Tyres effect on Speed

kiwi · Post by **kiwi** » Mon Jan 29, 2007 9:33 am

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

BX Bandit · Post by **BX Bandit** » Mon Jan 29, 2007 9:52 am

Were they always 175's originally Kiwi? As in, what was the original tyre size for the car 165 70s?

Post by **docchevron** » Mon Jan 29, 2007 11:15 am

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

Oscar · Post by **Oscar** » Mon Jan 29, 2007 12:02 pm

5mph = 8kph

Post by **docchevron** » Mon Jan 29, 2007 12:07 pm

Oh yeah...should have known that!

DOH!

Cheers
Chris G

jeremy · Post by **jeremy** » Mon Jan 29, 2007 12:12 pm

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!

kiwi · Post by **kiwi** » Mon Jan 29, 2007 7:09 pm

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?

BX Bandit · Post by **BX Bandit** » Mon Jan 29, 2007 8:04 pm

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

kiwi · Post by **kiwi** » Tue Jan 30, 2007 12:55 am

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

Jaba · Post by **Jaba** » Wed Jan 31, 2007 7:39 pm

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 ?

kiwi · Post by **kiwi** » Thu Feb 01, 2007 6:37 am

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.

Post by **docchevron** » Thu Feb 01, 2007 1:04 pm

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

kiwi · Post by **kiwi** » Wed Feb 14, 2007 10:35 am

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?

kiwi · Post by **kiwi** » Mon Feb 26, 2007 9:30 pm

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

Post by **docchevron** » Tue Feb 27, 2007 2:33 am

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