What are the key issues to consider when deciding whether to fit 175 or 185 tyres on an MGBGTV8?
A question often posed by V8 members is "I need to fit some new tyres shortly and had a look around at what fellow members have on their cars. Currently I have 185s but am considering going back to 175s. Are there any strong views on what is best?"

V8NOTE187 - Tyre aspect ratio
A useful insight to tyre aspect ratios. 010591 V8NOTE187


Tyre sidewall markings
Understanding the markings on the sidewalls of tyres will help you to choose the right tyres for your car. Here's a guide to those sidewall markings using a 175/80 R14 88H tyre as an example. 290608
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Updated: 140705
Posted: 110715
There are at least four sets of issues you will need to review in assessing your tyre choice: a number of objective issues on tyre size, aspect ratio and gearing, the subjective issue of how the car appears with a lower diameter tyre and ride height, the available information on tyre performance and the natural concerns over price and availability of the tyres you might wish to choose.

Tyre size, aspect ratio and gearing

Tyre width
The 185 tyre has a wider tread width of 185mm (7.28 inches) so is 10mm (almost 0.4 inches) wider than the original 175 tyre. That additional width will put more rubber in contact with the road (depending on tyre pressures and vehicle loading) which should provide greater grip, but there will also be more drag from the greater frontal area of the wider tyre which will tend to increase the fuel consumption (mpg) to some extent. The tyre width is referred to as the "section width".

Aspect ratio and rolling circumference
The aspect ratio of a tyre is the ratio of the "section height" of the tyre to the "section width", expressed as a percentage - see the sketch in V8NOTE187. Traditionally radial ply tyres have nearly always had an aspect ratio of 82% and because this was so universal, it was never marked on the tyre. Thus for example the correct tyre for the MGBGTV8 is a 175/82 although the 82 is rarely referred to. Nowadays of course we see a proliferation of different "low profile" tyres with aspect ratios of 70, 65, 60 and even lower. A 185 tyre for a 14 inch wheel will typically have an aspect ratio of 70. So the section height on a 185/70 tyre is lower than that on a 175/80 and consequently the rolling circumference is lower too. For the 185/70 it is 2019mm and for a 175/80 1931mm or 4.36% less which of course affects the gearing of the car.

Gearing
So matching overall diameters if changing wheel and tyre sizes is necessary to ensure ride height and gearing do not change. The recommended maximum variance in overall tyre diameter is plus or minus 2.5%.
Changing tyres sizes - tyre size change calculator

Example of tyre choice calculations

Appearance of the car with 70 profile tyres

As the section height is 14mm (0.55 inch) lower than that for a 175 tyre, the car will appear lower on the road and the wheel and tyre combination will appear smaller as the diameter will be 28mm (1.10 inch) less. Some V8 enthusiasts will feel that the period appearance of the car with the higher 175 tyres is part of the car. Others will see the lower 185 tyres as having advantages in terms of a slightly lower ride height together with a little more rubber on the road through the larger contact patch.


Examples of tyre choice calculations

The following calculations use the standard MGBGTV8 tyre - 175 80 R14 88H - as the base for the comparisons. In this example the comparisons were made for a V8 Roadster conversion.

Compare a 185 75 R14 102R
Whilst the calculator shows a small variance in the overall diameter of the tyre of minus 0.47% the tyre shown on the etyres website as available in the size is speed rated as "R" which has a maximum speed of 108mph whereas an "H" is rated to 130mph. The load rating is 102 (850 kg) which is well over 88 (560kg) - some 52% higher as the tyre is for a van.

So whilst this tyre in this size shown on the etyres website (Bridgestone R360) is a good match for rolling circumference and hence gearing, the speed rating of "R" is below the "H" needed for an MGBGTV8.

Compare a 185 65 R15 88H
Changing the wheel size to 15" and the aspect ratio to 65, the calculator shows a variance of minus 2.36% which is within the recommended maximum variance of plus or minus 2.5%. The etyres website shows this tyre is available (Continental Eco Contact 5) in an "H" speed rating and with an 88 load rating too.





Compare a 205 60 R15 91V
Changing the aspect ratio to 60 but still with a 15" wheel, the calculator shows a variance of minus 1.42% which is within the recommended maximum variance of plus or minus 2.5%. The etyres website shows this tyre is available (Continental Premium Contact 5) in an "V" speed rating (up to 149mph so well over H at 130mph) and with a 91 load rating which at 615kg is well over 88 at 560kg.



Compare a 185 70 R14 88H
Changing the aspect ratio to 70, the calculator shows a variance of minus 3.3% which is over the recommended maximum variance of plus or minus 2.5%. The etyres website shows this tyre is available (Michelin Energy Saver) in an "H" speed rating and with an 88 load rating too.


So with this tyre when the speedometer will show 70mph you would be travelling at 67.7mph.



Compare a 205 50 R15 86V
Changing the tyre to a 205 on a 15" wheel with an aspect ratio of 50, the calculator shows a variance of minus 7.86% which is well over the recommended maximum variance of plus or minus 2.5%. The etyres website shows this tyre is available (Continental Premium Contact 2) in an "V" speed rating (up to 149mph so well over H at 130mph) and with an 86 load rating which at 530kg is a fraction under 88 at 560kg.

With a variance of minus 7.86% the effect on the standard V8 speedometer is substantial - with 70mph on the clock the road speed would be 65.5mph.

See the tyre size calculator on the etyres website
Tyre calculations tabulated

Prices quoted are from the etyres website on 5th July 2014 and include fitting and VAT plus the discount for buying two tyres.

Recommended maximum diameter variance is plus or minus 2.5%
Tyre size, type and brand
Tyre diameter inches/mm
Variance %
Diameter change
175 80 R14 88H
Continental Eco Contact 3
£80.60


25.02 inches
636 mm


Standard tyre

Standard tyre
185 75 R14 102R
Bridgestone R360
£80.60

24.93 inches
633 mm

- 0.47%

- 3 mm
185 70 R14 88H
Michelin Energy Saver
£86.40

24.2 inches
615 mm

- 3.3%

- 21 mm
185 65 R15 88H
Continental Eco Contact 5
£88.10

24.47 inches
621 mm

- 2.36%

- 15 mm
205 50 R15 86V
Continental Premium Contact 2
£100.10

23.07 inches
586 mm

- 7.86%

- 50 mm
205 60 R15 91V
Continental Premium Contact 5
£88.90

24.69 inches
627 mm

-1.42%

- 9 mm
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