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  #1  
Old 03-01-2006, 04:08 PM
m.messenger
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What is the true speedo....

I currently run 18" wheels with 245 x 45 x 18 Vredestein Ultrac tyres - they fill the arches to the max but just OK, except on full bad bumps when they sometimes tickle the plastic shrouds at the back of the front arches and the outer sides of the rear arch shrouds - otherwise great. Before that I had Bridgestone run-flat 225 x 40 x 18 - too harsh because of the reinforced run-flat sidewalls.
The point is thet I have an Origin B2 GPRS radar thingy which gives a constant speed read: with the standard 16" wheels and 225 x 50 tyres the diff between the dash speedo and the GPRS speed ( once it catches up) was about 4-5 mph - with the bigger radius 225 x 40 Bridgestones it was about 3 mph - with the Ultracs I am spot on - 0 mph difference.
So there we go!

MM
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  #2  
Old 03-01-2006, 04:19 PM
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Mathematically the 245/45-18's are 7.333% faster than the speedometer indicates.
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  #3  
Old 03-02-2006, 02:49 AM
m.messenger
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Quote:
Originally Posted by Earthworm
Mathematically the 245/45-18's are 7.333% faster than the speedometer indicates.
Mr J
Respect to your Einsteinian reply - I'm a bit quanderied then, assuming that the GPRS is accurate - does that 7.333% equate to the diff between the original tyre diameter/speedo, when the speedo was really showing around 5mph too fast, in true speed? Obviously not across the whole speed range but average around 75 -100mph.
I know this overread is true of most cars and most model ranges now have a big wheel option. It seems strange that the calibration isnt based on the standard wheel size. Obviously there is benefit to the manufacturer if the car seems a bit faster than it is, but that differential alone could tip you over a radar camera/gun limit.

MM
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  #4  
Old 03-02-2006, 10:47 AM
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ok ok, 7.333% faster w.r.t. the stock size tires. Idealy the speedometer would be calibrated. Vehicle speed is irrelevant.
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  #5  
Old 03-02-2006, 12:55 PM
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Exclamation Same difference....

Quote:
Originally Posted by m.messenger
I currently run 18" wheels with 245 x 45 x 18 Vredestein Ultrac tyres - they fill the arches to the max but just OK, except on full bad bumps when they sometimes tickle the plastic shrouds at the back of the front arches and the outer sides of the rear arch shrouds - otherwise great. Before that I had Bridgestone run-flat 225 x 40 x 18 - too harsh because of the reinforced run-flat sidewalls.
The point is thet I have an Origin B2 GPRS radar thingy which gives a constant speed read: with the standard 16" wheels and 225 x 50 tyres the diff between the dash speedo and the GPRS speed ( once it catches up) was about 4-5 mph - with the bigger radius 225 x 40 Bridgestones it was about 3 mph - with the Ultracs I am spot on - 0 mph difference.
So there we go!

MM
Hi Michael.

Thanks for the comparatives. Here's one that may interest you. It is interesting you show about 4-5 mph difference between true speed and speedometer reading with stock tyres and rims. I keep 5 rims shod because I won't go a long run, such as the UK to JAE, with that piddling get-u-home space saver.

With my last SVX, I changed from a set of [4] Savas, which I liked because they were quiet, to a set of Michelin. Naturally, because I would only use the fifth rim for long runs, and only as a spare, I did not bother to buy a fifth tyre at $165 or so each. Just to check on rolling radius, I stood both wheels together, part worn Sava alongside brand new Michelin. Imagine my surprise when the Michelin shod wheel with full tyre tread depth stood 1.5 cms shorter than the half worn Sava! Both with regulation tyre pressure!

To me, this means that your rolling radius is going to vary between makes. This also partly explains why Subaru insists that we change all four tyres together. Buy only two at a time, and a bigger or smaller set on one axle will very soon put loads of pressure on the differentials.


Just one to watch.

Joe
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  #6  
Old 03-02-2006, 04:06 PM
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I can go with that one too Joe, I have two set off rims for the SVX one set with Uniroyal and the other with Pirelli, both correct size with a good 5-6 mil tread depth.....the pirelli is quite visably larger!

