SpitfireS

The Modifry manual has a short list of settings to use for different final drives.
It states to use setting 21 for the 4.57's
It also states the difference in % is 10.3
(10.3 X 2 = 20.6, so 21 is the closest value)

But I'm thinking....
I start with OEM gears being 4.10
I now have 4.57's
That is a 11.463 % change (or increase.)
So my setting should be 2 x 11.46 = 22.92 => 23.
Not 21.

Bottom line : what do you use for reference, the 4.10 or the 4.57.
The difference between the two is easy, thats 0.47.
Then you divide that by 1% of the reference, being 0.041 or 0.0457.
That gives you 11.463 or 10.284.

Note that I had a 2% difference with a 21 setting what makes me conclude I am right using 11.46 / 23 to start with.

Thoughts anyone?

xviper

I've always used 4.10 as the reference since that's what you are going "from". The 2% is simply the natural error of the stock speedometer. Digital speedos can have anywhere from 0% to 2% error to begin with. I calculated the error of the gears themselves based on the original gears, then added 1% to average out the stock speedo error. I'd then compare this to another known calibrated source like a police car or radar to confirm the need for more correction. If you adjusted the speed correction solely on the gear change, then you would "mathematically" be right back to what the speedo was when stock (including what the error was that was already present).

jerrypeterson

...and you guys forgot to include rolling diameter for the tires in the calculation.

This might help remove some of the human error factor:
http://www.5speedtransmissions.com/gear_calculator.html

afwfjustin

Reread the section in the instructions called "Determining a correction factor for a new differential ratio:"

The second paragraph states "at first glance you may think it's about 16% high (4.77s are used for the example), so I'll use a correction factor of 16%. But that would not be the right number to use. the part to remember is that we need to lose 163 pulses for every 1163 that are generated, so the speedometer sees what's left over (1000 pulses)... which in this case is .163 divided by 1.163, which yields .140 or 14%."

4.57/4.10 = 1.115 and .115/1.115 = .103 or 10.3

You need to lose 115 pulses for every 1115 that are generated, and the change is only 10.3.

10.3x2=20.6 and yes, 21 is the closest It would actually be better for you to make it 23 though, so you can get rid of the natural speedo error. For example, my 4.77s yield a 16% increase in gearing, but a correction factor of 14% is used. 14x2=28, but I used 30 to make it more accurate, since our car reads slightly high anyway.

HTH

SpitfireS

Hmmmm...
I'm confused now
(ashamed to be honest, for trying to reinvent the wheel)

xviper : I did a speedo check before (OEM gears and different tires) and I was surpriced it was so accurate, it was dead-on.
Not a km/h difference, measured at different speeds, 50, 80 and 120 km/h.

afwfjustin : I completely forgot to read that part of the manual again

jerrypeterson : I do have different brand and new tires now.

I was confused by thinking the 4.57 generates 11.46% more pulses => I need to correct 11.46% and this flaw in thinking was backed up by the results this afternoon

I will "arrest" my colleague & GPS tomorrow to see how its doing with the setting I use now, being 24.
It will probably be ok with the approx. 2% difference I had then @ setting 21.

SC_Highlander

I didn't bother calculating. I turned on the GPS, set the cruise control for 70mph, then adjusted the dials until both the speedo and the GPS said 70mph. I have the 4.57's and an '04 transmission, and I think I'm around 25 or so. I had to adjust it up a couple of turns after swapping the tranny.