[Lcode70]

Hello everyone, I'm brand new to the forum and this is my first post. I think my 12 bolt 410 is too low for cruising and would like some experienced input. Just bought my 70 SS and it's singing about 3k rpm at 60mph. I will spend 50% of my time at highway speed and 50% around town just cruising with no plans to race. I drive about 100 miles a month. What gears would be best? BTW, Local racing shop quoted me $700 for changing them.

 

[big gear head]

Is your car a L78 with the original rear end? It would be a CKK code on the axle tube. This is a very rare combination and worth preserving. If it's not an original 4.10 rear end then you're going to have to remove the cover and find out exactly what parts you have in it. If you have a 4 series differential then your options are very limited unless you change that to a 3 series. If you already have a 3 series with a ring gear spacer or a thick 4.10 then you have a lot more choices. The 3.31 is a very popular gear ratio for people who drive a lot.

 

[Lcode70]

Thank you for your very informative reply. I was told by the previous owner who did a frame-off restoration and owned it for 25 years that it was the original 410 posi rear end and he put new Strange axles in it. My lift is being put in Thursday and that's when I plan on cleaning it and checking the code. Unfortunately it's not the original motor but that's the way I bought it. The gentleman said he had around $10k in the motor when he had it built by a performance shop. I am unfamiliar with the three series or four series so I will do some more research. Thanks again.

 

[blm]

Another way is to go overdrive transmission although that route is more expensive. It allows for steeper gears around town but good to excellent highway rpm. There are many rpm calculators available online. Figure out what rpm you would be turning with certain gear ratios and then go out and cruise your car at that rpm. See how you like it.
I have a 3:73 rearend. I was considering changing to a 3:08 rear end. I calculated at low highway speed i would be turning about 2650 rpm so I went out and cruised my car at that rpm for about ten minutes on the highway. In my case I found the exhaust to be too loud at that rpm. I got an overdrive transmission and now I can cruise 70 mph and turn 2200 rpm and that is with the 3:73s. Exhaust is much more agreeable at the 2200 rpm.

 

[Lcode70]

I had no idea there were RPM calculators. I will definitely research that. My exhaust is also much more tolerable in the low 2000s. If I end up going with an overdrive transmission do you have any recommendations? Thank you so much for your experienced input.

 

[blm]

Tremec has a brand new 5 speed overdrive trans called the TKX. There is a current thread on that trans here.

Here we go...Tremec TKX

Well, I just ordered a TKX from Modern Driveline. I have a 67 136 car, currently a Muncie M21. The unit is the #17805, Gear ratio 2.87, 1.89, 1.28, 1, .68. I also ordered it with MDL Hydraulic Clutch Master Kit, Hydraulic T/O bearing, Tilton 6000 Slip-on, and their own MDL Clutch Set, GM...

www.chevelles.com

 

[buenymayor]

As the gear ratio gets lower, the pinion gear gets smaller. To offset this, GM moved the flange on the carrier closer towards the pinion to compensate for the smaller pinion gear. This is where the 4 series, 3 series, 2 series carriers come into play. 3:31s are a nice street gear, but will require a 3 series carrier. A 3.31 gear will bolt onto a 4 series carrier, but you won't be able to get it installed into the rear end because the ring gear will hit the pinion gear. Get a picture of your setup once you get the cover off..... we'll know more of what you have, then.

 

[weidhopper]

I have that same drive train except with a Muncie and I run a Gear Vendors overdrive. Very happy with the setup.

 

[Johnny Bravo]

I have a 406 with a Richmond super T10 (2.64 first) and 3.31 posi. I love it. Perfect street car gear, if you ask me.

 

[Lcode70]

Tremec has a brand new 5 speed overdrive trans called the TKX. There is a current thread on that trans here.

Here we go...Tremec TKX

Well, I just ordered a TKX from Modern Driveline. I have a 67 136 car, currently a Muncie M21. The unit is the #17805, Gear ratio 2.87, 1.89, 1.28, 1, .68. I also ordered it with MDL Hydraulic Clutch Master Kit, Hydraulic T/O bearing, Tilton 6000 Slip-on, and their own MDL Clutch Set, GM...

www.chevelles.com

Thank you. I'm hoping to just change the rear end gears mainly because it's the cheapest option (with the least side effects from my beautiful wife). But, after using a online rpm calc and its very interesting to see the difference in the rear end gears and a 5-speed. I will hopefully take the yoke cover off and post pics Friday. I can't thank you guys enough for all the help as I am new to all of this. I'm a 30 year career professional fireman, but in my next life I want to be a gearhead!

 

[big gear head]

This will keep you busy for a while. Wallace Racing - Automotive Calculators

 

[Lcode70]

As the gear ratio gets lower, the pinion gear gets smaller. To offset this, GM moved the flange on the carrier closer towards the pinion to compensate for the smaller pinion gear. This is where the 4 series, 3 series, 2 series carriers come into play. 3:31s are a nice street gear, but will require a 3 series carrier. A 3.31 gear will bolt onto a 4 series carrier, but you won't be able to get it installed into the rear end because the ring gear will hit the pinion gear. Get a picture of your setup once you get the cover off..... we'll know more of what you have, then.

