Tyres & Wheels a Calculation

[thebadleys]

Hi All,

Need some help and advice on tyre / wheel combinations if anyone help with the calculation

I have a 3.45 diff and a speedo from a 2.5pi but the SPEEDO reading is 10% low compared to my Tom Tom.

I am considering fitting 15" Minilites as a number of others have.

So what size tyres should I fit to compensate for the speedo reading.

Thanks in advance.

Nigel

[Kevin Garrod]

We need to know what size tyres you're running now?

[V Mad]

The diff will count for about 7%, don't know about the 2.5PI speedo, but you can read the revs/mile on the dial and compare it with the Stag speedo.

I wouldn't mess about with the tyre profiles just because the speedo reads wrong. Get the right tyres for 15 inch wheels so that the rolling radius is unchanged from original. There are online calculators you can use for that, but I suspect you should fit something like 205/65 R15 tyres. Then if speedo is wrong, get it re-calibrated or just live with it.

[thebadleys]

We need to know what size tyres you're running now?

Sorry 175 (assume 82 profile) 14

[Kevin Garrod]

Chris is right of course, you'll end up with a ridiculous tyre size if you try to correct the under read this way.

[jagojules]

Are the existing tyres new? Dont forget a new tyre has (I think) 8.4mm of tread depth , and we change them around 2mm - so even if you get it spot on with new tyres it will gradually read slower as the tyre wears.

[Kevin Garrod]

Are the existing tyres new? Dont forget a new tyre has (I think) 8.4mm of tread depth , and we change them around 2mm - so even if you get it spot on with new tyres it will gradually read slower as the tyre wears.

True, but not anything like 10% though.

[StagJonno]

Sorry 175 (assume 82 profile) 14

Are you sure about the 82 profile - thought those weren't available for a very long time (until recently when Michelin have started remanufacturing the original XAS type, at nearly £180 a corner). If your tyres are older than 10 years, the general wisdom from forumites is not to use them beyond 10 years old (some even advocating 5 years). This is despite decent tread and no visible cracking. See thread "Loose in the Wet" (latest post today).

As mentioned on that thread, I've just fitted 195/70 R14 as my considered best attempt to get close to the rolling circumference of the original 175/82 R14 - still ~2.4% less than original. Some have chosen to go for 205/ 70 R14. Suggest that you consider this before getting your speedo recalibrated.

Jonno

[thebadleys]

Are you sure about the 82 profile - thought those weren't available for a very long time (until recently when Michelin have started remanufacturing the original XAS type, at nearly £180 a corner). If your tyres are older than 10 years, the general wisdom from forumites is not to use them beyond 10 years old (some even advocating 5 years). This is despite decent tread and no visible cracking. See thread "Loose in the Wet" (latest post today).

Hi Jonno,

The current tyres are Nankang 175 R14 88H with no profile, this is, I understand, generally accepted as 80 or 82 profile when none os shown.

I am aware of the age issue and commented on it in your post, out of interest my tyres shown a date of 2707, week 27 of 2007, which is why I am looking at changing them as although there is plenty of tread I believe in having good tyres.

Based on their age and the amount of tread left, some 5.75 - 6.00mm, I imagine they have sat around quite a bit and as I intend to tour Europe next year I will feel happier and safer with new tyres that I know how they have been used. 15" wheels will save having the existing wheels refurbished ( which is similar cost as the 15") and give me access to a better choice of tyres.

What size tyres were previously on your car?

[HersnotHis]

Just as an observation my wifes 2 yr old Astra indicates a speed upto 10% higher than her Tom-Tom Sat nav. Despite this clever sat nav she recently got flashed by a camera that had just been moved some 100 yds back from its previous position and consequently had to attend a "Speed Awareness" course. At this course she raised the issue of this difference and was told that on a flat level road the sat nav is accurate but as the speed it displays is based on lateral displacement as viewed by the satellite the sat nav will be increasingly in error on any increasing incline. IMHO a valid point!

As a personal preference I think the stag looks best on the 14" original type alloys.

