The Opposite of Overdrive

[ski4evr]

I have really high rpm's at highway speeds. For 70 mph my engine is at 4300 rpm. I'm happy that the engine can get up that high and still have power and room to go, but that's too high for that speed.

I started looking at rpm/gear ratio/speed calculator's online and I noticed that my c4 has an "effective ratio" (that's what I'm calling it) of 1.21. I figured in my tire size (185/75 R13) and 3.50:1 rear end. I have to have a 1.21 ratio with the transmission to get a speed of 70 mph with the engine at 4300 rpm. I trust the tachometer. It's new and it reads the same as my tach/dwell meter for tuning up to 2500 rpm (that's as high as the tach/dwell meter goes).

It makes sense to me that automatics won't have a true 1:1 ratio, but 1.21:1? Could this be right? Is this an indication that the transmission is hurting? It is 41 years old with 96,000 miles on it (no rebuilds).

ski

 

[JOHN G]

Someone correct me if I'm wrong, but I think effective ratio includes the slippage of the torque converter combined with the gear in the tranny.

 

[Anlushac11]

8)

I could be wrong but the C4 is correctly listed as having a 1.00:1 final ratio.

Your calculator does not take into account such things as parasitic power loss and aerodynamic drag.

 

[ski4evr]

Some of my numbers above are off. I rechecked them on the highway and crunched the numbers again - and they got worse...

The c4 does have a final drive ratio of 1:1. However, at 4300 rpm, a tire diameter (185/70 R13 - I listed the wrong series above) and a rear gear ratio of 3.50:1, I should be going 85 mph. But...I'm only going 65 mph! My speedometer is on the money. I timed myself over several interstate mile markers.

To get that 20 mph difference, my transmission must have an actual final drive ratio of 1.3:1. This means it's slipping pretty bad, right? Torque converter slipping and possibly the internal gears?

I used this page to do the conversions for me:

http://www.4lo.com/4LoCalc.htm

I used the "Find MPH" on and the tire diameter one at the bottom. I raised the transmission ratio up until I got my actual MPH that I was going (65).

Wind has nothing to do with the above calc's. The more wind/resistance the more gas to keep the engine going 4300 rpm. Hmmm..., actually... If there is more wind resistance or resistance in general, the transmission would slip more becuase it's under more load. Regardless, I don't think my transmission should be giving me a 30% reduction in rpm's.

Thoughts? Anyone experienced this?

ski

 

[PLP6400]

Are you sure rear end is 3.50? Turning that high of a number of RPM's @ 70 MPH, seems more like 4.11 gears.

 

[ski4evr]

My shop manual does not show a 4.11 ratio for a 65 mustang with a 7.25 rear end. The highest it shows for a 7.25 rear is 3.50:1 for a 170 cid Falcon, Comet or Fairlane.

Although a 4.11 rear with a transmission ratio of 1.1:1 works out when crunching the numbers.

ski

 

[PLP6400]

My 65 Comet, Falcon, Fairlane and Mustang shop manual show a 4.10 rear end. It is identified as a axel code 9 or I. It is easy enough to check what you have. If you don't have posi, just jack up one rear wheel. Put the trans in neuteral and spin the wheel two revolutions while watching your drive shaft. If it's a 3.50 ratio the drive shaft will turn 3 1/2 turns. If you have 4.11's it will be a little more than 4 turns.

 

[ski4evr]

Just jacked up the rear of the car and checked the ratio. I spun the wheel two times and the drive shaft rotated ~ 3 and 1/2 turns. I was kind of excited thinking a had a weird 4.11 ratio and not a slipping transmission. Bummer.

ski

 

[MustangSix]

13" tires? Those were standard on 64-65 cars I think, but you rarely see them. They are very small.

Is it possible the tach might be set for 4 cylinders? That would cause the RPM to read higher than it should.

You should expect about 400-500 rpm extra as torque converter slippage at highway speeds.

 

[ski4evr]

13" wheels were original on the 64 1/2 Mustang sixes. They are tiny. The tach is set on 6 cyl and it tracks with my other tach I use for tuning. I think I'm in for a transmission rebuild and a new convertor.

ski

 

[StraightSixer]

I had 13 wheels on my coupe when I got it last year. It is a T code 65 though, everyone I talked to had 14's (at least) on their stangs.

 

[Ronn]

That car can fit 225 60 R15. Just buy a set of 15 X 8" wheels and use that size tire..kinda like adding another gear. Not to mention better traction and looks.