Overdrive options

[Dakar Timm]

I drove the '66 coronet up to a shop in Columbus wisconsin to have an overdrive installed in the original transmission .
the owner told me there were severaloptions and brands i could pick from .
One style has the overdrive unit attached to the rear output shaft.
The other style ,would require swapping out the transmission for a unit that already has an overdrive .
Currently i'm running about 4000 rpm in third , with the 8 3/4" rearend . I'm not sure what ratio that is.
The motor is a 318 polyhead that has been breathed on with a mild cam ,headers ,650 ,4 barrel Holley and a Pertronix ignition .
I'm looking for 2500-2700 rpm at 65 mph. Thanks

 

[1 Wild R/T]

To have the engine spinning 4000 RPM's at 65 MPH with a 26.5" tall tire & a 1-1 standard non-O/D high gear would mean 4.88 gears in the back... Which is doubtful... Might want to find another tachometer... Honestly 25-2700 RPM's at 65 is more than I like...

 

[4speed68rt]

I don't think that they can "install an overdrive in the original transmission". As far as I know you can either bolt a Gear Vendors unit to the back of the original and cut the driveshaft, or put a modern transmission in place of the original. If it was a 4 speed you could install a A-833 overdrive trans from a 70s or 80s Dodge or Chevy truck. I would want to understand the rear gear ratio first. I'm with 1 Wild R/T on the tachometer check. At 4000 RPM on the highway at 60 MPH, you must have an old drag strip car. Changing the rear gear to a 3.23 or something like that would be more cost efficient and might get it where you want it on the highway. Probably make your engine last longer too.

 

[mgoblue9798]

What size are your tires, and what transmission do you have now?

 

[diesel_lv]

2.76:1 w no OD is 2,300 at 65. That is what mine came w factory. I have a GV OD w 3.73 now and it's 2,400 at 65. My area interstate is 75, I'm 3,000 at 80 on the interstate.
Use this site to figure what you want.
Speed/Tire/RPM calculator

 

[Dennis H]

2150 @ 70 feels about right.

 

[Gary Gaz]

Be alot easier to change the diff ration to a lower one, say 3.23

 

[Dakar Timm]

Hopefully some more information will help. At first I thought the transmission wasn't shifting out of 2 gear . I manually shifted the transmission through all three gears to make sure it was actually in third . The transmission is a 727 . The bore is 30 over at 3.94 ,with the stroke labeled at 3.31. The cam is listed at intake 1.844 with .397 lift .Exhaust is listed at 1.563 with .403 lift .Compression is listed at 9 to 1. I believe the carb is the Holley 650 street dominator. .TTI 3" stainless headers move the gases towards the back . I do not know if the rear ratio has been changed .
The tach ( Huge Sun unit ) shows 750 rpm at idle and shifts at about 3000. The third gear comes in at about 40 mph . Using Diesel lv link , I should be doing either 95 mph at 3000 or 2100 at 65 .. That sounds more realistic .However ,it isn't what I'm seeing .I'm not ruling out the rear end hasn't been changed . I just can't find a receipt for it ...yet.. one final note 14" up front with 15" in the back

 

[1 Wild R/T]

If you have access to another tach connect it & compare the readings... The Sun could be set up for a six cylinder... Or just wrong...

 

[icetech]

The tach might be set to six cyl (as stated above) BUT.. you need to find the approx rear end ratio, no receipt needed for that.. get under the car and mark the yoke and one of the drums, turn the yoke and count the revolutions until the tire makes 1 full revolution, that will get you close enough to guess 3.91 or whatever it may be. That honestly is the first thing you need to know.

 

[Dakar Timm]

If you have access to another tach connect it & compare the readings... The Sun could be set up for a six cylinder... Or just wrong...

Please explain how the tach could be set up for a 6 cylinder . There is nothing on the back of the tach for any cylinder setting

 

[RemCharger]

Please explain how the tach could be set up for a 6 cylinder . There is nothing on the back of the tach for any cylinder setting

If you can take the back cover off there may be a switch.

 

[Dakar Timm]

If you can take the back cover off there may be a switch.

Thank you , I was not aware of this . I did not install the tach , it came with the car

 

[1 Wild R/T]

Also some tachs have a wire loop in the wire harness, it the loop is left intact it's calibrated for a 6 & if you cut the loop it's set up for a V8..

 

[Dakar Timm]

Thanks for the feedback . The shop that might be doing the transmission swap called and said the rearend ratio was 3.23 .
This translates to 3600 at 60 mph . He is recommending A518RH as a replacement . The overdrive is .7 He is thinking a converter lockup should drop the rpms a few hundred more . Is this the same as a stall converter ? Thanks for all the help so far ...

 

[Dave6T4]

I have 3.23's in my '67 Coronet with 15" tires, and it runs about 2400 rpm at 60 mph. I think your tach is lying to you. ...or you have really small tires on your car. LOL
An A518 will not fit in your car without a lot of modification to the transmission tunnel and t-bar cross member. GearVendors O/D conversion might be less expensive, and it fits under existing floor. At one time, I was considering an O/D conversion, but ultimately decided that for the mileage I put on my summer car, for the $$$ involved I could buy a lot of gasoline.
A lockup converter is just the opposite of a stall converter. There is a clutch inside that locks the 2 elements together so that there is no slippage at a certain speed.

 

[diesel_lv]

Thanks for the feedback . The shop that might be doing the transmission swap called and said the rearend ratio was 3.25 .
This translates to 3600 at 60 mph . He is recommending A518RH as a replacement . The overdrive is .7 He is thinking a converter lockup should drop the rpms a few hundred more . Is this the same as a stall converter ? Thanks for all the help so far ...

I'm not sure what tire diameter you are running, but w a 26.5" tire and 3.25 gears on a 1:1 drive ratio 60mph is 2,500 rpm. For 3,600 at 60 mph on a 1:1 final drive w 3.25's would be an 18" diameter tire. Get a better shop that knows how to use an RPM calculator. I posted a link earlier that is very simple to plug in the numbers.

 

[diesel_lv]

67 Coronet with 15" tires, and it runs about 2400 rpm at 60 mph

You have 15" tires???? That's about the size of the tire on my wheelbarrow. You may have 15" rims an 26.5" tires.

 

[Dakar Timm]

The shop installed a replacement tach . The one that was installed previously was off by almost 1000 rpm at 4000. I didn't know the rearend ratio until this morning . Now I can use the RPm calculator you gave the link for

 

[Dave6T4]

So, when you go to a tire store, you order your tires by o.d. size? If I went to my local tire store to order a 26.5" tire, they would tell me they would have trouble finding a rim that big. I think most people understood what I was saying.