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MG Midget and Sprite Technical - Rostyle query

Hi all,
a quick & easy question for you out there!
Are MG midget Mk III Rostyles all the same size?
My Midget mk III arrived on 165/70 R13 tyres - are these the usual factory fitted size?
Cheers all
colin
colin frowen

Yes
No 155/80 r13 is original

Onno K

I thought the factory size was 145x13.

I run 155/80's on mine and have since the '70's, as I think it fills the arches better, but it slightly alters the speedo readings.

R.
richard boobier

According to my handbook, 145SRx13 are original.
Dave O'Neill 2

145s are original; 155s were an optional extra when new.
David Smith

My sales Brochures only note cross plies or 145's.

R.
richard boobier

145s here also

But I like the looks of 155s over the 145s any day of the week...it fills out the wheel wells much better

Prop
Prop and the Blackhole Midget

Richard is correct, 145 is the best size for a 4.5 inch wide rim.
Nick Nakorn

Who says so Nick?

Car bibles says a 165x70x13 is good for a 4.5x13" rim.
Your tyre size: 165 80 13 good for 4.5x13 up to 6x13
http://www.carbibles.com/tyre_bible_pg4.html

So does the NZ LVVTA advisor PDF.
LVVTA_Info_01-2009_V2_Tyre_Size_to_Wheel_Size_Compatibility_Guide.pdf
http://tinyurl.com/mnyaaz2




Lawrence Slater

Yokohama agree.
http://www.yokohamatire.com/tires_101/customizing/rim_wheel_width/

Table from the bottom of the page.
80 Series
P155/80R13 4.5-5.5

P165/80R13 good for 4.5-6.0

P175/80R13 good for 4.5-6.0

P185/80R13 5.0-6.5
Lawrence Slater

They don't exactly agree, as Car bibles say that 155 & 165 will fit on a 4" rim, whereas Yoko say 4.5"
Dave O'Neill 2

But the general concensus is that 165's are fine on 4.5" rims.
Lawrence Slater

I like 165/70s and have fitted them on various wheels. They are fantastic on 5.5 rims (Compomotive) and pretty good on 5 inch rims (Minilites) but I found there was too much sidewall movement when on my 4.5 Rostyles and the handling felt loose and squirmy. Interestingly, a 165/70 gives almost exactly the same rolling circumference as the original tyres so give an accurate (?) or similar speedo reading to the originals
Chris.
Chris Hasluck

Just because a tyre fits a rim doesn't mean the tyre will perform well on a rim that is on the small side or large side of the specs issues by the tyre manufacturer. In other words, for the designed side-wall movement, it's much better to have a rim and tyre that are matched to the median widths of the ranges. Most tyre manufacturers specify the range of rim widths and often one might choose to over-tyre or under-tyre at the extremes of the range for aesthetic reasons. Within range safety is still fine but performance tails off the further away one is from the median.

Lawrence; who says so? 'Twas in me training m'lud.

Nick Nakorn

yes, what Nick said.

Just changed the wheels on the P6 from 205 tyres on a 5.5 inch rim to 205 tyres on a 7 inch rim.

Much better. As above, first tyre wheel combo within safety range but actually rubbish compared to the second combo
Rob Armstrong

Hi Nick. OK, so where does it say that 145/13 is the best size for a 4.5 rim?
Lawrence Slater

Lawrence, it doesn't. Manufacturers publish their range charts and they all vary a bit. Since the 4.5 inch wheel is so rarely used these days, many manufacturers now have that size at the lower end of their safe range for a wider tyre than the 145 - indeed some brands don't even sell a 145 because the market for that size is now so small. But if you cast your mind back to purchasing tyres in the 1970s, you'll remember just how narrow were some wheel widths and how specific were the tyre dimensions when most people bought cross-plies.

In those days, driving a car hard might even force the tyre off the rim even when it was the correct size (we were hooligans).

Of course, the ability for over-sized and under-sized combinations to work safely is now better than ever with high-tech radial tyres so I'm not saying a tyre bigger than 145 on a 4.5 rim will be rubbish, I'm saying the tyre is less stressed away from the limits of it's safety margin and so performs better.

Nick Nakorn

I can barely cast my mind back to lunch time, let alone the '70s. lol. But I do recall driving like a nutter on more than one occasion. ;).

I used to run 165's on my original 4" rims. Drove all over Europe with them and never shed a tyre. I've got a 165 on my 4" wheel as a spare on the Sprite, as I've only got 4 Minilites. And as you can see below, the NZ LVVTA reckon that's pucker.

I thought that since the rim shape was changed back whenever that was, shedding a tyre was far less likely.
Lawrence Slater

This thread was discussed between 07/03/2015 and 09/03/2015

MG Midget and Sprite Technical index

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