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madmatsmadhouse

How to id what diff is in my s15 / speedo prob

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Hi guys.

I brought another s15 at the start of this year.

Iv have just realised my speedo is out of wack. the speedo will read 100 when i am actually doing 105.. not good.

I have bigger wheels but using a calculator it should only read a 2kmh difference and are legal as they are not 15mm bigger in dia acording to the adr.

 

I have suspecion my diff could be the problem but cant figure out if its been changed or not. I can not see any markings on top of the diff but on the bottom it has k31 2626.

 

Does anyone know if this is at least the origenal oem diff housing at least.?

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You can sorta tell by marking the tailshaft end of the diff and marking 1 halfshaft and count how many turns it take of the tailshaft to make on revoloution of the halfshaft

Thats your ratio

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I will have a go at calculating the dfff ratio over the weekend

Difference sounds well within specs man.

sorry difference of what is well with in specs.?

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The speedo difference. They allow like 10% from the factory

 

A diff ratio change will change the speedo more than 3kph at 100

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wheel diameter has nothing to do with it.

 

you can put on 17s instead of oem 15s and keep the same rolling diameter.

 

what are you tyre specs?

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Pretty sure the adr says the speedo can read 10% higher then your actual speed but can not read lower then your actual speed....

My speedo says i am doing 100kmh but my true speed is actually 105kmh..pretty sure thats a no no.

I could underatand the speedo reading 2kmh under my true speed because of the tyre size but not 5. Thats why im susoect theres been a diff change

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The wheels on the car now are 235 35 r19. The adr says your not allowed more then 15mm bigger in diameter then factory. The wheels i got are about 12mm bigger in dia so all legal.. Just the speedo issue

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a 19" wheel is more than 15mm larger in diameter than a 16" wheel.

 

I was of the understanding the wheel diameter (not rolling diameter, just the wheel) cant exceed 2" increase

 

how are you measuring your actual speed?

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Its the rim and wheel put together to get the measured dia.

The car comes factory with a 205 55 r16. Diameter is 24.9imch.

I got 235 35 r19 so dia is 25.5 inch.

Difference is .6 inch. Thats 15.24mm

 

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Also i am measureing my actual speed with gps... Gos is pretty accurate.

 

Also ill find where in the adr is says about ya wheel size when i get home

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Jacked the s15 up this morning and had a look at the diff ratio.

For 10 revolutions of the rear wheel I got about 18.5 revs on the diff yolk so I got a diff ratio of around 1.85.

Am I correct that the stock s15 6 speed manual diff is a 3.9.?

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Jacked the s15 up this morning and had a look at the diff ratio.

For 10 revolutions of the rear wheel I got about 18.5 revs on the diff yolk so I got a diff ratio of around 1.85.

Am I correct that the stock s15 6 speed manual diff is a 3.9.?

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Iv just had a look at the s15 service manual and for the manual transmission diff R200H the final diff ratio is 3.692. the auto tranny diff R200V has the 3.916 ratio.

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So the 1.85 ratio didnt make much sense to me so i tried again.

The wheel had to turn 26 times for the diff yolk to turn 48 times. At these numbers and wheel and the diff yolk endered up at the same starting point.

48 devide 26 is 1.846. But if i times thay by 2 i get the 3.692 so i am guessing that the diff is factory.

Why do i have to times my1.846 by 2 to get the ratio.?

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Because you are only turning one of the drive shafts. If you were turning both equally the yolk would turn twice what it did.

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Ahh yep that makes sense thanks stock. I dont know to much bout diffs. Was just going by some vids i seen on youtube.

 

Anyine had there cars speedo calibrated before.?

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Im still finding it hard to understand why my speedo is so far out. I have the oem ecu still installed. factory diff amd gearbox. only thing thats different is the wheel size but that wont make the huge speed difference i am seeing. the bigger wheels will be having a small effect of 2% but really the speedo from factory would of been over anyway meaning at 100kmh on the clock the actual speen may be like 97 or something. so im getting a under speed of like almost 10km/h...

 

anyone got any other ideas

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I hate the 19" wheels on it now, they came on the car. there way to big and look shizz. Just didn't wanna worry about it until I needed new tyres..

I'm going to get some enkei rpf 1 rims front 17X8.5 +30 and rear 17X9.5 +38 and see how it goes. just gotta find a place with a good price

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Got the wheels on today. Speedo is still out. Im my speedo is at 100kmh my gps says im doing between 104kmh and 105kmh. Didnt fix the prob but is better theb it was

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