Tyres effect on Speed
Tyres effect on Speed
Does anyone know if having a differant profile size than what is on the spec sheet will affect the speedo very much?
Basically my BX19TZS is fitted with 175/70 R14 instead of the 65.
What I am finding is that a trip distance covered is lower than what it should be? For example at trip I do daily for work from home is 23 km that is confirmed by the tripo in my other Car and a company travel sheet.
The same Journey in the BX is consistantly coming up as 19km which means when I am driving at 100kph I am actually traveling faster? Could explain how comes I seem to be catching everyone up (not that I see many cars )
Very weird
Basically my BX19TZS is fitted with 175/70 R14 instead of the 65.
What I am finding is that a trip distance covered is lower than what it should be? For example at trip I do daily for work from home is 23 km that is confirmed by the tripo in my other Car and a company travel sheet.
The same Journey in the BX is consistantly coming up as 19km which means when I am driving at 100kph I am actually traveling faster? Could explain how comes I seem to be catching everyone up (not that I see many cars )
Very weird
- docchevron
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The rule of thumb goes something like this:
If the car was running 70 profile's and the speedo is correct (assumption), then you out 60 profile's on, the speedo will OVER READ by 5 mph.
Sorry mate, I know you guys work in KMh but, well, I dont!
What size wheels are you running??
Cheers
Chris G
If the car was running 70 profile's and the speedo is correct (assumption), then you out 60 profile's on, the speedo will OVER READ by 5 mph.
Sorry mate, I know you guys work in KMh but, well, I dont!
What size wheels are you running??
Cheers
Chris G
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Far too many BX's, a bus, an ambulance a few trucks, not enough time and never enough cash...
Far too many BX's, a bus, an ambulance a few trucks, not enough time and never enough cash...
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I worked this out for someone the other day - 175/70 x 14 has a circumfrence 3% longer than 165/65 x 14.
If you're collecting mileage allowance what about some low profile 13's?
You work it out like this.
the 65 - 70 - whatever is the aspect ratio which is the relationship the tread with has to the wall height.
So if the tread is 175 mm wide and the aspect ratio is 65, the wall height is 113.75mm (175 x 65/100)
Convert the rim diameter to mm - multiply by 25.4 - so 14 in = 355.6mm
So diameter is 2 x wall height + the rim diameter.
The circumfrence in mm is pi x diameter or if you haven't got a scientific calculator - use 22/7 which is very close.
Now you have the circumfrence in mm - divide by 1000 its in metres - then divide it into 1000 to see how many times your wheel turns every km
If you're at work you could make a nice spreadsheet and see how small a wheel you would need to become a millionaire!
If you're collecting mileage allowance what about some low profile 13's?
You work it out like this.
the 65 - 70 - whatever is the aspect ratio which is the relationship the tread with has to the wall height.
So if the tread is 175 mm wide and the aspect ratio is 65, the wall height is 113.75mm (175 x 65/100)
Convert the rim diameter to mm - multiply by 25.4 - so 14 in = 355.6mm
So diameter is 2 x wall height + the rim diameter.
The circumfrence in mm is pi x diameter or if you haven't got a scientific calculator - use 22/7 which is very close.
Now you have the circumfrence in mm - divide by 1000 its in metres - then divide it into 1000 to see how many times your wheel turns every km
If you're at work you could make a nice spreadsheet and see how small a wheel you would need to become a millionaire!
err yea I understood that
Sorry guys should clear it up a bit.
The Tyres on the Car at present (aka when Purchased) 175/70R14 the reason I queried it is because when I fitted a pair of snow chains one wheel chain was tighter. Turns out it was a 175/65R14
Well that any anybodies language will fail the WOF (Mot) the spare is also 175/70R14.
When I looked at the Citreon BX Glovebox Handbook The sizes mentioned are
Standard 175/65 R 14 MXL 81T
Approved 165/70 R 14 M+S or MXV 81 H
As you can see only the tyre that now sits as my spare matches this.
Chris, I am running the 14 wheel I think you maybe on to it when you say OVER READ by 5 mph? If the profile is higher then that would be a under read? Which sounds about what happened when we did a 100 kph test behind another car my BX was showing just over 90KPH.
-9% in anyones language so factoring in tyre wear (3mm) roughly 5.6mph error?
Sorry guys should clear it up a bit.
The Tyres on the Car at present (aka when Purchased) 175/70R14 the reason I queried it is because when I fitted a pair of snow chains one wheel chain was tighter. Turns out it was a 175/65R14
Well that any anybodies language will fail the WOF (Mot) the spare is also 175/70R14.
When I looked at the Citreon BX Glovebox Handbook The sizes mentioned are
Standard 175/65 R 14 MXL 81T
Approved 165/70 R 14 M+S or MXV 81 H
As you can see only the tyre that now sits as my spare matches this.
Chris, I am running the 14 wheel I think you maybe on to it when you say OVER READ by 5 mph? If the profile is higher then that would be a under read? Which sounds about what happened when we did a 100 kph test behind another car my BX was showing just over 90KPH.
-9% in anyones language so factoring in tyre wear (3mm) roughly 5.6mph error?
