How many revs

[Route--66--]

..maybe a bit of a geeky question but hooked up a generic rev counter to my Mustang '65 6cyl 200ci today just out of interest to see how many revs I was doing at 50/60mph... (yeh it was a lazy Sunday sunny afternoon!)
at an indicated 50mph (real speed about 46) it was about 3000rpm
and
at an indicated 60mph (real about 56mph) it showed 4000rpm

I have a 2.83 rear axle and C4 standard box

would this be about right..seems really high???

Cheers!

 

[anotherhassel]

It sounds very high. My 68 200 c4 turns like] 2300 @ around 62-65 roughly. If you installed a generic tech check and make sure you put the switch to the "6" position. God luck let us know how you make out
.

 

[Route--66--]

Cheers, yeh it was set to 6 cyls ..
to be fair it does return about 23-24mpg on a run(English gallons are a bit more than US though .. so that's about 19-20mpg U.S.) so maybe the gauge is a bit off ..?.. wouldn't be my torque converter wearing out would it ...?

 

[62 Comet]

My rear is 3.20 with the ford o matic 2 speed auto and at 50 mph it turning over 2200 and at 60mph its at 2800, yours sounds pretty high, arent both c4's and ford-o-matic's final gear 1:1? converter could be not pumping as well but my first thought was your rear end gearing...just my 0.02

 

[Route--66--]

Have checked out how many revs I should be doing with
http://www.csgnetwork.com/multirpmcalc.html
and it comes out about 2350rpm at 60 . . .as the mpg is pretty good for a car of this type I now think the rev counter itself must be out ..
car has only done 40,000 miles indicated (which I think is correct) so am as sure as I can be that the diff is the original 2.83 as marked on the data plate .. .
so ... I guess it is the rev counter that's out . ...I hope !
Cheers again!

 

[MustangSix]

Add another 500 rpm for torque converter slip to the gear calcs unless you have an AOD or other lockup transmission.

 

[66 Fastback]

Are your tires and wheel sizes the stock size equivalent?
Doug

 

[FalconSedanDelivery]

Add another 500 rpm for torque converter slip to the gear calcs unless you have an AOD or other lockup transmission.

If your converter is slipping 500 rpm's its Junk , most converters slip 6% or less even my Pinto converter behind my Mild 351W only slips 300-350 and that's at wide open throttle not at 60, there is no way your engine is turning that rpms unless your math or specs are off

 

[Route--66--]

Add another 500 rpm for torque converter slip to the gear calcs unless you have an AOD or other lockup transmission.

If your converter is slipping 500 rpm's its Junk , most converters slip 6% or less even my Pinto converter behind my Mild 351W only slips 300-350 and that's at wide open throttle not at 60, there is no way your engine is turning that rpms unless your math or specs are off

Yeh, am now pretty certain the old rev counter was off, or I just didn't connect it properly after losing the instructions sometime in the late 80's !!!

Yeh tyres/wheels are correct size Doug

Cheers Both!

 

[66 Fastback]

Well the other possibility is that the speedometer driven gear is incorrect and the indicated speed is not correct. I would tend to believe that an electronic tachometer has a better chance of being accurate than the speedometer. The speedometer has several components that have to function together to come up with an indicated speed that at best may only be within 2 to 5% accurate. What are the odds that the rear axle ratio, tire size, transmission, speedometer drive gear, speedometer driven gear have never changed from stock values. Have you verified you indicated speed by some means.
Doug

 

[Route--66--]

Cheers Doug
I checked out the speedo using a satnav and the speedo is out.. is indicating around 4-5 mph higher than actual speed.. not too much though..
Still to get a rev counter .. birthday is in a few weeks though...
Cheers!
D

 

[xctasy]

Satnav is always out a few miles per hour from my experience. Everytime I calibrate my 82 Mustang, 98 Explorer or 96 RAV4, by the elasped time between five 1km markers, the readings it gives is 2% lower than what I get by actual distance covered. Funny thing is my 82 Mustang has almost exactly the same gearing as your car with its 190/65 390 tires and 2.73:1 diff. 2241 rpm at 60 mph. And both my RAV4 (4-stage auto) and my Exploder (5-stage auto) loose less than 200 rpm max at 60mph when the lock-up clutch loops out, so thats about what I'd expect for maximum torque converter loss.

Three things to remember are that
1) the loaded wheel radius differs by about 2.2 % from the static radius.
2) a C4 has clutch slip,as stated
3) The correct formula for a 23 to 27" tire must take acount of tire crush, so it is:- calculated radius in meters (good ole imperial times 0.0254) times 60 times pi, all over the diff ratio. That gives km/h per 1000 rpm. To get out of the Kingdom of France back into Olde Worlde Imperial, divide by 1.60934.

So the correct modern rendition for an old Mustang cross ply is most likely a P185/78 SR 14 replacement, which would be ((185*0.78*2)+(14*25.4))=644.2mm, or 0.6442 m or 25.36 inches
Divide by 1.022 for wheel crush (0.6303 m or 24.82 inches)
Then curn through 0.6303*60*3.141 and divide the answer by 2.83.

41.98 km/h per 1000 rpm, or 26.08 mph per 1000rpm. At 60 mph, you should be doing 2300 rpm exactly. Based on what I've seen in lock up clutch cars, T/C slip is indeed 6% max under load, but even lockup clutches still have 1% slip unless its an AOD.

 

[Route--66--]

Good grief!
Thanks for that comprehensive response!
Will be getting a counter soon so will put those numbers to the test !
Cheers!
D

 

[100_2023]

I guess you've never read one of Xctasy's posts before. I always enjoy them. He always has a good perspective on things.

 

[ludwig]

Xctacy has about five jobs and if he ever feels the sniffles coming on, he had to call the government because there will be a significant fallout at the harbor, road section, transport and engineering fields.

 

[STEVEN.]

He's a Man of MANY vices 8) . ~OO6.

 

[woodbutcher]

Good Lord.When xctasy retires they will have to restructer the local govt.
Good luck.Have fun.Be safe.
Leo