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Discussion Starter #1
Okay many years ago i knew the answer to this but it currently illudes my frazzled brain.

I have suspected since i got the car the speedo isnt correct, what on earth makes me think its wrong. The abundance of cars right up my ass, and with the cruise control set to 60 trucks just walk away from me.

Reported Speed (Speedo) - Actual Speed (Race Logic GPU)

30 Mph - 27 Mph
40 Mph - 36 Mph
50 Mph - 45 Mph
60 Mph - 54 Mph
70 Mph - 63/64 Mph
80 Mph - 73Mph
90 Mph - 82 Mph
100 Mph - 90 Mph
110 Mph - 99 Mph
120 Mph - 109 Mph
125 Against the limmiter - 115 Mph


No the car i suspect is on the wrong wheels, i suspect this because the faster i go the more wrong it gets, so does any one know what the car left the factory with

Its a 2005 2.0 TDCI Ghia with every toy they sold.

Currently fitted are R16 205 / 55 v

I suspect i need R17 205 / 55 W or R16 205 / 65 W.


Because if it aint wheels its gearbox, and if its gearbox the one on it isn't the origonal gearbox.
 

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Just put your phone on a speed app when you feel the need. Just think yourself lucky, if you get caught doing 110mph you wont auto loose your license cause you will be just getting 6 points for 99mph. It may be configurable using something like elm config via the OBD interface but other than that, changing wheels is a bit OTT :)
 

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Discussion Starter #3
Yeah i feel pritty lucky its out that way, but i dont like other road users trying to climb in the boot, and i do use the Cruise control alot.

If i can figure out the right size wheels ill get them, i'm not an owner opposed to spending some money on it, afterall i parted with peanuts to buy it,a few more peanuts to get it almost fully repaired. Im probably in this for about £800 so far all up including a clutch and DMF. Id be happy to keep spending to £2000 total to get it as good as i possibly can. For its age its a very low mileage example (in the context of say auto trader, where there 180k plus miles,only have a hair over 110k on this

Im fairly sure it should have 17" rim's on it from factory, and i think the reason it has 16's on it, is a side effect of some frontend damage, when i "rescued" it, front right spring was sheared into 3 parts, upright was kinked, shock was useless, top mount was a disaster, the drop links snapped and the end of the roll bar bent upwards, so its hit something hard at speed, and im thinking they broke the alloy it, and just replaced them with 16" and used it like that (at least to bring it to me cos it drove here somehow)

I wanted a project car during lock down this is it :)
 

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Discussion Starter #5
Hi Gerald.

I know i can correct 16" to 17" buy simply getting a 65 tire, as opposed to the 55's i prefer, but on just about any car, it makes them to soft for me, i dont want to feel like the front end is shoving its way around the corner as the sidewalls flex.

Im just getting more and more irritated with the car, other drivers attempting to climb in the boot, theres been a few instances where its annoyed me so badly ive though if i just hit the breaks now....

Being out on the motorway dont bother me so much, i drive with the traffic its less of an issue, but around town its a NIGHTMARE.

I had a quick look under the car, and it looks suspiciously like an electronic sensor for speed, meaning it can be calibrated in the ECU.

When i have time ill get out there with fords software and take me a look around all these PCM, BCM#s and the rest of it in the car I was able to remove the limmiter so i must be able to calibrate the speedo, or see its calibration and thus the wheels it thinks are on it
 

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Discussion Starter #7
I hadn't thought of that, it would screw the mileage and it will screw with a lot of other things besides


MPG for example - im sat here this morning with a high pressure hydraulic pump and an Arduino making a high pressure injector tester ( i put low MPG down to a leaky injector ).

Okay so that about seals it for me, lets go see if i cant find me a set of 17" Alloys, if anyone knows anywhere in particular, Id prefer OEM Wheels, ideally from a place that can fit me some Pirelli Rubber to go with them
 

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Your speedo looks correct to me. They are normally optimistic by about 10% so at around 70mph you are about correct.
 

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Discussion Starter #10
Got to speak with the previous "abuser" last night of the car.

He eventually confirmed he'd fallen asleep at the wheel, hit the kerb at a round about with the front right, and destroyed a wheel and the front right suspension, due to money constraints he put 16's on it from his garage.

I'll have a set of 17s on it by end of the week and then ill report back.

I wont accept a speedo thats 10% wrong to be acceptable, and if i have to spend a few hours to make it accurate so be it, its one of them things that will drive me personally around the bend, I only started this cos of morons attempting to climb in the boot, that speedo should be much closer to the real thing, i shouldnt have to sit in the drivers seat doing math to work out of im speeding or not, my eyes should be on the road ahead :)

My company A3 is out by 1 Mph
The wifes new car is out by 1 Mph
Checked the old dears car, its out by 3 mph (25 yr old purgeot)
Dads BMW is perfectly accurate to a tonn

mines out by at least 3 mph at all times, and as much as 10 at speed, i dont find that acceptable.
 