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Basically,
the 175/65 gives you the smallest diameter = standard
165/70 is 2% larger and so compared to above, you will be going 2% faster than a speedo calibrated for 175/65
The 175/70 is 7% larger than the 175/65, so you'll be going 7% faster than the speedo indicates
Alternatively, distances will seem shorter so a 23km journey on 175/70 will show as 93% of 23 which is 21.5 (to nearest 1/2 mile)
Gorr Blimey, sorry to be so boring
the 175/65 gives you the smallest diameter = standard
165/70 is 2% larger and so compared to above, you will be going 2% faster than a speedo calibrated for 175/65
The 175/70 is 7% larger than the 175/65, so you'll be going 7% faster than the speedo indicates
Alternatively, distances will seem shorter so a 23km journey on 175/70 will show as 93% of 23 which is 21.5 (to nearest 1/2 mile)
Gorr Blimey, sorry to be so boring
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That makes sense, cheers
I still have a niggling 2 km missing on that trip but maybe that could be put down to tyreware from 8mm new and tyre pressure?
Now you made me think more I do ascend from 400m to 800m in that trip and a further 800m to work. So the Tyre pressue I assume would change? Well the empty plastic Drink bottles get crushed
I also only have 3mm of tread left on these tyres so before the First snows possibly strike after Easter they will be changed.
Interesting you made me think about the other couple of factors which would be a great defence maybe to being stopped for Speeding on the high altitude roads
I still have a niggling 2 km missing on that trip but maybe that could be put down to tyreware from 8mm new and tyre pressure?
Now you made me think more I do ascend from 400m to 800m in that trip and a further 800m to work. So the Tyre pressue I assume would change? Well the empty plastic Drink bottles get crushed
I also only have 3mm of tread left on these tyres so before the First snows possibly strike after Easter they will be changed.
Interesting you made me think about the other couple of factors which would be a great defence maybe to being stopped for Speeding on the high altitude roads
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If you check out the tyre diameters here :
http://www.tyresave.co.uk/tyresize.html
It shows a 3% difference. My experience of quite a few BXs is that the speedo is usually about 4% optimistic over the true speed i.e at 110kph the true speed is 104kph or 70mph and 67mph.
This does not square at all with your loss of 4k on your work trip. This is around a 15% difference and cannot be accounted for by tyre size difference. You appear to be considerably overgeared as far as the speedo is concerned. What does the rev counter indicate (if fitted) at a steady speed ?
http://www.tyresave.co.uk/tyresize.html
It shows a 3% difference. My experience of quite a few BXs is that the speedo is usually about 4% optimistic over the true speed i.e at 110kph the true speed is 104kph or 70mph and 67mph.
This does not square at all with your loss of 4k on your work trip. This is around a 15% difference and cannot be accounted for by tyre size difference. You appear to be considerably overgeared as far as the speedo is concerned. What does the rev counter indicate (if fitted) at a steady speed ?
The Joy of BX with just one Citroën BX to my name now. Will I sing Bye Bye to my GTI or will it be Till death us do part.
Interesting enough we tried the follow trick to check the speed not easy with corners and rises, but we think that at 100kph of the lead car we were showing about 91-92kph. Irony of this situation is I have not yet been overtaken on our quite Central North Island roads yet
I have not yet checked against the Rev counter, that will also prove difficult with the driving conditions as well. Get back to you on that.
I have not yet checked against the Rev counter, that will also prove difficult with the driving conditions as well. Get back to you on that.
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If you ever take the instrument binnacle apart check the number on the speedo head on the top right hand corner of the face and let us know what it is, that is all revealing!
Cheers
Chris G
Cheers
Chris G
Smokes lots, because enough's enough already!
Far too many BX's, a bus, an ambulance a few trucks, not enough time and never enough cash...
Far too many BX's, a bus, an ambulance a few trucks, not enough time and never enough cash...
Treated myself to two new Front tyres today
Whilst I had the wheels off ordered a set of Brake Pads as they were looking a bit low.
What got me was standing the two wheels together and seeing the profile differance between a 175/70 R14 and a 176/65 R14. To think I had been running around like that since October when I got the car
Any other car and the suspension would have clearly shown that, Should mean less tendancy to slide out to the left at corners in the wet now as well (it was the passenger wheel that was 65).
Got the correct size on the front of the car now so speedo discrepancy should be less and Fuel consumption improved?
Gone now is the Only tyre made in France on the Car, probably was the original spare?
Whilst I had the wheels off ordered a set of Brake Pads as they were looking a bit low.
What got me was standing the two wheels together and seeing the profile differance between a 175/70 R14 and a 176/65 R14. To think I had been running around like that since October when I got the car
Any other car and the suspension would have clearly shown that, Should mean less tendancy to slide out to the left at corners in the wet now as well (it was the passenger wheel that was 65).
Got the correct size on the front of the car now so speedo discrepancy should be less and Fuel consumption improved?
Gone now is the Only tyre made in France on the Car, probably was the original spare?
The tyres have made little if any differance to the speedo variation from actual speed
How do I know?
Yesterday crusing along at the standard 100kph and the Cop car heading in the opposite direction gave me the friendly (yea Friendly) flash of Blue and Reds to slow down
On a plus it was to much hassle for him to be able to expect to turn around and catch me up without smashing the speed limit himself to pull me
How do I know?
Yesterday crusing along at the standard 100kph and the Cop car heading in the opposite direction gave me the friendly (yea Friendly) flash of Blue and Reds to slow down
On a plus it was to much hassle for him to be able to expect to turn around and catch me up without smashing the speed limit himself to pull me
- docchevron
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Ok, so it sounds more and more like it's got the wrong speedo head fitted.
Can't tell of course without the big number in the top right corner of the head unit...
Cheers
Chris G
Can't tell of course without the big number in the top right corner of the head unit...
Cheers
Chris G
Smokes lots, because enough's enough already!
Far too many BX's, a bus, an ambulance a few trucks, not enough time and never enough cash...
Far too many BX's, a bus, an ambulance a few trucks, not enough time and never enough cash...