H

[KOY 23]

Just as an observation my wifes 2 yr old Astra indicates a speed upto 10% higher than her Tom-Tom Sat nav. Despite this clever sat nav she recently got flashed by a camera that had just been moved some 100 yds back from its previous position and consequently had to attend a "Speed Awareness" course. At this course she raised the issue of this difference and was told that on a flat level road the sat nav is accurate but as the speed it displays is based on lateral displacement as viewed by the satellite the sat nav will be increasingly in error on any increasing incline. IMHO a valid point!

As a personal preference I think the stag looks best on the 14" original type alloys.

H

Did they say what the error is?

[StagJonno]

Hi Jonno,

The current tyres are Nankang 175 R14 88H with no profile, this is, I understand, generally accepted as 80 or 82 profile when none os shown.

I am aware of the age issue and commented on it in your post, out of interest my tyres shown a date of 2707, week 27 of 2007, which is why I am looking at changing them as although there is plenty of tread I believe in having good tyres.

Based on their age and the amount of tread left, some 5.75 - 6.00mm, I imagine they have sat around quite a bit and as I intend to tour Europe next year I will feel happier and safer with new tyres that I know how they have been used. 15" wheels will save having the existing wheels refurbished ( which is similar cost as the 15") and give me access to a better choice of tyres.

What size tyres were previously on your car?

Hi Nigel,

Sorry, your name looked familiar, but hadn't realised you'd contributed to the thread I started. My previous tyres (as were on the car when I bought it a year ago) were 185/70 R14 - like yours with ~6mm tread. I discovered through this forum about differences to the original "82" tyres in terms of rolling circumference. From the Tyresave.co.uk calculator the 185/70 R14 tyres were making the overall gearing even worse by -4.35%. Even with O/D the car feels as though it needs another gear, so I was hoping to reduce this effect. Going for 195/70 R14 gives -2.18% (halving the difference, and every little helps). 205/70 R14 would have reduced this difference to 0%, but I wasn't totally happy with going that wide.

Had I not just recently had the wheels refurbed (£240 cash for 4), plus significant additional cost for spraying the black bits, I would have been interested in minilites, though I do like the originals.

Did you change the diff ratios, or was it the PO? Any idea of likely cost? I may be interested in going this route at some future point.

Looking forward to hearing how you get on.

Jonno

[flying farmer]

Just been and measured the rolling radius of my two Stags wheels, one set are 195/65x15=304mm, the other are 195/60x15 = 288mm or a 5% difference rather unsurprisingly.

The 65 series are on a car with the original 3.7 diff, the others on a car with a 3.45 diff so an overall difference of 2%, and I can confirm that the 195/60 and 3.45 diff combo gives a mileometer accuracy of better than 1% on the mk2 Stag.


I suggest measuring your rolling radius with your existing tyres and comparing it with these figures as a starting point.


Also, check your mileometer reading between two known points as I have often noticed Triumph speedos getting lazy with age and reading slow, whereas the mileometer with its gear driven drive will retain its accuracy. Sometimes if the under read is consistent throughout the range, ie it is under reading by 5 mph at 30mph, but is still under reading by 5 mph at 70 mph, dismantling the speedo and twisting the needle on its spindle while firmly holding the rotating bits at the back, will sort the problem.

My Stag engine Triumph estate has a 10% under read by virtue of the 195/65x15 tyres fitted, and it is overall too high geared for its weight and relatively feeble near standard Stag
engine. I think a diff swap between the estate and project Stag this Winter would be beneficial, especially as I am thinking of going down to 60 series tyres when the current ones on the Stag are knackered (one more track day will do that)!

Neil

[HersnotHis]

Did they say what the error is?

No they did not but now having looked at it from basic maths if the hill is 20% (1 in 5) its about 2% and at 25% (1 in 4) its 3%. The police certainly gave my wife the impression that this was more significant!
H

[Kevin Garrod]

Just been and measured the rolling radius of my two Stags wheels, one set are 195/65x15=304mm, the other are 195/60x15 = 288mm or a 5% difference rather unsurprisingly.



Neil

You might want to check those figures, 300mm is about 1 foot!