Speedometer not reading correctly

[Frog7151]

I am having trouble getting my speedometer to read correctly. I have a GMPP LS3 and E67 ECU. The transmission is a TKO 600. I have changed the tire size to 26.7? (27540R18) and the final drive ratio to 1.59 to correct for the stock 40 VSS reluctor GMPP ECU setting. The speedometer is approximately 10-12 mph off. I am using Dakota Digital VHX gauges and their BIM 2 OBD module.

Question, In Speedometer setting- Hardware, Speed output type- should it be set to Vehserial, wheelspd or vehrep TOS? What should the setting be in Vehicle Speed Sensor - Vehicle Speed Output- Disabled, Serial or Rep TOS?
Thanks

[kevin87turbot]

We need just a little more info to help you get it dialed in. What rear gear ratio are you running? Is the speedo reading too fast or too slow? At what speed it the speedometer off 10-12 mph?

[Frog7151]

The rear end is 3.73. The speedometer is reading too fast and it seems to be above 30 mph that it happens. Thanks for the help

[kevin87turbot]

The rear end is 3.73. The speedometer is reading too fast and it seems to be above 30 mph that it happens. Thanks for the help

That helps a little. In order to get it dialed in 100%, just check the speedometer error at a certain mph. Let's say that it's reading 12 mph too fast at 60 mph. Just divide 12 by 60 in order to determine the error percentage. The error % is 20% in this example. Just multiply your 1.585 gear ratio by 1.20. The new gear ratio is 1.902.

[Frog7151]

Thank you. That is very helpful

[blindsquirrel]

I use the GPS in my dashcam. Long flat section of road, drive it both directions at two different speeds (like 40 one way and 60 the other). Get home, review dashcam and scanner files and get the difference between dashcam GPS speed and scanner/ECM speed, figure out the error percentage, make sure the percentage is the same for the two different test speeds (error in MPH will be different but the % should be the same).

[Frog7151]

That’s a good ideal. I don’t have a dash cam but I do have a handheld GPS that I could have my wife monitor while at different speeds.
Thanks

[20Softtail00]

I have the same set up as you but with a Tremec T56 Magnum 6 speed transmission tucked into a 1959 Corvette. Your tire size (26.7") appears correct and I assume from the final drive ratio of 1.59 that your TKO 600 outputs 17 pulses per revolution. You did not mention any changes to the "Sensor Calibration VSS Tire Revs per" and that may need to be updated too. I have mine set using a per mile number. To calculate that on a per mile basis take your wheel diameter in inches (26.7) and multiply it by 3.1416 to get your tire circumference (83.88) in inches. One mile = 63,360 inches so divide 63,360 by 83.88 and you get 755.36 vss tire revolutions per mile. Enter that amount in both the "Dist" and "Dist - Err: fields and be sure you select miles (to the right of the input fields). Hopefully that will correct your speedo reading. I struggled to determine whether my transmission was outputting 12 or 17 pulses per revolution but once I straightened that out my speedo is dead on.

[Frog7151]

Thanks everyone for the help. I was able to get it dialed in.