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Really ****ty Tyre Question


Shackleton
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This one annoys me to post... think you'd need to know your RRC's and tyre sizes inside out as well as your R380's, plus have a maths degree, but no harm asking.

I want to change the tyres on my RRC. It's got standard 205/80/16's on the original Rostyles. It's running a standard Defender Td5 & R380 (iirc thats 1.41 ratio transfer box) driving original EARLY RRC axles - 3.54:1 ratio - I think.

I don't have a clue about this stuff so I have the feeling I might be mixing gearing ratios with axles ones?

Anyway, an uncle helped me figure out the percentage difference in the speedometer readings (speedo is from a '91 3.9) over the various commonly used road speeds. Then I lost the piece of paper. I'll have to do it again sometime. But I remember at cruising speed the speedo was, by his calculations, 22% out. Cruising speed would have been around 65mph (85mph indicated)

Does anyone here have the smarts to tell me which tyres I should get to narrow that gap? I don't want big f.off wide tyres. To get girly about it I think this one looks well proportioned and if I had to make an educated guess using BFG's spec page I'd say these tyres are either 225/70 or 225/75's. (my RRC is on police spec red and whites up front and Bearmach HD blues rear so it's sitting tall and my 205/80's look lost)

http://bringatrailer.com/2010/01/16/rarity-1972-range-rover-suffix-a-two-door/

Any advice gratefully received.

George

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Use a GPS to record the actual speed, and see what the speedometer says.

Work out the difference. You may be able to swap the drive gear in the TFcase

to get the speedo reading correctly, other wise you can swap tyres.

But if it is 22% out, you´ll need some taaaaaall tyres and then I´m not sure the

engine will cope.

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if you get the right drive gear for the speedo to stick in the t-case all will be well. Ralph will be along shortly with the part umbers. :)

Tyre size wise I would go for 235 x 85 with a 1.4 T-case. Just check the rear edge of the rear doors to make sure the tyre is not catching it (easily fixed) and wind out your steering lock stops.

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if you get the right drive gear for the speedo to stick in the t-case all will be well. Ralph will be along shortly with the part numbers. :)

Tyre size wise I would go for 235 x 85 with a 1.4 T-case. Just check the rear edge of the rear doors to make sure the tyre is not catching it (easily fixed) and wind out your steering lock stops.

part numbers for the speedo drive gears can be found by looking through the tech archive index or doing a search of the Defender forum for 'speedo drive gear' :i-m_so_happy:

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there are lots of tyre size calculators on the web, like the one martifers above has suggested. Just google 'tyre size calculator' and try a few out. But probably best to get the correct speedo gear just so you know your speedo is correct.

I found out recently that your speedo has to work to pass the MOT. However, it does not have to work correctly :rolleyes:

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Thanks folks,

I might get the speedo gear but given it's nearly four years since I did the conversion and I've been as far afield as Greece & Portugal without a correct speedo, I couldn't really give a rats. I tend to judge my speed on the motorway against whether I'm going faster than an artic or not. Good enough for me.

RE John Law. In Ireland classic vehicles have to undergo a general road worthyness by a Department of the Environment accredited test centre. It's not like the MOT / NCT though, it's more like 'yeah his chassis is straight and not full of holes and there's four wheels attached to it.'

235 is wider than I want. Think I'll just suck a set of 215's, maybe 225's and see what happens. Really more interested in getting better mpg so trying to balance the width for height against width for drag if you get me. She's never let me down on the beach with the 205's so that's not an issue.

Will check out the calculators. Also good to know the speedo gear P/N's are here too, wouldn't have thought of that.

Thanks again!

George

EDIT: Martifers, that Miata thing is very interesting! - not least cause it seems that it may be inaccurate. I'm on the Generaltires site, looking at an Ameritrac 215/85/16 listed as revolving 686 times a mile whereas the Miata calculator says it'll revolve 664 times a mile. Both sites list the same diameter for the tyre.

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EDIT: Martifers, that Miata thing is very interesting! - not least cause it seems that it may be inaccurate. I'm on the Generaltires site, looking at an Ameritrac 215/85/16 listed as revolving 686 times a mile whereas the Miata calculator says it'll revolve 664 times a mile. Both sites list the same diameter for the tyre.

A quick calc gives it as ~660 revs per mile so I'd be suspicious of the Generaltires site. Just because it's a big site doesn't mean it's correct... lol The Miata calculater is correct.

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As a guide I worked out that changing to 235/85s from 205 80s made 10% or in the region of. According to my sat nav the speedo was then accurate, more or less.

If I remember properly I figured out that a 1.4 transfer box would more or less undo the change, I find that with the bog tyres on it stuggles at top end, especially towing, but I think the engine needs attention too.

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Thanks folks,

235 is wider than I want. Think I'll just suck a set of 215's, maybe 225's and see what happens. Really more interested in getting better mpg so trying to balance the width for height against width for drag if you get me. She's never let me down on the beach with the 205's so that's not an issue.

What about a set of 750x16's then. Will fill the arches better than the 205's

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As a guide I worked out that changing to 235/85s from 205 80s made 10% or in the region of. According to my sat nav the speedo was then accurate, more or less.

If I remember properly I figured out that a 1.4 transfer box would more or less undo the change, I find that with the bog tyres on it stuggles at top end, especially towing, but I think the engine needs attention too.

Hi RRB, so you have a Range Rover that you changed from 205's to 235's? What engine and box?

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