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calculating tyre sizes - hugh smith
I am currently spec'ing an upgrade to the running gear of my 1956 morris
minor. It currently has a 948cc engine with 4.55 diff. On 14" wheels.

The proposed upgrade is to put a 1275 Marina engine with a 5 speed Siera
box and a marina back axle (with either a 3.7 or 3.9 diff). This means I
will also use 13" steel marina wheels.

I am trying to calculate what an acceptable cruising speed I will get out
this combination. The problem is factoring the tyres. If I know that the
tyres are, say 175/55 can I calculate the diameter of the tyre?

I need to know this so that I can calculate the speed that I can get on
the road.

Can anyone help, or do I need to measure actual tyres?

(the car will be running disk brakes, front roll bar, and spax
adjustable gas shockers to cope with the increase in power)
RE: calculating tyre sizes - John Slaughter
Data on tyre diameters is available from the manufacturers technical departments, and the useful figure is the 'revolutions per mile'. You'll find there are slight differences between different makes of the same nominal size tyre, but these aren't significant. Armed with that information, based on the axle ratio an accurate miles an hour/1000 rev/min can be calculated. (60,000/axle ratio = tyre revs/hour. Tyre revs per hour/ tyre revs per mile = mile an hour per 1000 rev per min) This is assuming a 1:1 top gear in the gear box. From my experience of Minors (I own a 1958 saloon) I believe the standard 4.55 axle and tyres gives about 15.25 miles an hour per 1000 rpm in 4th. I ran my car on 13" wheels for a few years and in practice found that a 'standard' 80 profile 165 x 13 radial was effectively the same diameter as a standard tyre (5.20 cross ply or 145/80 radial x14). That swap, then, leaves the gearing unchanged. If you want to run 70 series tyres, then a 185/70 is the same diameter as a 165/80.

So assuming you are maintaining your tyre diameter on the 13" wheels by using 165/80 x 13's, then the gearing will increase in direct relationship to the axle ratio ie a 3.9 will give you about 17.5 miles an hour/1000 revs and a 3.7 diff will give you about 18.75 mile an hour/1000 revs ASSUMING top gear is still 1:1. I once owned a Sierra and from memory a Sierra 5 speed has a 1:1 4th and some gearing up in 5th. Check with a Sierra workshop manual, remembering there are several different 5 speed Sierra boxes. This would push up the above figures, in direct relationship to the 5th gear ratio. The choice is then yours, a really tall 5th with the 3.7 (at a guess 21 or 22mph/1000) or perhaps the more useable 19 or 20 mph/1000 with the 3.9. Depends how much time you'll spend on motorways, or how much you value acceleration. By modern standards a Marina 1.3 isn't that powerful, so a really high 4th may not be a good idea.

Another point is that most (if not all) Sierra 5 speeds don't use heavy hypoid gear oil; they are designed for the more modern 'manual transmission fluids' such as Syntrax. Dearer than hypoid, but it maintains shift quality. Above all don't use engine oil as one does in the Minor 'box!


John Slaughter
RE: calculating tyre sizes - honestjohn
Dear John,

I'd just like to express my thanks for such a stunningly comprehensive answer to Hugh's question.

RE: calculating tyre sizes - John Slaughter
No problem - didn't realise it was that good!


RE: calculating tyre sizes - hugh smith

thanks very much, much clearer now. Just need to confirm the sierra box ratios, but looks like I can use either a 3.9 or 3.7 diff and get decent cruising speeds.

thanks for the help.


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