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 More
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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