Marks speedo correction units

[fatassgq]

Hi all,
Just after some advise as to how to set up these things when you have lower diff ratio's than standard.

For example I have a five speed box with 4.6 ratio's and run 33's all the time?

It has a program for 33" tires but I don't think this is taking into account the diff ratio's? Do I have to work out my final drive ratio or something? If so how is this done again?

I just wanted to fix the speedo up as it reads faster than what i am actually doing! Feels like driving miss daisy doing 60km/h!!!

Hope this makes sense
Cheers
Brian

[chimpboy]

Best bet is to actually get your speed measured properly, either with a radar gun (I happened to have someone who could help me out that way for a vehicle that I'd done a gearbox conversion in), or alternatively one of those marked 5km sections of the freeway - depends which city you are in but they are around.

Then your odometer vs the actual 5km travelled gives you a ratio that the box needs to correct for.

[fatassgq]

I have had it on the dyno and it reads about 20km/h faster when doing 100.
ie when actually doing a 100km/h speedo is reading about 120! does this help?

[GQ4.8coilcab]

I have had it on the dyno and it reads about 20km/h faster when doing 100.
ie when actually doing a 100km/h speedo is reading about 120! does this help?

means you are 20% over. Get the box to correct 20%

[chimpboy]

I have had it on the dyno and it reads about 20km/h faster when doing 100.
ie when actually doing a 100km/h speedo is reading about 120! does this help?

Yeah, that's all you need to know.

[evil_hitman]

another option is to grab the trusty gps and adjust the switches (on the fly with the help of a passenger) to the correct speed.

Another option to the marks kit if you're handy with the soldering iron is
http://www1.jaycar.com.au/productView.a ... BCATID=347

$45 each. (to make an equvalent to the marks kit you would need 2 of these) plus enclosure if you want one.

Matt

[RogTJ]

Just calculate the difference using ratio between tyre sizes (factory and current) multiplied by the ratio between factory and new diff ratios, that will give you approx setting, then fine tune using GPS. I have just done mine and very straight forward (I did diffs and tyres at the same time and ended up 9% out overall).