Speedo Calc, LS1 Corvette

[CR500]

I've read a few threads on the speedo calc but I still don't get it.

The car is a 2000 LS1 Corvette with manual transmission and 3.42 differential (H0103).

Stock rear tire = 275-40-18. When I plug that into the calculator it comes up with:
H0101 = 18802 (edit: fixed previous number what was in the wrong units)
H0102 = 11.70

When reading the stock tune it shows:
H0101 = 19280
H0102 = 40

My current dyno tune shows:
H0101 = 19884
H0102 = 40

Are these just minor tweaks to fine tune the speedo? The dyno tune is off the stock tune by over 5%. I'm thinking the tuner just copied / pasted from another cars tune and missed that the other car had different size tires.

I'm changing the rear tires to 305-35-18, the calculator indicates:
H0101 = 18984
H0102 = 40

Should I just start with 18984 and adjust according to my GPS at highway speed?

What is the calculation performed by the calculator?

Thanks!

[joecar]

Is the Transaxle checkbox checkmarked...?

[joecar]

Post your tune file.

[turbo_bu]

FWIW - I've played with the speedo calculations on the full sized trucks. From what I have found, yes, the factory "tweaks" the cal to get the speedometer to read right. I did the hand calculations, and compared it to what was in the stock files from the factory. Yup, GM tweaks them to get the right :).

[CR500]

Joecar, yes the box is checked!

Turbo_bu, thanks. Guess I'll go with the calculator value for the new tires and adjust according to my GPS. What is the calculation?

Rob

[turbo_bu]

Example of what is calculated:

{H0101} Vehicle Speed Sensor Pulses per Kilometer 62763 Pulses/Km
{H0102} Vehicle Speed Sensor Pulses per Revolution 40 Pulses/Rev
{H0103} Differential ratio 3.71997 :1

{H0101} is calculated using the following formula:
tire size (T), H0102 (P) and H0103 (D) using the equation:
Pulses per mile: H0101=P*D*63360/(T*pi) where T is specified in inches.
Pulses per km: H0101=P*D*1000000/(T*pi) where T is specified in mm.

If you start plugging in your numbers you will probably see what the "theoretical" value is ... and what the actual factory value is.

[CR500]

I must be doing something wrong.

T = Tire diameter (inches)
305-35-18 = 26.41"
T*pi = circumference = 82.96"

63360 = inches / mile

D = H0103 = Differential ratio = 3.42
P = H0102 = Pulses per rev = 40
H0101 = 40*3.42*63360/82.96 = ~104480 (calculator shows 18984)

[turbo_bu]

Not exactly sure, but seem to remember that under the Speedo Calculator there is a check box for Corvettes. The pop-up info for the check box says that the pickup is on the axle and not the transmission output. Guessing that this might be part of the issue??? I have not played with corvettes yet, mainly trucks, Camaro's and swaps.

[Gordy M]

When measuring your tire diameter it also makes a difference on the width of the rim. The 26.4" diameter for your tire is measured on a 12" wide rim. If you have hoosiers/kuhmo autocross tires on 10.5" wheels they will be a slightly larger diameter. The Corvette PCM will make the corrections based on axle rotation. I have changed tire sizes on both my Escalade and Corvette and the GPS shows the spedo to be very close to its reading based on 10 mile runs.

[CR500]

Turbo_bu, Joecar had asked about the checkbox and it is checked. It looks like when it is checked the calculator uses an H0102 of 11.7 (instead of 40). When I use that and calculate by hand (spreadsheet), I get close to the same result as the calculator. Curious why it isn't exact. I'm not rounding off and I'd expect the calculator isn't either.

I noticed that I made a mistake in post 1. I gave pulses / km from the calculator and pulses / mile from the tune. They are now both in pulses / Km and not that far off.

OEM Tune = 19280
Calculator = 18802
Difference = 2.5% probably due to GM tweaking for the actual tire rather than calculated values.

Calculated by hand = 18808, fairly close to the calculator.

Thanks

Rob