O.K. gear experts< Why does my rig defy the charts and formulas??

[reamer]

All right, here are the facts, C-4 with 1:1 output; Dana 20 with 1:1 output:
3.50:1 gears; 33 x 10.5 BF Goodriches.

Now I have verified the mph going down the road with My navigation system, the speed is correct. I have verified the Sunpro Tach against a Snap-on Engine anilizer (both read the same). I have verified the Axle tag ID by rotating the tire 1 revolution and getting 1.75 revs of the driveshaft (then miltiply by 2) = 3.50:1

Sooooo According to the charts at 65 MPH I should be tach-ing at @2300 rpm; at 85 MPH it should be 3000 rpm.

But in all actuality 65 MPH is tach-ing 2700 rpm and 80 MPH is 3100 rpm

With the true readings the truck is acting like it has 4:11's
What's up with this????
reamer

 

[Broncobowsher]

Converter slipping?
What RPM do you turn at these speeds with no throttle applied?

 

[Apogee]

X2 on the torque converter...it's the only thing between the engine and the tires that can slip. I would mention the tires are probably not true to their advertised size, but with GPS that pretty much removes them from the equation.

 

[reamer]

So Broncobowsher, Should I get the rig to 65 mph and then coast and see where the tach is? How to test a converter for slipping?
Thanks
Reamer

 

[Broncobowsher]

For the 65 test, get up to 70 and coast. Downhill is perfectly acceptable. When you get down to 65 note the RPM on the tach.

As mentioned the advertised tire diameter is rarely the actual tire diameter. Then the loaded rolling radius is shorter still.

 

[reamer]

So that tahc value "should" equal the value in the chart if not slipping?

 

[Broncobowsher]

Should be close. Doubt it will ever be "equal" to the therotical value. Too much variability in the tire.

 

[reamer]

Thanks
I'll give it a try tomorrow.
reamer

 

[Socal Tom]

I see a few hundred RPM drop when my T/C locks up on the 4R, so those numbers look about right to me for T/C slippage.
Tom

 

[bronko69er]

Also, dont believe the sidewall of the tire, measure the circumference with string or something for your calculations.

 

[Steve83]

It's moot with a slushbox. If your speedo & tach are correct, then that's all you need to (or can) do. And since they are correct, there's no need to measure the tires.

 

[70_Steve]

All right, here are the facts, C-4 with 1:1 output; Dana 20 with 1:1 output:
3.50:1 gears; 33 x 10.5 BF Goodriches.

Sooooo According to the charts at 65 MPH I should be tach-ing at @2300 rpm; at 85 MPH it should be 3000 rpm.

But in all actuality 65 MPH is tach-ing 2700 rpm and 80 MPH is 3100 rpm

According the the gear calculator I just looked at ( http://www.grimmjeeper.com/gears.html ) at 65 mph with your setup, you should be taching almost right at 2500 rpm, not 2300.

 

[blubuckaroo]

All right, here are the facts, C-4 with 1:1 output; Dana 20 with 1:1 output:
3.50:1 gears; 33 x 10.5 BF Goodriches.

Now I have verified the mph going down the road with My navigation system, the speed is correct. I have verified the Sunpro Tach against a Snap-on Engine anilizer (both read the same). I have verified the Axle tag ID by rotating the tire 1 revolution and getting 1.75 revs of the driveshaft (then miltiply by 2) = 3.50:1

Sooooo According to the charts at 65 MPH I should be tach-ing at @2300 rpm; at 85 MPH it should be 3000 rpm.

But in all actuality 65 MPH is tach-ing 2700 rpm and 80 MPH is 3100 rpm

With the true readings the truck is acting like it has 4:11's
What's up with this????
reamer

Your gearing is too high, an since a C4 doesn't have a locking torque converter, it's slipping a lot. You would really enjoy 4.11s!

 

[Hal9000]

Tach not accurate?

Tire size not quite right due to worn tires or inaccurate manufacturer numbers?

Gears actually not what you think they are (EG, tag on axles reads one thing, but PO swapped gears.)?

 

[HoosierDaddy]

BFG says it's 33's are 32.7" tall.
http://www.tirerack.com/tires/Spec.jsp?make=BFGoodrich&model=All-Terrain+T/A+KO&tirePageLocQty=

Theres a little bit of it , not sure how much slippage a TC has.
edit : after some research , slippage can average around 10% , a tight converter will be as low as 8% , a loose one 12%..... or so I read.
if tires are 32.7 and gear is 3.50 there needs to be about 16% slippage to get 2700rpm @65mph
http://www.ringpinion.com/Calc_RPM.aspx

of course this a SWAG on my part.
(Scientific Wild Ass Guess)

 

[reamer]

So what would be a good new Torque Converter? Mostly street with some get up and go?? (which will lead to a new thread on R&R a c-4.
reamer

 

[HoosierDaddy]

I've got a funny feeling that picking the "right torque convertor" is very similiar to "picking the right cam" .
Lots of variables to deal with , plus your expectations ....
Might be best to call a tech line from a company you are comfortable with.

If you like reading ....http://www.pontiacstreetperformance.com/psp/Twisting.html

about halfway down the page it gets interesting at the "Slippage vs torque vs vehicle weight " section

 

[Broncobowsher]

For reference; the RPM drop when the convertor in my 4R70W locking up often drops more RPM then the gear changes.

The excess slippage could simply be you are putting too much torque into the trans for the cruise RPM. 33" tires and 3.50 gears I can see that happening. I run 4.88 and .70 overdrive which is equalivant to about a 3.4 ratio. Very close to the 3.50 you run. I also run a 33" tire. I remember the lockup dropping the RPM upwards of 500 RPM under the right conditions. That is a stock convertor (for the 4R70W). The slippage is about what you describe. I doubt it is a convertor issue, it is a gearing/tire size issue.

 

[ken75ranger]

So what would be a good new Torque Converter? Mostly street with some get up and go?? (which will lead to a new thread on R&R a c-4.
reamer

A stock rebuilt torque converter will be fine for your rig.

 

[stock1970]

also keep in account that the math calculation is based on perfect circle.. meaning that tires flex some due to tire pressure, heat, wind, ect ect.. it will never be exact.. just my .02 ..