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Discussion Starter #11
Kaboom.

Okay so i have this now sorted out.

I purchased a Delphi ODBII Diagnostic Kit, and have procured a copy of the ford main dealer ODB Software (ebays a wonderfull place).

During my couple hours playing around, i finally found what i was looking for. Speedo and Gearbox Data.

The car has it programmed into the ECU for 833 Revs per Mile, some quick maths (im good at that) tells me, there are 2 realistic wheel and tire combos to get to 833 Revs per Mile


Option 1 (My Chosen route) - I will be fitting, R17 205 45 W for 832 Revs Per Mile
Option 2 - R16 205 50 V - This is 838 Revs per mile, although a 50 tyre isn't standard and part of the reason ive now put 17s on it.
Option 3 - R15 205 60 V - 817 Revs per mile

Original wheels and tyres were R16 205 55 for 811 revs per mile.


The results

No more monkeys attempting to climb in the boot, MPG Reported has increased from a max of 44 mpg, to 48 mpg, and the car feels significantly stiffer to drive, less wallowy on the front end, almost exclusivly down to considerably stiffer sidewalls.

As for accuracy of the speedo, when compaired to a Race Logic GPS, is now has a maximum diffrence of 2mph all over the clock, and after pulling in a small favor, a laser speed check shows with 30Mph on the Dash my actually speed is 29.2 Mph

This ladies and gents is how accurate your speedo should be, IF YOU HAVE THE RIGHT SIZE WHEELS AND TYRES


I would like to thank everyone who chipped in.

Been fun!


Reminder. I aint no fricking mechanic, im a software engineer by trade, anything you do to your car based on this info is down to you to verify. Just because it worked for me dont mean it will for you, you might have more or less RPM set by default in the ECU
 

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My 2.0 tdci has 215 50 R17. Check this out

Not a great help when tyre manufacturers don't keep to the width and aspect ratio sizes stated on the sidewall .

What's the overall diameter difference between 205 55 16 and 235 45 17 ?
 

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Discussion Starter #13
Gaz

R17 205 45 W (Although if limited to 125 then its V not W) is 832 Revs per mile
R16 205 55 W (although if limited to 125 then its a V not W) is 811 Revs per mile

That's a 21 rev difference.

Assuming you are down sizing from 17 to 16 for example, and using the above

21 Revs * 78.1" (78.1 is the total circumference of the wheel including 205 55 16 tyre)

1640.1 " Inches per Mile
or
136.6 Feet Per Mile
or
45 Yards per Mile
or
0.0258 Miles per mile

Now for simplicity, for every 1000 miles (in miles) your mileage will be out by 25.8 Miles, and your speedo will show you going faster than you actually are (see my origonal post) wrong wheel sizes meaning the speedo "drift" (for lack of better word), will get worse the faster you go, as opposed to having the right size wheels, in which case fords 3 MPH Offset is the norm, the offset means your always 3mph less than whats stated.

If the wheels are the wrong size, the faster you go, the bigger that drift becomes, which is why i used to hit the limmiter at 125 and only be doing 114mph, not anymore, at 125mph now on the limmiter im moving at 122mph, and most importantly the whole reason i started this

No one is attempting to climb in the boot around town anymore, and that alone makes this whole thing worth it. Cos if theres one thing i can stand as a father with kids in the car, is the need of others to get right up the ass in the car in front
 

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Discussion Starter #14
A quick update to my last, its 8 am, and ive done most of the maths in my head, you might wanna just double check them
 

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But you can't apply the maths accurately because tyre widths and aspect ratios are not kept to by the tyre manufacturers !
 

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Discussion Starter #16
Tyre width is irrelevant in this senario

Tyre aspect ratio to the ground, is not relevant

All thats relevant is the total circumfrence of the assembly (that's wheel + tyre)

I need to make the wheel turn at 833 revs per mile, a single roatation moves X and X * 833 = 1 Mile


If you really want me to break down the maths for you, just say the word ill lay it out, using nothing but total circumfrence because that is all that matters
 

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Discussion Starter #17
Screw it.

Wheel 17"
Tyre - 205 45

1 Mile = 63360"

63360 Inches / 833 = 76.062 per rotation

Total Assembly wheel and tyre circumfrance is 76.2 inchs, so guess what, in ONE Rotation the car moves 76.200


Schools out
 

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It's OK there is the majority of people who do not know tyres are not produced to the width and aspect ratio sizes on the sidewall .
 
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