Great explaination. I didn't even know there was a two series. Hopefully pics coming Friday. Thank you

Another way is to go overdrive transmission although that route is more expensive. It allows for steeper gears around town but good to excellent highway rpm. There are many rpm calculators available online. Figure out what rpm you would be turning with certain gear ratios and then go out and cruise your car at that rpm. See how you like it.
I have a 3:73 rearend. I was considering changing to a 3:08 rear end. I calculated at low highway speed i would be turning about 2650 rpm so I went out and cruised my car at that rpm for about ten minutes on the highway. In my case I found the exhaust to be too loud at that rpm. I got an overdrive transmission and now I can cruise 70 mph and turn 2200 rpm and that is with the 3:73s. Exhaust is much more agreeable at the 2200 rpm.

I have a 406 with a Richmond super T10 (2.64 first) and 3.31 posi. I love it. Perfect street car gear, if you ask me.

What are your rpm when cruising around 65mph? Thank you

 

[Johnny Bravo]

What are your rpm when cruising around 65mph? Thank you
[/QUOTE]

I'm around 2,700 at 65 with a 27 inch tire. 70 mph is about 3,100. It's perfect for me. With the 2.64 first and the torque of the 406, there's plenty of bottom end gittyup and there's no exhaust drone when cruising (I have sound deadener throughout the interior, so it never bothers me). You have all the torque you'll ever need for 3.31's. Granted, overdrive would be nice, but It's a 70 Chevelle. If it was that big of a concern, I'd buy a newer car to cruise at low rpm's. Just this guy's opinion.

 

[blm]

Thank you. I'm hoping to just change the rear end gears mainly because it's the cheapest option

Well it is definately your call. Another option you could consider is a Tremec TKO. That was the version that preceeded the TKX. They have been around for a while so used units are fairly common. Only problem with those is it requires cutting the trans tunnel and enlarging it a little. By the time you pay for the gear change you could be half way to a new trans. Take your time and consider all possibilities. Make the right decision for you.

 

[Lcode70]

Okay guys I had my Rotary lift installed yesterday and was able to take the rear end cover off and get several pics (attached). I have been using online calculators and I am now leaning toward a overdrive transmission and leaving my 4;10 gears, but..........
I ran into a problem! While I was under the car (on the lift) I noticed my rear coil springs were about an inch short from touching at the top and I pulled it out by hand! The spring appears very new, and measures 13" out of the car. My shocks are no good and they are Monroe Sensatrac PO68Z and they measured 21.5" fully extended and 13" fully compressed. Any thoughts which is wrong; the springs or the shocks? I have the F41 suspension. I just want the stock look and probably whatever came with the F41 originally.

Thanks

 

[blm]

Well i can tell you this much. That rear end is not original to the car. It is from a 69 that originally had a 3:31gear ratio.

1969 Chevelle Rear Axle Codes

Decoding Chevrolet VIN, trim tags, cowl tags, engine, engine block casting numbers, cylinder head casting numbers, intake manifold casting numbers, transmission, interior codes, and paint codes.

www.chevellestuff.net

 

[David Bates]

Okay guys I had my Rotary lift installed yesterday and was able to take the rear end cover off and get several pics (attached). I have been using online calculators and I am now leaning toward a overdrive transmission and leaving my 4;10 gears, but..........
I ran into a problem! While I was under the car (on the lift) I noticed my rear coil springs were about an inch short from touching at the top and I pulled it out by hand! The spring appears very new, and measures 13" out of the car. My shocks are no good and they are Monroe Sensatrac PO68Z and they measured 21.5" fully extended and 13" fully compressed. Any thoughts which is wrong; the springs or the shocks? I have the F41 suspension. I just want the stock look and probably whatever came with the F41 originally. View attachment 703384 View attachment 703384 View attachment 703385 View attachment 703386 View attachment 703387 View attachment 703388
Thanks

The 3917124 Casting number is for a late 1967 to early 1969 production year vehicle and is reflected in the casting date L-28-7 (not L-28-70) which is December 28, 1967 which would be for a 1968 Chevelle fwiw and not original to your 1970 Chevelle. The CF0208 stamped assembly code is for 3.31 posi assembled on February 8 (1968). The issue with your springs is common and most likely related to the extended length of your shocks. My shocks are 19-1/4" extended and 13" compressed. A lot can happen in the 50+ year life of these vehicles!

 

[Lcode70]

Well i can tell you this much. That rear end is not original to the car. It is from a 69 that originally had a 3:31gear ratio.

1969 Chevelle Rear Axle Codes

Decoding Chevrolet VIN, trim tags, cowl tags, engine, engine block casting numbers, cylinder head casting numbers, intake manifold casting numbers, transmission, interior codes, and paint codes.

www.chevellestuff.net

Agreed, thank you

 

[Lcode70]

The 3917124 Casting number is for a late 1967 to early 1969 production year vehicle and is reflected in the casting date L-28-7 (not L-28-70) which is December 28, 1967 which would be for a 1968 Chevelle fwiw and not original to your 1970 Chevelle. The CF0208 stamped assembly code is for 3.31 posi assembled on February 8 (1968). The issue with your springs is common and most likely related to the extended length of your shocks. My shocks are 19-1/4" extended and 13" compressed. A lot can happen in the 50+ year life of these vehicles!

I was hoping it was my shocks, since they need replacing anyway. Could you please tell me what shocks you recommend? Thank you

 

[blm]

Could you please tell me what shocks you recommend? Thank you

What I would suggest is you do a search in the Brakes,Suspension,Steering forum. That way you will be able to get the opinions of many and possibly those opinions will be backed up. Should be able to find plenty of info there.