’ve likely got a 3.73 or3.90 gear in it now.
‘Leave it alone and enjoy it.

 

After determining the axle ratio, I suggest you check tach accuracy. I recently had an old SunPro tach that seemed to be reading high on the highway. My initial suspects were the TH350's converter or a clutch pack. I finally compared the tach to a digital timing light and found it was reading around 400-500 RPM high at an actual 2500 RPM.

Pretty good chance your tach is fairly accurate, but it's something to check if it turns out your axle ratio is actually 3.08.

That's a nice-looking car.

 

When you shoot for a target cruise RPM, just remember that lower is not better... correct is better. 1800 rpms on the highway is fine for a big block with a stock cam, but not with aftermarket stuff. I agree that 3000 is too high, but not crazy depending on the cam specs.

Best cruise RPM should never be too far below the peak torque RPM. It can cause the engine to lug, which not only requires more right foot (and fuel) to maintain speed, it can cause high piston temps and detonation. I like to know the peak torque RPM and the peak VE RPM. Put your cruise RPM in between those two revs. Don't just shoot for lower, shoot for right.

 

As far as the trans is concerned, I wouldn't hesitate to do a beefed up 700r4. The only real difference is the driveshaft length, and shortening your existing shaft isn't cost prohibitive. I think that a stout 700r4 plus the cost of shortening is probably cheaper than a 200-4r built to take the abuse, plus the differences in shifter linkage might mean having to adapt a lot to get the geometry right.

Another thought... what is the brand of converter? A cheapy converter doesn't just stall at 3000, it provides very little torque until 3000. I've driven 3000 stall converters that act just like a stocker until you really ask it to stall. I've also driven converters that it feels almost like the trans is slipping until you get up to the stall speed. If you have the latter, no amount of gearing will change the cruise RPM... unless you do a lockup TC.

 

Find your ratio then decide.

I am in the leave it alone group.
My butt likes around 3000-3500rpm on the highway.
Not having overdrive keeps me going 75 not 95. With something like 3.42 gearing and .8 or .7 overdrive I will catch myself pushing 100 near the bottom of an on ramp more then I perfer.

 

700R4 only requires a small mod on the trans cross member to accommodate the slightly more rear mount, often this is not more than elongating the trans mount bolt holes. It can get into shortening the drive shaft a cut and weld process any driveline repair shop can do.

You will need to order the 700R4 set up for a cable driven speedo. They were made both ways so most shops will easily supply it the way you need it.

The 700R4 uses a cable operated Throttle Valve (TV) where your TH350 is vacuum, so you will need the kit that supplies these parts. Here is a link on how to set it up and tune it.

https://www.cpttransmission.com/tech_tvcable.htm?vm=r&s=1

Keep in mind that a carbureted engine need to spin faster than an EFI engine so when combined with a big cam the engine may not be happy at all. Carburetors depend on velocity through their venturi to meter fuel so inertia of sir and of fuel are important elements of metering. EFI measures the airflow through the engine with 3 prime sensors of throttle position, RPM, and either manifold vacuum which engineers call Absolute Manifold Pressure with a MAP sensor or with direct air flow from a gadget called a Mass Air Flow (MAF) sensor. This is used to compute the ratio of on to off time of an electric injector operating under constant fuel pressure. So EFI supplies very accurate fuel metering across all RPM ranges even with big cams; carburetors are not this good especially under low air flows that come with reduced RPMs.

Bogie

 

2800-3000 rpm isn't exactly " screaming" down the street ..

 

Actually, with a 3000RPM stall converter, that could be cruising on the stalled converter, which generates a ton of heat in the transmission fluid. I've seen lifted trucks with high stall speed converters burn up a transmission or overheat the engine cruising on the highway because they were cruising (and pushing the aero load) with a stalled converter.

@mclark1983 you may want to check this after you determine your actual rear axle gear ratio. With a 3,000RPM stall converter, you really want to be at 3,300RPMs or higher at your slowest cruising speed. Here's how to check:

• Determine your actual gear ratio
• Find your actual tire diameter at 225/70R14 Tires (that was a guess, click the "by size" option in the dark grey bar below the menu bar and logo.
• Go to a "Engine RPM/gear ratio calculator" like Engine RPM / Engine Speed Calculator and enter your actual numbers there, probably with a vehicle speed of 50mph or 55mph. Then hit the "calculate" button to find the input shaft RPM (marked "Engine Speed" on that calculator)

If the input shaft RPM is below 3,000 RPM, you're going to be running 2,800-3,000 RPMs with your torque converter, and that won't change much with speed until you hit a speed where the input shaft RPM is over the stall speed of your torque converter. And you'll be churning heat into the transmission fluid at speeds below the 3,000 RPM engine speed. Ideally, you want to be 10% over the stall speed at "cruising speed" to avoid heat issues stemming from a stalled torque converter.

