Speedometer Accuracy Issues on 1996 Scudo 1.9 TD

piko · 2024-07-14 20:27:30

Hi everyone,

As a new proud Scudo owner, I happily joined this community hoping to get some advice and ensure the best maintenance the car can get. The car is rust-free, despite being a 1996. Besides some normal maintenance stuff, I have a weird problem with speedo readings. When it reads 100 kph, my actual GPS speed is 13% less, 83 kph glasses . The car runs smallish 195/65r14 tires, but these are only 5% smaller than stock. Has anyone else encountered this problem? What were your solutions or adjustments?

Thanks in advance!

Best regards,
Piotr

AlvyLad · 2024-07-14 21:13:36

https://sizemywheels.com/tire-size/citr … atch/19d-i
AFAIK 5% discrepancies are okay, your wheels are 20% too small

RegW · 2024-07-14 23:53:03

83 is 17% less then 100

Casper · 2024-07-15 00:20:49

Speedo in any motor tends to read a tad higher. I notice if doing 70mph i am actually doing 66mph. My bus (different motor) runs 14" wheels as standard and i put on 16" wheels and now when doing 70mph i am actually doing 74/5 so yeah, between a wrong profile tyre or smaller wheel your speedo will be out. I tend to use my sat nav for a speedo when out in the camperbus but thankfully Scotland has switched off all its speed cameras.

piko · 2024-07-15 08:30:54

AlvyLad wrote:

20% too small

Mine are actually 195/60r14s, but still that's 9% less circumference than stock 195/75r14. Anyway, should the odometer be accurate by then, the speedo might be casualy exaggerating. Could you check the difference between gps and speedo readings?

RegW · 2024-07-15 12:01:20

I did a 600 ish mile trip this weekend mostly on motorways and noticed my 2006 Expert speedo
consistently read 2mph higher than my sat nav

AlvyLad · 2024-07-15 18:19:38

piko wrote:

AlvyLad wrote:
20% too small
Mine are actually 195/60r14s, but still that's 9% less circumference than stock 195/75r14. Anyway, should the odometer be accurate by then, the speedo might be casualy exaggerating.

Regularly I get showing 79mph when on GPS navigation it's 70
Your maths are WAY OFF the mark

Just get right wheels on and see there from

Last edited by AlvyLad (2024-07-15 20:05:04)

piko · 2024-07-16 08:22:23

Your speedo is exagerrating by 13% and I consider that a lot, than's for checking that mate, I'll calm down my suspicions of having the wrong gearbox.

AlvyLad wrote:

Your maths are WAY OFF the mark

My math is spot on. First number (195) is tyre width, while the second is sidewall height given as percentage of the first value. Sidewall height does not translate proportionally to function of circumference, you must add rim diameter to the equation.

TYRE 1:

195/60r14
width: 195 mm
sidewall height = 0,6 × 195=117 mm
rim diameter: 14" = 14 × 2,54 = 355,6 mm
tyre diameter = 355,6 mm + 2 × 117 mm = 589,6 mm
tyre 1 circumference = diameter * pi = 589,6 × 3,14 = 1851,344 mm

TYRE 2

195/75R14
width= 195 mm
sidewall height = 0.75 × 195= 146.25 mm
14" rim diameter = 355,6 mm
tyre diamenter = 355,6 + 2 × 146,25 mm = 648.1 mm
tyre 2 circumference = diameter * pi = 589,6 × 3,14 = 2035.014 mm

2035.014/1851,344 × 100 = 1,099 – the bigger tyre is 10% bigger than the smaller one.
…or
1851,344/2035.014 × 100 = 0,91 – the smaller is 9% smaller than the big one.

Cheers!

RegW · 2024-07-16 14:35:09

All this talk of rims and side walls is not irrelevant but does somemwhat
overcomplicate the issue.
Tyres are not all round, they distort (flatten) slightly at the bottom.

I made an Arduino based speedo for a motorbike and found it was simpler and more accurate to ignore rim/tyre sizes and just measure the distance between the wheel centre and ground to get a more realistic running radius.
From this you can calculate the running diameter and circumference.

The wheel sensor gave one pulse per revolution so in my case it was easy to calculate
Distance: number of pulses
Speed: number of pulses in a given time interval.

I dont know how many pulses the Dispatch/Expert/Scudo gearbox pulsers send but
reckon its more than one which should make it more accurate.
If I really wanted to know, I'd probably jack up a wheel and measure them as I turned the wheel one revolution.
You can pick the signal up from the back of the speedo or in my case also from the ECU
or from the wiring harness at any accessible point

OAT · 2024-07-17 21:54:03

My speedo is spot on.
But only when I stop.
neutral

kenbw2 · 2024-07-18 00:08:35

OAT wrote:

My speedo is spot on.
But only when I stop.

I got mine to an impressive 140mph once before it started being like yours

JohnDragonMan · 2024-07-21 07:57:04

ohh i did that on mine once when i had it up on axle stands looking for the cause of a vibration. it was a wheel balance issue.. not driveshaft like i thought it was.

piko · 2024-08-04 16:04:01

Thanks guys, still looking for bigger wheels. On these smallies and I'm getting 5.2 l/100 km's (fake kilometers).

RegW · 2024-08-05 13:41:49

5.2 l/100Km is 54mpg
That sounds pretty good to me if your kilometer reading can be relied on.

kenbw2 · 2024-08-05 19:16:43

Yea my best ever was 52MPG (5.4L/100KM) but generally more like 35-45 MPG (8.1 - 6.3 L/100KM)

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#1 2024-07-14 20:27:30

Speedometer Accuracy Issues on 1996 Scudo 1.9 TD

#2 2024-07-14 21:13:36

Re: Speedometer Accuracy Issues on 1996 Scudo 1.9 TD

#3 2024-07-14 23:53:03

Re: Speedometer Accuracy Issues on 1996 Scudo 1.9 TD

#4 2024-07-15 00:20:49

Re: Speedometer Accuracy Issues on 1996 Scudo 1.9 TD

#5 2024-07-15 08:30:54

Re: Speedometer Accuracy Issues on 1996 Scudo 1.9 TD

#6 2024-07-15 12:01:20

Re: Speedometer Accuracy Issues on 1996 Scudo 1.9 TD

#7 2024-07-15 18:19:38

Re: Speedometer Accuracy Issues on 1996 Scudo 1.9 TD

#8 2024-07-16 08:22:23

Re: Speedometer Accuracy Issues on 1996 Scudo 1.9 TD

#9 2024-07-16 14:35:09

Re: Speedometer Accuracy Issues on 1996 Scudo 1.9 TD

#10 2024-07-17 21:54:03

Re: Speedometer Accuracy Issues on 1996 Scudo 1.9 TD

#11 2024-07-18 00:08:35

Re: Speedometer Accuracy Issues on 1996 Scudo 1.9 TD

#12 2024-07-21 07:57:04

Re: Speedometer Accuracy Issues on 1996 Scudo 1.9 TD

#13 2024-08-04 16:04:01

Re: Speedometer Accuracy Issues on 1996 Scudo 1.9 TD

#14 2024-08-05 13:41:49

Re: Speedometer Accuracy Issues on 1996 Scudo 1.9 TD

#15 2024-08-05 19:16:43

Re: Speedometer Accuracy Issues on 1996 Scudo 1.9 TD

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