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weight39
New Member

Canada
25 Posts

Posted - 09/02/2002 :  8:54:34 PM  Show Profile  Reply with Quote
As you know I just got my 78 Mustang 302 back on the road.

On a highway test run I encounter some problems. At 60 mph my RPM's were around 3500. Is that normal?

Also my speedometer was way off. It said that I was doing 80mph but I was really doing 62mph according to my friend following me. What can be done about this? Tires account for a slight variation (I'm getting 195/70R13 tires this week...the guy before me had 175/70R13 on the front for some reason) but not for that big of a difference. I'm wondering if it could be a gear in the transmission or the speedometer is messed up...I don't know.

My temperature went above normal and I had to pull over and let it cool. The thermostat I installed is 193degrees. What do you guys use?

I might need a new radiator because it may be restricting flow. I am trying to way out the cost of getting professionally flushed vs getting a new one. Are 3 core rads hard to find for these cars?

Thanks all.

cobracmdr
New Member

USA
25 Posts

Posted - 09/02/2002 :  11:10:56 PM  Show Profile  Reply with Quote
Do you know the rear gear ratio? It sounds like it might have been changed before. You can buy a new speedometer gear to correct it. I know mustangs unlimited had a few different speedometer gears to choose from.

As for you cooling problems, I used a 180 degree thermostat. I still had overheating problems because i had a 2-row radiator. When i installed a 3-row radiator, the temperature improved alot. Modine makes a 3row radiator for a 77-78 mustang II. It fit perfectly in my 75. The radiator cost me around $260.

Hope i was of some help

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weight39
New Member

Canada
25 Posts

Posted - 09/03/2002 :  12:26:30 PM  Show Profile  Reply with Quote
I'm going to give the 180 degree thermostat a shot first and have a shop look at the old Rad before I invest in a new one (I have a 3 row rad in it now...maybe it can be saved).

I'm not sure about the rear gear ratio. I will look into that.

I forgot to mention that I have an Automatic transmission and the RPMs were around 3500 for 60 mph. I was wondering if that is too high? I just got this car so I'm not sure what is normal.

Thanks

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weight39
New Member

Canada
25 Posts

Posted - 09/03/2002 :  9:13:33 PM  Show Profile  Reply with Quote
I guess I will have to rebuild this transmission. 3500 rpm is way to high. The tach and speedometer that much off is insane.

Does anyone know the part number for the automatic transmission on the 1978 302? I want to make sure the correct one is in there before I get it rebuilt (Although I imagine it is pretty difficult to put a different transmission in).

Also I would like to take a good kit to the transmission shop. Can anyone recommend a good kit?

Thanks all.

Edited by - weight39 on 09/03/2002 9:59:39 PM
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Percy Hazel
Junior Member

USA
374 Posts

Posted - 09/04/2002 :  12:50:19 AM  Show Profile  Reply with Quote
I run a stock totally reconditioned V8 radiator with
a Flow-kooler water pump and a "high flow" [Stat makes them & others]
thermostat....No problems
As to Road RPM if you are running 195/70X13 rear tires they have
a rolling radius [center of tire to ground] of about 10.5" or a effective diameter of 21".
With that kind of tire size and if you have the 3.4:1 rear end ratio
[common for V6 not V8 but fits] you would be turning 3200-3300rpm
@ 60mph.
If you do not know the ratio jack up the car and turn one of
the wheels until the drive shaft rotates one full revolution.
If both wheels are not turning together divide by 2.

Good luck


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Percy Hazel
Junior Member

USA
374 Posts

Posted - 09/04/2002 :  12:54:34 AM  Show Profile  Reply with Quote
Oops forgot
After you find out the ratio go to MustangII.org and use
their speedo gear calculator.
When you know which one that you need your local Ford dealer
can probably supply it......mine did.

Later

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Phughes
Junior Member

112 Posts

Posted - 09/04/2002 :  2:27:14 PM  Show Profile  Reply with Quote
is it possible that his tach is just set up for a 6 cyl?

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weight39
New Member

Canada
25 Posts

Posted - 09/06/2002 :  1:49:09 PM  Show Profile  Reply with Quote
Percy I jacked up the car and turned the tire. Only one tire rotated and the tire turned 3/4 a turn for every 1 axle turn. I'm not sure how to interprete those numbers. Any help is appreciated.

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weight39
New Member

Canada
25 Posts

Posted - 09/06/2002 :  4:24:00 PM  Show Profile  Reply with Quote
I checked the number on the transmission and it says D4ZP-A040-AA. I need to confirm that I have the right transmission in there before I get it rebuilt.

I called Ford and they said I should have a C4 in the car.

Thanks

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weight39
New Member

Canada
25 Posts

Posted - 09/06/2002 :  6:41:29 PM  Show Profile  Reply with Quote
I went to Ford with that part number and they told me that the C4 transmission on my car was from a 1974 4 cylinder. They told me that it was geared differently and I was going to burn it up unless I get a new tranny.

The guy at the transmission shop told me that still does not account for my speedometer being out (he said in 3rd gear they should be the same).

I know that the rear end ratio's were different in 1974 then they were on my 1978. According to my Haynes book the rear ends on 1974's had a 3.55:1 gear ratio. After 1975 they had a ratio of 3.0:1. I believe that I have the original rear end on the car.

I'm wondering if this transmission could account for the speedometer being off so much?

Thanks.

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Bobby Aldrich
Advanced Member

USA
4017 Posts

Posted - 09/06/2002 :  9:50:15 PM  Show Profile  Reply with Quote
If you feel your car has the origial rear end. You can check the drivers door information decal for the axle code to find the ratio your car came with. If your rear axle still has the I.D. tag it will have the ratio stamped in it. My 1975 MustangII was running at about 3,400 RPM at 65 MPH . It had 3.55 gears . I changed the rear end to 3.00 gears. I then had to change my speedo gear. The engine now runs at about 3,000 RPM at 65 MPH. It's alot nicer to drive. If your speedo is reading 18 MPH fast. You will need a gear with more teeth. Each tooth will = about a 3 MPH change. Count the teeth on the gear at the end of the speedo cable. Then see if you can get the gear with the correct teeth that are needed. If the gear you have aready has alot of teeth, say 20 , you may need to change the drive gear inside the trans. The Ford driven gear teeth numbers seem to be between 16 and 23. Good luck.

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