One option not mentioned yet is the TH350C transmission with a lockup torque converter. If your calculations indicate an acceptable engine speed (2000-2500RPM) at your desired "cruising speeds," the lockup torque converter (you'd have to get a 3000RPM stall speed one for that application) would bring your engine down to the input shaft speed and would resolve the excess heat issue from driving on the stalled torque converter.

 

Here is the calculator I use. It allows for easy side by side comparison. (It is ad free).

http://www.grimmjeeper.com/gears.html

It has a drop down menu of several transmissions(and transfer cases/underdrives) and lets you choose your tire size several diffrent ways. Even allows for sae or metric inputs if that suits your part of the world.
Taylored more for the 4x4 crowd. But you can leave those 4x4 inputs blank if your running 2wd.

 

What are you guys driving that's comfortable at 2800-3000 RPM on the highway, especially for anything more than a few miles?

Also, cruising at just under peak torque RPM would put even a mild 350 Chevy at 3500 RPM or higher. That would buzz my brains out unless maybe it was a 283 or 327.

 

I have had a few c30 with 4.56 14 bolts and sm465's turning 235/75R16 tires.

Cruise down the highway between 3500 and 4200. Get 8mpg at 9mph and around 6mpg at 80 mph with a sbc pulling something or empty. Big block will only cost you one or two more mpg.

Good cooling/oiling system, exhaust that is not to loud, and a good radio.

Pushing 3000 is nicer then 4000 though so for long distances I would usually sit 50 to 100 feet back from a semi (just enough to keep people from cutting in front of me) and cruise at 60-65 mph.

 

I have a 1970 Nova with a 700R4 Transmission and I love it. I am Running a posi 307 rear-end Gears. My car has a little power it runs high 12 in the 1/4 mile track. yes the trans mount you have can be used you will need to drill 2 new holes for the trans support on the subfram. I think there is an adapter for the shifter so you could keep the original. But do some research to find out. Your drive line will need to be shorten to fit. Also I put a truck Transmission oil pan on mine because it's deeper and a big oil cooler. I have Run this car hard for 6 years no Transmission problems. I have had it on the open Road over 150mph no problem. The biggest problem people have with this transmission is not adjusting the ITV cable correctly. I picked my transmission up from a place called pick and pull on half price day and it was $200 had a rebuild sticker on the side.

 

You need to have the cam, torque convertor and gears to to be compatible for how you want the car to perform. I once had a 83 Mustang with a built 460 (don't remember the cam specs), C6 stock convertor and 4.10 gears. It was okay around town and I did very little highway. Well I swapped in a 3000 stall and the car really sucked driving around town, went back to the stock convertor.

 

I think much of your problems can be solved by simply swapping to a TH350C and rigging up a switch for lockup. If you're rear ratio is something like 3.08, then it's obvious that you're at 3000 rpms because of the high stall. Adding a lockup TC will be much like adding a fourth ratio in that case. If you have 4.10s, you're at 3000 rpms because you're legit at 3000 rpms and a lockup won't do as much. Need to find that rear ratio

 

I drive 2800-3000 stall converter on the street for 40 to 120 miles a stretch city and country with 2000 rpm at 70 MPH a lot without issues. While it does have a lock up clutch, I don’t allow engagement very often as it’s not really up to heavy traffic nor getting on the power so having a manual override unless I‘m on the high plateau east of the passes or running along the coast where it’s pretty flat I just don’t use it.

The big thing on steep grades you can see the effect of the higher stall as the engine takes the load of climbing the revs will come up a little but nothing serious. To really see the stall move to its limits takes a pretty serious incline and a lot of throttle.

Vehicle weighs 2800 pounds with me in it with a 3.08 posi out back well built 350under the hood into a complementary built 700R4.

Bogie

 

You like the engine as is, keep it. Most of these cars had 3.31 gearing which is a nice compromise for what he is wanting unless he is looking for 140 mph romps. Do you need that much Convertor? 700R4 is by far the better way to go. Aftermarket ring and pinions are not the quality they were 40 years ago and your OE set attest to that. If you like how it runs at lower speed now, you won't want to change the gearing. 700 has a 3.06 first gear, th 350 has a 2.52. Again, do you need all of that convertor? You need enough for idle quality but with the lower first gear, you really won't need 3000 to move this car. I say 700R4. The science is readily available to do this and probably the easiest and dollar wise. Selling your old transmission setup will help with reducing $$$$.

 

3.08 gears would help. Your rpm at 60 mph sounds like 3.73 gears (assuming the trans is good and not slipping).