2F,4:88,37'

[Mace]

I'm pretty sure most of us know about the speed we are going. 60 mph vs 90 or 40 mph. So, we can guess that he is going about 60 mph. It's an assumption but...

How do you know it's in high range? Was anything else done to your rig other than the tires/gears? What are you verifying the engine RPM with?

 

[Output Shaft]

Known facts:
Gears are 4:88. Verified
Tires are 37"
Engine sounds like it's gunna blow at 60mph = 3500 rpm. Verified.
Clutch is probably not slipping since this "issue" turned up the moment after the new gears were installed (statistically improbable).
Tachometer is probably reading correctly as there was no problem before the gear swap and the engine is now racing at 60 mph.

There's only one ducky left...

I'm not going to come right out and say it, but the answer has already been stated in earlier posts....

 

[SteadyT3ddy]

SOA/Acel igniter. My tach shows 3500 rpm(havnt checked any other way).. I do have a little exhaust manifold and few exhaust leaks ( that probably add to the "engine about to blow up" sound) other then that nothing has been done to the vehicle.

 

[elripster]

How would I hook this up?

I comes with a clamp on feature that goes on a spark plug wire just like a timing light.

Frank

 

[Shebamacalla]

As everyone has said the numbers don't add up. It can't be in low range because then it would be at 3500k closer to 35mph than 60. Did we determine if it's a 4 speed manual or auto? Were you running the 37s before the regear? Did you start with 3.7s? What was your rpm at 60mph before the regear. The tachometer has to be wrong. I never liked pushing my 2f past 2800. So maybe it's around there which would make more sense and just sounds different than it did before the regear which was probably closer to 2000k at 60mph.


Sent from my iPhone

 

[Shebamacalla]

I think maybe it's been said before but with your gps mph app take a mph reading in every gear at 2000rpm on the tach and report back your findings.


Sent from my iPhone

 

[elripster]

Are we to assume that your gps and speed agree after gearing to 4.88's and switching to 37" tires?...

Frank

 

[SteadyT3ddy]

Tires are 37/12.5R16.5 Wrangler military MTRs.

 

[elripster]

So..... playing with numbers... as mentioned you can't be 4lo, that's a 2X delta. Even assuming the tire deflection gives you a 36" effective diameter, you would need a 6.25:1 diff ratio to get the engine up to 3500 RPM in 4th at 60MPH. The diff is likely 4.88 as you measured. At effective 36" tire diameter 4.88's would be 2733 RPM in 4th at 60 MPH. The RPM difference is about 22% (2733/3500) unless you assume true 37" tire diameter in which the real RPM drops to 2659 and the difference becomes 28%. The difference (1/1.41) between 4th and 3rd should be 29%. You sure you didn't try to shift her into 5th out of habit?

Tachs are simple gauges and usually work correctly or don't work at all.

Frank

 

[Output Shaft]

Tires are 37/12.5R16.5 Wrangler military MTRs.

Those tires when brand new have a diameter of 36.5 in.

And if worn, they could be 36 in. or less if real worn.

That would get you up above 2900 rpm at 65mph.

 

[SteadyT3ddy]

The tires are 95%+ thread.

So..... playing with numbers... as mentioned you can't be 4lo, that's a 2X delta. Even assuming the tire deflection gives you a 36" effective diameter, you would need a 6.25:1 diff ratio to get the engine up to 3500 RPM in 4th at 60MPH. The diff is likely 4.88 as you measured. At effective 36" tire diameter 4.88's would be 2733 RPM in 4th at 60 MPH. The RPM difference is about 22% (2733/3500) unless you assume true 37" tire diameter in which the real RPM drops to 2659 and the difference becomes 28%. The difference (1/1.41) between 4th and 3rd should be 29%. You sure you didn't try to shift her into 5th out of habit?

Tachs are simple gauges and usually work correctly or don't work at all.

Frank

5th? i was going to throw it into Reverse from 4th hoping would be a overdrive hack...

 

[elripster]

Isn't reverse on the 1st gear side? I have the h55f so the 4 speed shift pattern is fading in memory...

 

[SteadyT3ddy]

Was totally kidding.. Yes Reveres is like

 

[elripster]

Had you succeeding in reverse at 60mph this would probably be a very different thread.

 

[cruisermatt]

@Mace

Is it possible for the transmission shifter to be indicating 4th gear but the shift collar to stay on 3rd? And when he goes it downshift it grabs 3rd and goes the rest of the way down?

 

[SteadyT3ddy]

Was totally kidding.. Yes Reveres is like View attachment 1150590

Lets save that for another time...

@Mace

Is it possible for the transmission shifter to be indicating 4th gear but the shift collar to stay on 3rd? And when he goes it downshift it grabs 3rd and goes the rest of the way down?

Doubt that lol

 

[RockDoc]

What does your cable driven speedo read? The change from 3.73 to 4.88 gears should balance pretty close the change from stock (28" I think) to 37" tires. Have you done more than one run to rule out wonkiness on the GPS figures?

Are you forgetting to mention the fact that you are laying down massive rubber at highway speed? Transfer case has been regeared with underdrive gears?

 

[cwwfj60]

I vote for a different gps app or possibly a buddy to drive next to you to verify speed, somethings gotta give here

 

[Mace]

Crawlpedia calculator..
Engine RPM / Engine Speed Calculator

Fun toy to play with. Even if it was stuck in 3rd gear (1.516:1 ratio) that would put him at about 4000rpm at 60 mph


So you've had this truck for a while now (bought at auction right) and have been driving it with smaller tires and Stock (I assume) gears in the axles correct?

 

[onemanarmy]

3500 RPM?!

I'm scared to go anywhere near 3k.