Speedo Correction

[DORIFUTO_The Future]

Hi guys,

Probably making a dick of my self here but anyways. I did some searching around and didnt come up with anything about speedo correction, so after sitting around scratching my head i came up with an idea which i thought i would run by you guys.

Basically i want to be able to get an accurate reading from a speedo that is getting its readings from a R34 GTT manual Gearbox which in turn is connected to a s13 auto diff (wheel and tyre size is the R34 GTT standard 17x7.5 and 225/45).

Im guessing that the problem is the speedo cable running from the gearbox to the speedo, i would assume that since the R34 GTT has a diff which has a ratio of 4.111.1 (correct me if i am wrong) that the speedo cable is spinning at that ratio but the actual final drive of the driveline is 3.9.1 so the speedo is reading wrong.

So i thought that if i some how was able to intercept the speedo cable before it reaches the speedo itself and change the ratio of the cable from 4.111 to 3.9 that it would read correctly.

I scratched out these little pictures in paint to illustrate my idea.

Or with the standard s13 tyre size.

Well anyways let me know what you think.

Edited February 17, 2006 by DORIFUTO_The Future

[AlexCim]

I think if you want to fix your speedo, the easiest way is to do it with changing tyres to suit the % difference in ratio's

Find a set of tyres that read 3.9/4.111, so at a 6% error to your standard wheels. As the 3.9 ratio will give more top speed, you need to balance that with faster acclerating tyres (smaller) (i think)

http://www.1010tires.com/TireSizeCalculator.asp

So, using the stock GTT wheels, i deduce you need 225/35R17's to solve your problem.

Could also be in the other direction, i am not sure, i am confused. But thats probably the cheapest/easiest way to fix your issue...

[mr 32]

you need to go to a speedo cable shop and they will be able to rest everything for you, they will do a wind test will driving and see how far out it is. i can recomenned someone if your in sydney.

[DORIFUTO_The Future]

AlexCim - Wouldnt matching tyres only work for a car that hasnt had the final drive ratio changed but which you are sticking bigger rims on?

mr 32 - oh ok cool sounds pretty good, so they will change the speed cables ratio to match what it actually needs to be? I live in Tasmania so yeah unless i get on The Spirit and come up for a trip then i dont reckon sydney would be viable anyways thanks for that ill have a hunt around down here and see if i can find one.

[AlexCim]

Well, the way i see it, you are chaning the ratio of the engine to the rear driveshafts (like, the half shafts)...sooo... to compensate for the change in ratio of the spinning of the axle, you change the ratio of the axle to the rolling diameter to compensate..

Lots of annoying maths, but i think i had it right.

[Wogboy]

R34GT-T Trasnmission doesn't use a cable, it's electronic thru the ECM.

[neo32]

r34 is a pulse no speedo cable try pulling the sender out of the box get one with different teeth to suite