3,500 rpm 2 55 mph with 12.5 x 33? Is this normal?

[oldracerguy]

I have a sbc mated to what I think is the stock FJ40 1978 tranny using a Marks of australia adapter. I am turning 3,500 going 55mph as verified with my GPS. I hear some guys with their 2F's are running much lower rpm and can run 70 mph down the highway. Is this right? I have the 4.10 gears still installed. Your thoughts?

 

[FF LC FREAK]

I'd say no. Something is wrong. I have 33" (figure 32 or so tall for calculator accuracy) tires with 4:11s and turn about 2700 rpms at 60 mph. Are you sure your diff ratio is 4:11 and not 4:88 or taller? Check out this website to see some calculators that can help you out - Gear Calculator. Something is definitely not right if you are turning 3500 rpms at 55 mph.

Good luck!

 

[Mace]

shift into 4th?


You check that the Tach is adjusted properly for your motor (i.e. 8 cylinder instead of 4 or 6 cylinder)

 

[fieldsken1]

take out of low range

 

[Splash]

The chart at the attached link is very helpful.

4Lo.com :: Gear Ratio & Tire Size Chart

It shows that 33s with 4.10s should run at 2720 RPM at 65 MPH.

 

[oldracerguy]

You guys crack me up.....

Okay, it is in 4th gear, it is in high range

I'm going to go back out to the shop, jack it up, and see how many times the driveshaft rotates for one revolution of the tire. I thought I did this already to verify that it still had the 4.11 in. Be back soon.........

 

[Degnol]

Make sure your tach is not set on 6cyl I'll bet it is and you are actually turning about 2600rpm.

Ed


EDIT: Whoops! Did not fully read Mace's response

 

[oldracerguy]

tach is good but what did I find?

Okay so I just went out and jacked up the rear end. I marked the driveshaft and made one complete revolution with the rear tire. The driveshaft made just over 2 revolutions?!?!?!?! this doesn't make any sense to me as it would indicate I have a 2.10 rear gear? what am I missing?

 

[Mace]

Okay so I just went out and jacked up the rear end. I marked the driveshaft and made one complete revolution with the rear tire. The driveshaft made just over 2 revolutions?!?!?!?! this doesn't make any sense to me as it would indicate I have a 2.10 rear gear? what am I missing?

open diff, the other tire is not turning so the spiders are working overtime.

You have a 4.10 diff.


Check your tach..

 

[Mace]

BTW, do you have an additional lever in the floor somewhere (underdrive instead of a marks adapter??)

 

[oldracerguy]

Thanks1

open diff, the other tire is not turning so the spiders are working overtime.

You have a 4.10 diff.


Check your tach..

tach is good. Thanks for the observations on the open diff. No ther levers. My paperwork from the prior owner says Mark's adapter. what would I be looking for. Big aluminum housing between the smallblock bellhousing and the 4 speed tranny. I'll keep looking.................. No other levers anywhere. I'll try to take a picture of the adapter and see if you guys see anything funny.........

 

[Mace]

take a picture behind the tranny as well.

I am still betting it is the tach tho..

Does the motor sound like it is revving to the moon? Like really spinning?

 

[Degnol]

I am still betting it is the tach tho..

Does the motor sound like it is revving to the moon? Like really spinning?

X2! Its all gears and spinning stuff...not rocket science. Either your clutch is slipping or your tach is fubar'd. Its all about X times X times X......

If you have a timing light that shows rpm, check it against the tach that you are reading from. You keep saying "tach is fine", but is it selectable and is it set on 8cyl?

Ed

 

[oldracerguy]

Tach is spot on.

My tach is not selectable. I can get my idle to run right down to around 650 rpm. The motor is spinning really high. It runs like a $#%& ape. clutch is a heavy duty Center Force with only a few thousand miles on it and it does not slip. Driveshafts are stock length front and rear so I am thinking no extra adapters in there. Could my actual gears in the transmission be different. This seems unlikely as the V8 adapter looks like it is the only thing different than stock. It all looks original to me, even has the factory skid plate still on it. What other tests can I do to confirm gearing in the tranny or transfer case?

 

[Degnol]

My tach is not selectable. I can get my idle to run right down to around 650 rpm. The motor is spinning really high. It runs like a $#%& ape. clutch is a heavy duty Center Force with only a few thousand miles on it and it does not slip. Driveshafts are stock length front and rear so I am thinking no extra adapters in there. Could my actual gears in the transmission be different. This seems unlikely as the V8 adapter looks like it is the only thing different than stock. It all looks original to me, even has the factory skid plate still on it. What other tests can I do to confirm gearing in the tranny or transfer case?

Well.................I have to ASSuME that the tachometer is electronic....and we all know that anything ELECTRONIC does not fail But find a way to verify your rpms, other than the fine, good tach you have. Otherwise, it just does not make sense.
Something is not telling the truth.

Ed

 

[oldracerguy]

Well.................I have to ASSuME that the tachometer is electronic....and we all know that anything ELECTRONIC does not fail But find a way to verify your rpms, other than the fine, good tach you have. Otherwise, it just does not make sense.
Something is not telling the truth.

Ed

Okay ok, enoug already I say. You have shamed me into buying a replacement tach. I needed an excuse to do this anyway as the one I have is too small and I would like the option of having a shift light, just to warn me of my rpm's when I am in the sand dunes. I am still thinking I will find nothing as this thing is really singing going down the road. ...but let's confirm before we chase the proverbial geear mystery. Thanks!

 

[Mace]

Okay ok, enoug already I say. You have shamed me into buying a replacement tach. I needed an excuse to do this anyway as the one I have is too small and I would like the option of having a shift light, just to warn me of my rpm's when I am in the sand dunes. I am still thinking I will find nothing as this thing is really singing going down the road. ...but let's confirm before we chase the proverbial geear mystery. Thanks!

Where are the pics

btw, other than a stock Tach, I have never seen an aftermarket tach that was not adjustable for 4,6,8 cylinders..

 

['74 UA FJ]

My money is on the tach-o-meter.

 

[Mr. Toad]

Okay ok, enoug already I say. You have shamed me into buying a replacement tach. I needed an excuse to do this anyway as the one I have is too small and I would like the option of having a shift light, just to warn me of my rpm's when I am in the sand dunes. I am still thinking I will find nothing as this thing is really singing going down the road. ...but let's confirm before we chase the proverbial geear mystery. Thanks!

You can confirm your axle ratio by lifting one rear wheel and rotating it twice while counting driveshaft revolutions.

To determine tranny gearing, I suppose you could put the tranny and transfer in hi gear/range, lift both rear wheels and rotate the engine from TDC to TDC while your helper counts the driveshaft revs. Should be 1:1 if it's a stock tranny.

 

[oldracerguy]

I'm and idiot............

My money is on the tach-o-meter.

I pulled the back of the tach off and thee is a switch that selects from 4 to 6 to 8. It was on 6. I switched it, took it for a test drive, 2,650 rpm at 55 mph! I guess I can spin that little 383 a little higher...............
Thanks for all the feedback!!