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What size tyres can I maybe hopefully get away with?

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Hi. I need some new boots to replace the 185/13/60's on Vitesse and would like something skinnier and larger overall diameter if possible to get away from the lower profile look.

The car has a 3:27 diff and J Type overdrive which I understand is a bit overgeared, though will keep pulling up Motorway inclines in O/D top, as long as I start going up at 65 mph.

Also car will start off on a 20 degree gradient without undue clutch slip and seems to be cruising in O/D top at about 1000 rpm, per 20mph, which doesn't seem too long legged and the car feels quite revy at say 60mph at 3000 rpm (I'me aware my speedo and taco could be innacurate).

Current tyres have about 2mm tread left on them

I'm wondering what difference/how noticeable, it would make to the overall gearing, if I fitted 165/13/70, or even 155/13/80.

I've looked at the overall tyre diameters for these sizes, though don't know how this would relate to driving it.

A bit expensive to try them and see what's like.

Any thoughts would be brill.

Cheers, Dave


Edited by daverclasper
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155/80 is the original spec and works well if you're "pootling" (for somebody's definition of pootle). 175/70 works out to be the same rolling radius, within a gnat's, and is good for rotoflex cars. 165/80 is a little taller but a good compromise if you feel the gearing is OK. I don't think you'd really notice that difference - I didn't on the GT6.

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Having a tyre whose tread is narrower than the rim it's on may have advantages.  See below:


Tread wider than the rim (left) can wobble about, especially on flexible radials, like a pyramid on its point.

But the same tread on a wide rim(right) is supported better, despite the radial walls, like a pyramid on its base.

Allan Staniforth, the hillclimb guru, used 8.6" (220mm) treaded tyres on 11" (280mm) rims.  But you might need a lot of spacers, or rims with an extreme offset to fit those to a Triumph!

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Dave, I had 185/60R13 on my old Spit and I completely agree with you - they are noticeably smaller diameter and don't fill the arches. It spoilt the look of the car. These days I have 185/65R13. They are the correct diameter and look great but are now unobtainable even in Europe.

So your choice is the original size of 155/80R13, or 175/70R13 which is the same diameter or wider, or as suggested you could have 165/80R13 which is larger diameter but would perhaps be similar to the crossplies which might have been fitted in the 60's.

Cheers, Richard

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Dave, That 5% difference in the aspect ratio of the 185/60 & 185/65 tyre makes (does a quick calculation) 9.25mm difference in the radius. If my maths is correct, that's a 3.4% reduction in the speedo and this agrees with my memory (we are talking 10 years ago) that it used to read a few percent fast, just like most cars are intended to be. It reads bang on now.

Hope this helps, Richard

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10 hours ago, daverclasper said:

Thanks for all the very handy info.

Richard. Did you notice any/much difference in the overall gearing when driving?, as it's quite an increase in diameter, which is what I'm looking for.

Cheers, Dave

https://www.tyresave.co.uk/tyre-size-calculator/ is a great tyre comparison site

http://www.mintylamb.co.uk/gearspeed/ will give speeds, revs etc in every gear, you will need to alter the tyre sizes and indeed diff ratio, then you will have ALL the info you will need. 

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