8.1 V8 Gear Ratios

[Sid447]

Hopefully this is in the right part of the Forum,

(pls move if not)

Truck in question is a 2002 Silverado with Allison LCT 4-speed txmsn.

Under "general" in the trans calibration it lists the four gear ratios and {H0103} has the vehicle listed with a 3.73 diff.

Yet when I do a gear calculation using these parameters I come up with this:

1] Gearbox:- Allison LCT & 3.73FD
Tyre:- 2431mm
Ratios: 1] 5.61, 2] 3.04, 3] 1.67, 4] 1.00
-------------------------------------------------------------------------------------------
Top Gear gives 39.105 KPH/1000 RPM and a top speed of 211.165 KPH at 5400 RPM

Engine speeds in top gear:-
40 KPH = 1023 RPM 50 KPH = 1279 KPM 60 KPH = 1534 RPM 80 KPH = 2046 RPM
100 KPH = 2557 RPM 110 KPH = 2813 RPM 130 KPH = 3324 RPM 160 KPH = 4092 RPM

Top Speed in 1 gear = 37.641 KPH
And changes into 2 gear at 2926 RPM dropping 2474 RPM
Top Speed in 2 gear = 69.462 KPH
And changes into 3 gear at 2966 RPM dropping 2434 RPM
Top Speed in 3 gear = 126.446 KPH
And changes into 4 gear at 3234 RPM dropping 2166 RPM
Top Speed in 4 gear = 211.165 KPH

===============================================

Checking with the two short logs (attached) the truck clearly doesn't seem to have this gearing.


Also attached is the stock (untouched) tune file from the vehicle along with the two logs.

There are also two graphs;
Graph (1) has been plotted using the calibration details from the tune file.
Graph (2) has used the shift stabilization ratios from {D4601} just to see if they were any closer.

It really needs a log where the TCC is sure to be locked up and in 4th gear, but that will have to wait a while.
Just wondered if anyone knew the answer in the meantime to what I must be overlooking.

........Maybe the truck has a centre diff?

[Blacky]

Just so you know you're not being ignored, I am looking at this...
Regards
Paul

[Blacky]

Can you confirm by physical measurement that the tire circumference is actually 2431mm?
Also can you confirm that the diff gear ratio in the truck is actually 3.73 as sometimes the calibrations can be different to the actual ratio.

The discrepancy appears to be a factor of about 1.75

Also, if you can take a log that includes the current gear PID, that would help.

Regards
Paul

[Sid447]

Yes, Okay Paul.

I physically read the tyre size of the sidewall. I remember only that they were a Michelin M + S tyre. Will double-check on the 3.73 issue and get another log as mentioned.

If I load up his stock tune file and compare it to the '02 Silverado tune file from Holdencrazy with the same OS; It shows {H0101 is different by 371units (0.5%).

When the tyre specs were fed into the "Speedo Calculator" it (H0101) went to a value of 61319, and H0104 went to 1533, which then made it require a 2.96% gear adjustment I think.

I'll do a double-check here.