Rpm to speed

[Chuckanut]

Hello everyone I need help from people that have a tach in there truck and have stock gearing/tire size with the stock th400 or if they swapped to a sm465

I have a gear venders in the truck and at 2000rpm at .78 OD in going 45mph. In 1 to 1 in going 35mph at 2000 rpm

A rpm calculator said im supposed to be going 52mph in overdrive ( IS THIS TRUE)

Everyone said its the weight of the truck with touque converter slip. I want to know if other people get similar results with a stock cucv.

If I have a manual trans ( no slip) will its go the supposed 52mph?

 

[Skinny]

How are you measuring engine rpm? I mean a gearing calc and physics aren't bendable. So either what you physically have in the truck vs. the numbers in the chart don't jive or your rpm measurement on the engine isn't accurate.

 

[Chuckanut]

How are you measuring engine rpm? I mean a gearing calc and physics aren't bendable. So either what you physically have in the truck vs. the numbers in the chart don't jive or your rpm measurement on the engine isn't accurate.

I have a a tach hooked up to the alternator, and we double checked the tach in the truck with a handheld one

 

[Skinny]

I have had issues with reading accuracy off idle with my alternator and also with other sources of interference like blower motors. When you say handheld tach like a pickup clamped to an injection line?

My tach output on my alternator requires a resistor in series because the signal would wander and go high when driving compared to idle. Something to think about.

 

[Barrman]

I have had the same issues with the pick up that clamps around the alternator. The Dakota Digital tach signal adaptor also pulls from the alternator but in my experience is much more reliable and accurate.

 

[Chuckanut]

I have had issues with reading accuracy off idle with my alternator and also with other sources of interference like blower motors. When you say handheld tach like a pickup clamped to an injection line?

My tach output on my alternator requires a resistor in series because the signal would wander and go high when driving compared to idle. Something to think about.

Sure buddy

 

[Skinny]

I don't know what that means?

Barrman - I added a small resistor on my pickup wire and it solved all my problems. I think it was just enough filter but my tach is spot on now and doesn't wander around like before. Worth a try if you are having issues.

I just did a reseal on my 6.2 and my timing cover had some cracking. So I replaced it with the latest cover which has a port like the 6.5 for a crank trigger. I added the time wheel as well. So if this doesn't work I have a back up.

Never thought a tach signal would be so difficult to work with.

 

[Tinstar]

NEIKO 20713A Digital Tachometer, Noncontact Laser Photo Sensor with 2.5 to 99,999 RPM Accuracy, RPM Gauge Marker with Batteries Included: Digital Tacometer For Measuring: Amazon.com: Industrial & Scientific


I used this to dial in my tach (using alternator for signal) and it worked perfectly.
Super easy to use.

 

[Chuckanut]

NEIKO 20713A Digital Tachometer, Noncontact Laser Photo Sensor with 2.5 to 99,999 RPM Accuracy, RPM Gauge Marker with Batteries Included: Digital Tacometer For Measuring: Amazon.com: Industrial & Scientific


I used this to dial in my tach (using alternator for signal) and it worked perfectly.
Super easy to use.

Yea we used that too

 

[Bob H]

The picture shows the gear ratio entered as 456 to 1 instead of 4.56 to 1
The picture shows MPH in the calculation of 0.52 MPH not 52MPH.

31" tire dia? to calculate the actual rolling radius of your tires, chalk a line down the sidewall of a rear tire to the ground @ 6 o clock, roll tire 1 revolution, measure the distance traveled between chalk marks & do some math.
these trucks do not have a lock up convertor so figure in a 5% slip for that (could be closer to 10% down at 2000 RPM
35 mph at 2000 RPM sounds about right

 

[Chuckanut]

The picture shows the gear ratio entered as 456 to 1 instead of 4.56 to 1
The picture shows MPH in the calculation of 0.52 MPH not 52MPH.

31" tire dia? to calculate the actual rolling radius of your tires, chalk a line down the sidewall of a rear tire to the ground @ 6 o clock, roll tire 1 revolution, measure the distance traveled between chalk marks & do some math.
these trucks do not have a lock up convertor so figure in a 5% slip for that (could be closer to 10% down at 2000 RPM
35 mph at 2000 RPM sounds about right

Never mind your right, thank you for explaining

 

[Chuckanut]

The picture shows the gear ratio entered as 456 to 1 instead of 4.56 to 1
The picture shows MPH in the calculation of 0.52 MPH not 52MPH.

31" tire dia? to calculate the actual rolling radius of your tires, chalk a line down the sidewall of a rear tire to the ground @ 6 o clock, roll tire 1 revolution, measure the distance traveled between chalk marks & do some math.
these trucks do not have a lock up convertor so figure in a 5% slip for that (could be closer to 10% down at 2000 RPM
35 mph at 2000 RPM sounds about right

The math for 1.1 works out but in overdrive (gear venders) it’s said 51.88 but I’m doing 45mph is that just torque converter slip?

 

[patracy]

Yes, the calculation is purely a mathematical one excluding TC slippage. That also would be increased by the overdrive due to the ratio advantage the OD effectively has over the TC. In 3rd gear a TH400 would be expected to have 5-8% slippage. Factor in your OD after the fact and that slippage gets multiplied to around 9-10% on the high end. Which absolutely backs up your 45mph observation.

If it bugs you that much the options are only to live with it. Spend several thousand with coan to add a lockup converter and pump to your TH400. Swap in a manual. Or move to a 4l80e to effectively get what you're specifically after.

My .02, just live with the slippage since you have a OD.

Also are you measuring speed with GPS or the stock speedo?

 

[Sharecropper]

If I were you, I would abandon that silly overdrive unit and install a Turbo Encabulator. Chrysler Engineering developed the unit several years ago and there are still a few around in salvage yards. Here is a video -

turbo incabulator