Tyre sizes

[Jim Haines]

This is my first post here - I found the previous entries interesting and having had my car now for 22 years + I can safely say I have experienced most of the problems with the possible exception of any fuel worries. I only use top grade Shell or BP, and seem to keep sailing on.I had the carbs. professionally rebuilt at the outset and I do have a filter in the line that seems to stay clean, so long may that situation continue. I also fitted a Lucas 45DElectronic distributor which I bougt new off the net for £45 - a cheap and very effective improvement

My car is a 1971 Roadster Automatic, which I had professionally restored by Paul Roach - now the Jaguar specialist - just after I bought it, with my retirement in mind. He did a good job - it is still in A1 condition and I get a lot of fun from it. However, I
am now at the stage where the tyres are over 10 years old, and I really think I ought to change them.

My question is - what is the largest circumference of tyre that I can fit to my wire wheels without fouling the body?

As I have automatic transmission the engine tends to rev higher than an equivalent manual, so I am wondering if I can do anything to reduce this slightly by fitting 'oversize' tyres and would be grateful for any advice that anybody may have. I do not want to modify the body in any way. I have changed the rear shock absorbers to modern piston type, but I don't think this will affect the tyre size.

Any advice would be gratefully received.

[George Wilder]

Jim
The easiest way to reduce revs is to fit bigger wheels 15 inch seem quite popular though bigger tyres and wheels can cause heavier steering - see castor correction thread in the MGB General Section.
I live in North West Essex not far from where Roach operated in Castle Hedingham (now in Acton Sudbury). I have two friends locally with BGTs and 15 inch wheels. As my roadster has O/D I have not looked at bigger wheels my self.
Where are you located?

[Peter Cresswell]

Hi Jim
The MGC uses 15" wheels with 165/80 x 15 tyres and they fit ok under the same design of rear wheel arch. As George says they would offer you a usefull increase in overall gearing.
Remember the speedo might read slightly low, but you can check this using a satnav on a level road.

[Jim Haines]

Many thanks for the replies.
I live in Upminster - in his younger days Paul Roach lived and traded in this area and was one of the few people I knew who was good at working on my old cars.
He did the MG for me, as well as a Mk6 Bentley I had at the time - happy days...
I appreciate the guidance but am trying to avoid the expense of a new set of both wheels and tyres. I only do 4/500 miles per year in the MG so my thinking was to seek advice on tyre sizes for standard wire wheels via this forum. If I can't find anything with greater circumference and simply have to fit the same size it won't be a problem - I was just trying to see my options before I actually go ahead.

[George Wilder]

Jim
The only way to increase the diameter of the tyres and wheels to slightly reduce the revs with the same wheels is to find tyre with a higher profile number than the ones you have - go from a 60 profile to 80 profile for instance. The tyre width ie 165/175/185 will not help the diameter.
MGCs had 15 inch wheels - could - dare I say it -a secondhand set of wires be an option?

[Jim Haines]

Thank you, Peter & George, for your responses.
Using the data you have suggested I have spoken to my tyre supplier (RoadRunners) and they can get 165 x 80 tyres as well as tubes and rim tapes so I think I will go along this route when the weather gets a bit better.
The question I have - does anybody know the mathematical formula that will give me either a radius or circumference figure for a 60 profile and an 80 - I would like to compare the 2 so that I can get an idea of what difference this could make if it can be worked out.
At this stage I plan only to change the rears and the spare, leaving the fronts to run on 60's and not affect the steering characteristics.

[Ian F]

http://www.etyres.co.uk/how-to-change-your-tyre-size/

Jim, this is probably too late for you to use now, but there is an on-line calculator which you can use to calculate the rolling circumference of tyres, and compare different tyre sizes. This should allow you to get what you want, either a close match or larger/smaller. easy to use as well,

IanF.

[johnbuckley]

I'm puzzled. On my bog standard '79 BGT rostyle wheels,the front tyres are labelled Michelin MX 165 R14, on the rear Hankook Steel Radial 880 165 R14. ie neither show a profile size. What profile are they likely to be ? If I want to replace like for like on size what size should I now look for?

[Peter Cresswell]

Hi John
Cross ply tyres used imperial (inches) measurements for the sizes, so for the MGB the size quoted was 5.60 x 14 where the width of the tread was 5.6 inches across, and the wheel diameter was 14inches
When radial tyres first appeared they had a rather strange mixture of metric and imperial measurements, so standard fitment for the MGB became 155 x 14 and later 165 x 14. This was probably to reflect that most of the wheels in the world used inches for measuring the diameter - and also the width of the rim, so early Bs used 4in wide rims, GT s used 5in which were also the width of Rostyles. The 155 (and 165 came from the width of the tread in mm. There was also a relationship between the height of the sidewall of the tyre and the width and in the case of early radials this was always 80% (so for example for a 165 x 14 tyre, the sidewall height was 80% of 165mm or 132mm.
Later in the 1970s as racing technology spread to road tyres, the height of the sidewall was reduced to produce a wider tread for a given diameter, and RS2000 Escort MK1 & 2 for example used 175/70 x 13 tyres as standard. Then in the search for more grip from even lower profile tyres, people like Pirelli introduced the P6 tyre which used 60% profile and the XR3 for example used 185/60 x 14 tyre. Now the overall diameter of the wheel and tyre for a 175/70 x 13 and a 185/60 x 14 was almost identical but the tread width was substantially increased.

Getting back to the MGB, your tyres are the correct size for car with standard width and diameter wheels, and the sidewall is 80% of the tread width. However if you want to use either wider wheels or wider tyres you have to consider the overall diameter of the wheels and tyres otherwise the speedo will read incorrectly. Hence most people use a 185/70 x 14 tyre with 5.5in wide alloy or wire wheels.

The letter in your size (165R x 14) is the speed rating and again this started simply with SR (speeds upto 115 mph) HR (upto 130 mph) and VR (150 mph), but these have been greatly refined in recent years using letters N (87mph) to Y (186 mph). Cars that can exceed 186 mph need specially made tyres that are available at a specially made price! R is rated at 106 MPH and is barely adequate for any modified MGB.

Of course many modern cars use even lower profile tyres, including the MG6. The TSE version uses 225/45 x 18 tyres, so gets a wide tyre from moderate diameter wheel, but the are very prone to damage from pot holes.

[johnbuckley]

Thanks Peter, So a set of 165/80 14 will be the closest in size to those I have on at present. Other than inaccurate speedo ( a minor worry) what are the advantages/disadvantages of changing from the original tyre size to a wider one say 175/75 14 or 185/70 14 utilising the same standard rostyle wheels?