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Discussion Starter #1
I've got a problem on my drivetrain

When I'm cruising at 3000rpm on the third gear of my C4 (ratio 1:1, rear axle ratio 3:1 and wheel diameter about 23") the speed is theorically about 120km/h (75 Mph), but in fact my speedometer (and my GPS) indicate 70km/h (45Mph).

The engine is a 302ci with a edelbrock performer top end kit, the transmission a C4 performance automatic with a TCI saturday night specal (stall at 2000 rpm) torque converter

I already change the transmission and the torque converter but the problem is stricly the same (same speed)

I check the rear axle ratio, the engine is running very well, the transmission shift all gears, but the speed is nearly the half it should to be on the 3 gear

Is there someone with any idea to my problem ?

Thanks !
 

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Discussion Starter #3
The problem is not the speedometer but the real speed (mesurate with my GPS)
is not normal that at 3000rpm on the 3rd gear, the speed was too slow


speedo gear , get the right one (s) or can do it with larger dia. tires
 

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Is the problem only at highway speeds? What is your actual and indicated speed at 3000 RPM in second gear? Are you sure you have 3.00:1 gears in the diff? It sounds like the problem is with the transmission. From what you are telling us the output shaft of the transmission is not turning 3000 RPM in 3rd gear when the engine is turning 3000 RPM. Get the trans fixed.
 

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Discussion Starter #6
no,
the problem is on all gears and at all speeds, and the speed seem to be the half it should to be (in all gears)

so I think it's a converter problem, but i'm not sure, is there a way ton check the torque converter ?

It' s not a fail of the rear axle because the speed is ths same on the speedometer and on the GPS

Thanks !
 

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Did this only happen with the installation of the new C4 performance transmission? Is the trans and torque converter full of fluid? Maybe the TC didn't fill with fluid. I think you would know if the TC didn't engage the pump when it was installed, but check that, and also check the fluid level. Don't drive it until you have it all checked out.
 

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He had the same problem with another C4 and another torque converter!
How is it possible to have the same problem both time with different parts??

Pas idiot le coup de convertisseur mal rempli d'huile, du coup, il ne peut pas se verrouiller et reste à son ratio de base...
Vu que çà l'aurait fait sur tes deux convertisseurs, cela viendrait de leur alimentation en huile?
Là je ne connais pas, comment sont-ils censés se remplir? (via la pompe de la boite?)
 

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I've got a problem on my drivetrain

When I'm cruising at 3000rpm on the third gear of my C4 (ratio 1:1, rear axle ratio 3:1 and wheel diameter about 23") the speed is theorically about 120km/h (75 Mph), but in fact my speedometer (and my GPS) indicate 70km/h (45Mph).

The engine is a 302ci with a edelbrock performer top end kit, the transmission a C4 performance automatic with a TCI saturday night specal (stall at 2000 rpm) torque converter

I already change the transmission and the torque converter but the problem is stricly the same (same speed)

I check the rear axle ratio, the engine is running very well, the transmission shift all gears, but the speed is nearly the half it should to be on the 3 gear

Is there someone with any idea to my problem ?

Thanks !
Deome,
There is always some slip in the converter of an automatic transmission.
your problem seems to be more in the calculations that you are making. In a car with a 3.0:1 final drive with 23 inch tires it is unlikely that you will get more than 45 MPH. I run 26 inch tires and it has a calculated speed of 62 MPH at 3000 RPM.
I suggest that you look closely at your calculations and measure from the ground to the center of your axle for your radius and then double that for your tire diameter.
 

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Discussion Starter #10
I measure the wheel diameter and i Found approximatly 25"
I'd calculated the speed with a software "powerdyn", and this calcul depends on rear axle ratio, trans ratio and wheel size, and of course engine rpm
I discuss with other mustang's owners with the same configuration, and at 3000rpm they're (all) cruising close to 75Mph
I know that a little difference is normal but it's not the situation
 

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I measure the wheel diameter and i Found approximatly 25"
I'd calculated the speed with a software "powerdyn", and this calcul depends on rear axle ratio, trans ratio and wheel size, and of course engine rpm
I discuss with other mustang's owners with the same configuration, and at 3000rpm they're (all) cruising close to 75Mph
I know that a little difference is normal but it's not the situation
In your first post you said the tire size was 23"... Is it possible you put in the wrong size? all calculators need precise data to give precise results.
 

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Are you sure the trans is running in high gear and not second? A C4 would figure out around 50 in 2nd gear, more or less. I don't think you can buy a speedometer gear that will run 50% off in a C4. If the engine is really running at 3000 and you're sliding along at 45, you'd know there was a problem without a GPS.
 

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He had the same problem with another C4 and another torque converter!
How is it possible to have the same problem both time with different parts??

Pas idiot le coup de convertisseur mal rempli d'huile, du coup, il ne peut pas se verrouiller et reste à son ratio de base...
Vu que çà l'aurait fait sur tes deux convertisseurs, cela viendrait de leur alimentation en huile?
Là je ne connais pas, comment sont-ils censés se remplir? (via la pompe de la boite?)
Bad luck and/or poor installation. Who installed the transmission? Who built the transmission? How much fluid was put in? Has he checked the fluid? Does it smell burnt? Does it actually take three revolutions of the drive shaft to get one revolution of the rear wheels?
 

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Discussion Starter #14
I've installed the first transmission a Performance Automatic with a TCI converter, I put 9 Liter of Dexron 3 according to the manufacturer,

I rebuilt a re-install the 2d transmission with my used stock converter
I put approximatly the same volume of fluid

I check the rear axle and he drive shaft take 3 revolutions when a wheel take 2 revolution (the other wheel stay on the floor)

The fluid not smell burnt, and fluid level is Ok, fluid pressure too

Bad luck and/or poor installation. Who installed the transmission? Who built the transmission? How much fluid was put in? Has he checked the fluid? Does it smell burnt? Does it actually take three revolutions of the drive shaft to get one revolution of the rear wheels?
 
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