vt low low diff speed

[pimpmyvt]

hey guys, ive purchised a vt a while ago and just wondering what diff i have at 100 on the speedo im doing 75 (tested on dyno) and produice 157hp in third and 156hp in forth. as i know the motor is stock tho has a sport exaust cat back

thx guys

[Tsunamix]

100 / 4.11 x 3.08 = 74.94 .

I suspect you have a 4.11 diff ratio.

Normally the VT changes up from first to second at about an indicated 70-73 kph under WOT. if you happen to know what speed you change up at now (acytual not indicated) this will confirm it.

4.11 ratio is not s street ratio, its a dragstrip ratio. 3.46 or 3.7 offer best road comprise for VT 6's with a 4 spd auto box.

Cheers

[drift-it]

correct me if im wrong but i thought the tone wheels and speed sensors were on the axles after the diff so a diff ratio change would not put the speedo out...or would the pcm just need to be re-calibrated to account for the higher ratio?

[pimpmyvt]

well i have the rag box (manual) 5 speed and at 110 kmh in 5th gear im sitting on 140 kmh on the speedo, and to shift outta first im doing 35kmh at 5.5k rpm, and from my guess its roughly 25% out.. thx for the help...

[Tsunamix]

The sender unit sends a set certain number of pulses per kilometer. For instance it may send 10,000 pulses per km. The PCM reads the pulses coming in, per second, to determine speed, which is then sent to the speedo.

if the diff ratio changes, you no longer get 10,000 pulses per kilometer. In fact, 10,000 pulses will only get you 749.39 meters. Consequently, your speedo says you are travel faster (more distance in the same time) than you actually are

So you have to tell the PCM things have changed. From 10,000 pulse per kilometer to 13,344 pulses per kilometer in this example.

I don't know how many pulses per kilometer the sensor actually sends, this is just an example.

This is also how the speedo adjuster kits from jaycar work - it changes the number of pulses per kilometer at the speedo. I think the sensor is in the gearbox, so if you change diff ratios you don't need to change your shift points if you have an auto. Unless of course you want your speedo to be accurate. If you do change your sensor's pulses per km setting, you will also have to recalibrate your shift points in the PCM. This is why the Speedo correctors from jaycar are popular - they only affect the speedo.

[pimpmyvt]

just looked at jaycars site there about $55. so i guess it plugs in to the computer? or into the box??

[drift-it]

thanks for the explanation tsunamix!

[Tsunamix]

The jaycar kit Ithink goes in the dash

[pimpmyvt]

cool thx guys.