Diff ratio change not exact percentage

[VX L67 Getrag]

Just wondering if anyone can explain why diff ratio change isn't exact for speedo difference?

I have been tuning a car & realized pretty quick the speedo was out by 7% the whole way through!

I know factories allow for 2-3% do assumed it had a diff change, but that would have made 3.2956 ratio (7% on top of 3.08).
I since found out it was 3.46 gears but not sure why it was out by so little?

It did have bigger wheels on it too so guessing this is what has come into play?

[immortality]

Fairly sure 3.45/46 is about 11%. Yes, the bigger wheels would have reduced that %age a bit depending on how much bigger the rolling diameter is coupled with the factory tolerance would be about right.

[VX L67 Getrag]

Thanks immortality, was pretty sure!

Just thought it would be good to post up for others reference!

I'm guessing if we're using a vt that's completely stock but diff change from 3.08 to 3.46 & used 11% diff the dash is generally 188C (6284) minus 11% gives us 15D9 (5593).
So should be able to program it to cluster, then if rolling diameter is changed should be able to do similar calcs!

[immortality]

I used a speedo corrector kit (jaycar) in my VN with 11% difference for 3.45 gears and it was fairly much spot on. I left the factory tolerance of a few km/h as my missus has a heavy foot.

When I use the cruise control function I always just set it a couple km/h over the limit and then I know I'm spot on, here in NZ we get a bit of leeway except on public holidays when the cops become speed nazi's.

Once I get bored with cruise control the foot goes down and the overtaking starts

[vlad01]

how much do the cops give u there?

[immortality]

Most of the time we about 10km/h on the motorways ( I think it's the tolerance for speed cameras). It varies and depends on the copper though.

Last year I was cruising back from down south, coming up through the central plateau, had the cruise control set on 110 (would have been 107/108 true road speed) and a cop coming the other way just flashed his headlights at me but that was it.

I've just been playing with the trip comp on the missus VX. This car has 225/50/17 tyres instead of the standard 235/45/17's and I worked out that it's just over 2% greater rolling diameter so I have altered the trip comp to suit. also thought I would have a look at the fuel cals because Last time it was showing 40km to empty, it had beeped to say to was getting close to empty. When I filled it up it only took 58 litres and I'm sure it's got a 75 litre tank? Today it was showing I had used almost 2 litres more that actual (or about 3% more). I'll see if this fixes the distance to empty

[vlad01]

wow! i might move to NZ then, here in Vic they are as tight as 2km over !, most cops go at you if you are 5k over but not all do, some even 10k but they are the good cops who think is BS to fine for 5k or under.

but 2k is complete BS coz most new cars are 6k out or more. I can get away with is cos my speedo is less than 1k out so I can push it to the limit and know that they can't do anything lol.

[VK_3800]

immortality might need to watch out for tickets in the mail! Nowhere in NZ has tolerance greater than 9km/h and they regularly find excuses to bring it down to 4km/h (in some places like outside schools this is permanent - and there often seems to be a camera in these spots for some reason... even when school is out)

[immortality]

ATM, I average 1 speeding ticket every 10 years, I never speed around a school (we have one just up the road) and I pass to 2 going to and from work. Just need to watch out for when it drops to the temporary 40km/h.

Funnily enough, on public holiday weekends cops are very scarce around here, they are all out on the highways where I'm not

Fortunately I'm fairly good at spotting the old speed cameras.

[vlad01]

fortunately i only ever got one ticket, for 5K over with error, 8k clocked speed. not sure how I got it as i had cruise on and went though few other cameras on the same road without ever switching it off or adjusting the speed.

since then I am a hawk on the cameras, i can spot cops hiding in bushes and down side streets while before I pass them, can spot undercover cops regardless of car make or model.

And know the hyundai i-load stopped at side of roads are mobile camera units nearly all of the time.