TH 350 RPM WTH Questions

[avejoe]

My truck seems to be turning really high (3.5k at 55mph). I don't have big tires. 3.08 or 3.07 gears (don't know much about truck gears). I feel my transmission shift from 1st to 2nd to 3rd. It feels like there should be another shift given the rpms. Am I missing a gear or is something else going on?

Thanks!

[Dean23]

Take it out on the road in drive...bring it up to speed then shift from drive to 2nd. see if you feel it downshift. if not you are stuck in 2nd. If it shifts down. then you were in 3rd and you have some taller gears then you think out back

[avejoe]

Sometimes the simple solution alludes..

Thanks!

[Longhorn Man]

Other things to look at...
How do you know you have 3.07 gears....sounds more like 3.73 to me...even lower.
Is the speedo right? You might be doin 80...but you'd still be doin less than 3500 RPM with 3.07 gears.
Is the tranny slipping?
Is the tach reading right? (don't trust 35 year old tachs, they were wrong 35 years ago)

[avejoe]

Confirmed 3.07 gears on the front diff. Trans not slipping. Speedo is ok. If I'm doing 80, then everybody else is doing 100!

Tach reads ok at idle. It seems to be close compared to my "ear tach".

My t-case cannot grab 4-hi. Could it be something related to that?

So what your saying is that with 3.07 gears, I should not be doing 3500 rpm at 50-60 mph?

Oh, and I do have three forward gears.

Thanks!

[mrein3]

Originally Posted by avejoe

Confirmed 3.07 gears on the front diff. Trans not slipping. Speedo is ok. If I'm doing 80, then everybody else is doing 100!

Tach reads ok at idle. It seems to be close compared to my "ear tach".

My t-case cannot grab 4-hi. Could it be something related to that?

So what your saying is that with 3.07 gears, I should not be doing 3500 rpm at 50-60 mph?

Oh, and I do have three forward gears.

Thanks!

You never know any more how fast everybody else is going. Today's cars are well equiped to go that fast. My daily driver is a 94 Escort. I go 80-85 every morning without even thinking twice about it. And folks do pass me.

The best thing to do first is to determine if your speedo is correct. Take your GPS for a ride. If you don't own one I'm sure you have a friend who does. My mutt truck (70 cab, 71 chassis, 72 tranny and transfer case) is within 1 mph from what the GPS says at all speeds.

You have a 4x4. What I found with my 4x4 is that if you have 3.73 differential gears, and a NP205 transfer case, your speedo should read correctly with 28-29 inch tires and no gear reducer/increaser in the speedo cable. The attached photo is a picture of a deal I found on a Blazer that came from the factory with 3.07 gears. This contraption was screwed into the transfer case where my speedo cable goes. It changes the speed of the speedo cable to make your speedo read correctly.

I think your first step is to determine if your speedo is correct.

Another way to determine how accurate your speedo is would be to time how long it takes you to go from one mile marker to another. Get up to 60 mph on your speedo and time how long it takes you to go from one mile marker to the next. Obvioulsy it should take 60 seconds to go from one marker to the next. [I'm not responsible for any tickets incurred while doing this]. If my math is right it should take 1 minute and 5.45 seconds to go 1 mile at 55 mph.

Before you go replacing your tach or anything else make sure you really are going 55 mph when your speedo says you are going 55 mph.

[Longhorn Man]

These pics were taken with 3.07 gears, 28 inch tall tires, and a TH350 with NON-lockup converter.





That's why I say something isn't right.

[avejoe]

I have the speedo gizmo that goes into the t-case like in your picture. The only needle that compares in your picture to mine is the fuel.

I do have 30" tires.

[mrein3]

Originally Posted by avejoe

I have the speedo gizmo that goes into the t-case like in your picture. The only needle that compares in your picture to mine is the fuel.

I do have 30" tires.

Take a GPS for a ride. 28" tires are stock. 30" tires are nearly 10% taller than stock.

Taller tires make your final gear ratio smaller (taller).

For example if you have 3.73 gears and 33" tires you have, in effect, 3.07 rear end gears. 33" tires are approximately 20% taller than 28" tires and 3.07 gears are approximately 20% "taller" than 3.73 gears.

You need to figure out how fast you are going.