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MG MGB Technical - Wire wheel upgrade - 14' to 15'

Hi to all,
have acquired a reasonable set of 5.5 72 spoke 15" wires which I have had checked for trueness etc.
My current set up is 14" wires running 185/70/14 tyres of indeterminate age.
I assume that originally the MGB would have run 80 section tyres as the speedometer is about 8% out.

Question is, if I upgrade to 15" wires, what tyre section should I look to get to maintain the correct speedo reading and rolling circumference.

55 section tyres have been suggested but I don't fancy the ride quality on rubber bands this thin!

Will 70 section still be OK or do I need 65 / 60?

Appreciate your thoughts.

Andy.
The mg-tart
(11 MG's and counting!)
Andrew Tyson

Andrew,

I'm on 185/65 x 15 and very happy. They look much better than the 14 inch wheels I think. You will find calculators on line to tell you the difference from your present tyres.

David
D Balkwill

Andy,

Using the following tire comparison website you can put in your current size and compare specs with other sizes of tires;

<mintylamb.co.uk/?page=tyre.htm>
David Kalp

Andy,
You can also try http://www.miata.net/garage/tirecalc.html
Tony
Tony Shoviak

Andrew-
I'm running P195/60R15 tires. The ride quality is good and, using highway mile markers and a watch as a gauge, the speedometer is pretty accurate. Of course, due to variances in tire design, the rolling radius of any given tire size also correspondingly varies. Some tire manufactures give the rolling radius of each tire on their company website.
Stephen Strange

Andy, I'm running 185/70-14 on my MGB and 185/65-15 on my MGA, 60 spokes on both sets. The wheels on the MGA look far nicer being 1" more in diameter. Choose the tyre size that looks the best and have the speedo recalibrated for the new ratio.

Andy
Andy Preston

I am running 15" wire wheels on my 67 BGT. Using a GPS I got the following mph readings on speedometer versus true speed:

1st column speedo reading
2nd column true speed (GPS)

25 30
30 35
33 40
41 50
50 61
52 65
58 70
60 74
65 80
70 86
75 92

I have this list taped to my dashboard. It's much easier and cheaper than recalibrating the speedometer.

Bernie
Bernie Lowe

I was advised that if you go lower than a profile of 60 with inner tubes they don't like the flatter shape and fail more regularly.
Ian Buckley

Tire sizes, including for 15" wheels, have been discussed extensively, check archives; notably last fall: "tire recommendations for 15" wheels". Tire manufacturer catalogs give rolling radius as mentioned above. Use the catalog numbers easily available on the web to figure it out, and use your head.
A 1" dia bigger wheel with the same section tire will be about 4% larger radius since these sizes all are 24"+ dia, and will cause a similar change in speedo readings. Roughly, 165/80/14 = 185/70/14 = 195/60/15 within the limits of normal treadwear.
The 8% error Andrew gives for 185/70/14 is too much , and Bernie's 20-23% is way out for any section that will reasonably fit on the car (he didn't give it)- it would be a 30" OD tire. I expect Bernie at least has the wrong speedo.

FRM
Fletcher R Millmore

My 68B still had the OE spare wheel in the boot with the old radial still in good nick, so I could easily compare the rolling circumference and my 185*15-65 Brigestones were spot on. Denis
Denis4

This thread was discussed between 22/02/2010 and 23/02/2010

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