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MG Midget and Sprite Technical - Tyre size vs. Diff

Hi,
Question: I have changed the diff last summer, went for 3.7 from worn out 3.9. My current tyres are 175/70 R13 (Uniroyal Rain something) on Minilites copies. These are almost 10 years old and it is time to replace them due to the age (rubber degradation).

Should I stay with 175/70 or should I go for lower profile tyres due to the new diff?

Car is MKIII RWA, standard power, standard 4-speed gearbox. The change to 3.7 has improved cruising (lower rpm in higher speeds), but slowed down quick starts a bit. Is there a way how to calculate the impact of lower profile wheels?

Thanks,
Stepan
Stepan Marek

Hi Stepan,
I guess the simple question is which do you value more - the quieter high speed or the more rapid acceleration.

As regards the acceleration, you can generally compensate for the reduced gearing by holding the car fractionally longer in each gear before changing up - unless you had an engine that is tuned to give a very narrow power band and sharp cut off in torque.

The Tyre bibles site has a vast amount of information on just about every possible topic on tyres. There is a tyre size calculator here: http://www.carbibles.com/tyre_bible_pg4.html (towards the bottom of the page, 4) which allows you to see the effect of different choices (eg of sidewall height /profile). You could use this to calculate what reduction in sidewall would compensate for the percentage decrease in the diff ratio.
Guy W

To find out how different various tyre profiles are, use a calculator like this one
http://www.etyres.co.uk/how-to-change-your-tyre-size/

The calculator shows the percent difference in rolling diameter. A smaller diameter tyre lowers the overall gearing, ie. more revs are needed for any given speed. It all depends what you want from your car. Do you want more relaxed cruising or faster acceleration? You can't really have both.

One other thing, changing the diff ratio and/or altering the tyre size both affect the accuracy of the speedometer.
Mike Howlett

I would add that some would think a 175 tyre is oversized for an unmodified 1275. You could be using up more of the available power with no real benefit in grip from the extra width. So one option would be to keep the aspect ratio at 70 but reduce width to 165. the will produce a lighter tyre with a very slightly reduced circumference which may suit the car better. But also look at prices as tyres are significantly cheaper on the more common sizes than if you go for a more rarely specificied one.
Guy W

Do what Guy says in his last post. Make sure it doesn't look silly, or go too low on the profile or you'll knacker the low speed ride.
Rob Armstrong

Stephan,
if your Minilite copies are 5" wide then 165/70/13 would suit them well (see info in Tyre Bibles and specifications on actual tyres on manufacturers' websites (where shown)).

165/70/13 comes out at almost exactly the same as 145/80/13 which is the original standard size tyre for Spridgets with 3.9 and 3.7 diffs.

I'd go with 165/70/13 tyres too (they'd be OK for 5.5" rims too).

The differences in rolling circumference with your present tyres and the sizes discussed and recommended here are small so won't show much difference to cruising, acceleration and top speed. 155/70/13 tyres would double the small difference.

http://www.carbibles.com/tyre_size_calculators.html

Whatever size tyres you choose if you buy reasonably good quality replacements I'm sure compared to your present 10 year old tyres the new tyres will feel better and improve braking, road holding, ride and steering.

Within a sensible range you could also experiment with tyre pressures too (for ride and handling rather than diff difference).
Nigel Atkins

This is very useful as well http://www.fisc-racing.com/misc/ratios.html
John Collinson

Thanks for that John very interesting (and amusing if you press the first button), I've saved that link.

I expect Rob has seen it.
Nigel Atkins

Hi,
Thanks for all the information, especially the link from John is very useful, exactly what I needed to know.

What would you go for in my case (3.7 diff)?

1. Uniroyal Rainexpert 175/70 R13 (older type with "V" design)
70 mph at 3844 rpm (my current set up)

2. Yokohama A539 175/60 R13
70 mph at 4093 rpm

3. Uniroyal Rainexpert 165/70 R13
70 mph at 3940 rpm

I run on 175s for around 10 years, changed together with Rostyles to Minilites switch from 155/80 and although my engine is standard (except larger needles in SU carbs, K&N filters and tuned distrubutor with removed vacuum system) I have never noticed that there is not enough power, but I know this can be very subjective point of view...

Stepan

Stepan Marek

Stepan, one alternative to consider against changing your gearing and overall drive ratio is to lighten the flywheel.. I know in itsefl this has compormises - but in my experience ( from many mini's ) is that you can gain over the std flywheel with some knowledgeable machining.. will give a improved 1st / 2nd gear accel feel and not compromise your engine speed for motorways.. Obviously don't go too extreme or the idle can get lumpy and conversely make it easier to stall on sharp engagements of the clutch . I am sur others on here will have done this with good resuts. Does mean a strip down and engine/ gbox removal. But is a 'system' level option to consider...
John Barber

"This is very useful as well http://www.fisc-racing.com/misc/ratios.html"


John, why couldn't I find that?

I knew it was there, but couldn't seem to locate it.
Dave O'Neill 2

Stepan
If you are happy with your gearing like it is which it appears you might be-then why change
Your car looks great on the 175s
Do you think 165s might look a bit skinny on your rims
I'm thinking stay the same OR if you want it to rev a bit more go for the 175/60s
Honestly, after a day you wouldn't notice the 200 extra revs at highway speed but it will make it pull uphill a bit better so maybe 175/60 would be a better choice
willy
William Revit

If your Minilite copies are 5" I'd go for option 3.

Personally I like all the grip available in the wet and not so worried in the dry so that the car moves on the road to to give relatively low speed fun (compared to most modern cars that are no fun below highly illegal speeds).

Having said that, I don't know the Yoko A539s but I generally like Yokos and these claim to be good in the wet so if your wheels are 5.5" then option 2.
Nigel Atkins

This thread was discussed between 07/01/2017 and 08/01/2017

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