Whilst were on the subject of tyres, what was the SVX originaly issued with from new.....any thoughts

John
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  #7  
Old 03-03-2006, 12:26 PM
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all change

The 18" wheel with Toyo 225 x 40 was only 10-15mm bigger than the original 16" with 225 x 50, and the Toyos were awful, partly due to the lower profile, and one deformed, which is when I put 4 Bridgstones. Their demise was caused by v bad caster on the fronts, which Collins in Willesden did a brillaint job bringing within tolerance, so I decided to start over with a full set of Ultracs, and am quite happy to the tune of 7.333%, that my speedo speed = my true speed!
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  #8  
Old 03-04-2006, 08:51 AM
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Tyre ROLLING circumference

Have a play with this:
http://www.performancetyresdirect.co.../table10e.html

To measure the rolling circumference (divide by 3.14 to get radius):
Park the car, put a chalk mark on the tyre wall and the road, move the car 10 wheel revolutions, mark the road again. Measure the distance between the two marks on the road and divide by 10 !

Arthur
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  #9  
Old 03-04-2006, 09:44 AM
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Exclamation Roly Poly

Thanks for the info, but no thanks: I prefer the theory and will do everything I can to avoid being caught doing this!

MM
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  #10  
Old 03-14-2006, 11:46 AM
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Original Tyres

[QUOTE=blueji]

Whilst were on the subject of tyres, what was the SVX originaly issued with from new.....any thoughts?

According to the Autocar test linked by Pete, the original tyres were Michelin MXX3 225 x 50ZR16.

MM
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  #11  
Old 03-14-2006, 12:00 PM
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Just recently fitted a Sat Nav thing in the SVX,with 18" wheels and 225/40/18 Toyo's, the speedo reads about 4mph slower at 80mph than what the Sat Nav thing says.
Mal
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  #12  
Old 03-14-2006, 12:35 PM
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If the speedo says 80mph you SHOULD be doing 81mph with those tires but you're actually doing 84mph. Normally the speedo errors in the opposite direction.
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  #13  
Old 03-14-2006, 04:58 PM
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[QUOTE=m.messenger]
Quote:
Originally Posted by blueji

Whilst were on the subject of tyres, what was the SVX originaly issued with from new.....any thoughts?

According to the Autocar test linked by Pete, the original tyres were Michelin MXX3 225 x 50ZR16.

MM
Thanks M for pointing this out, I had noted the same. I would have thought Subaru might have brought it into the UK with Bridgestone's, but then again, was their tyre technology up to it in 1992!?.....or too dear

John
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  #14  
Old 03-15-2006, 01:26 AM
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Quote:
Originally Posted by svxistentialist
Hi Michael.

Just to check on rolling radius, I stood both wheels together, part worn Sava alongside brand new Michelin. Imagine my surprise when the Michelin shod wheel with full tyre tread depth stood 1.5 cms shorter than the half worn Sava! Both with regulation tyre pressure!

Joe
Keep in mind, with the weight of the car on the tires (US spelling), the radius will be less. Different models of tires with the same air pressure will compress at a different rate, stiffer sidewalls, etc.

Even different tires of the same model will compress more on the front of our SVXi than the rear due to front weight bias.

I measure the distance from the topmost edge of the rim to the ground and adjust the air pressure (within manufacturers specs) to match that measurement with the tires cold. Then I'll take a run to warm them up and tweak a little as needed.

Dan
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  #15  
Old 03-15-2006, 02:31 AM
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Quote:
Originally Posted by intelisevil
Keep in mind, with the weight of the car on the tires (US spelling), the radius will be less. Different models of tires with the same air pressure will compress at a different rate, stiffer sidewalls, etc.

Even different tires of the same model will compress more on the front of our SVXi than the rear due to front weight bias.

I measure the distance from the topmost edge of the rim to the ground and adjust the air pressure (within manufacturers specs) to match that measurement with the tires cold. Then I'll take a run to warm them up and tweak a little as needed.

Dan
It's a good point you make Dan, softer sidewalls might make the wheel sit lower in the tyre.

That's very thorough, your matching the rolling radius like that. I only ever keep the tyres at the correct pressures or maybe a pound or two higher all round for comfort.

Full marks!

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