Wheels and Tyres

DISCLAIMER: The infomation on these mods are a collection from experience (myself or friends), maginazine articles, mailing lists, internet web sites etc. So don't take these as correct gossibal, as I probably got my fax wrong on some them (sorry). Hence I don't any take responsibilty if you take one of these on and cock it up, its your fault and not mine.

One of the most asked questions, what's the biggest wheel that can fit onto a Nova? This depends on whether you're prepared to do little bit of work or not. Novas were fitted with 13" wheels while the GTE and GSi where fitted with 14" items as standard. All these had a 49mm offset. You do know what an offset is don't you? If not, click here.

WHEEL DIAMETER AND WIDTH

The most popular diameter of wheel is from 13" to 16". As mentioned before Vauxhall put 13" and 14" wheels standard on their cars. People who want bigger alloys put on 15" to 16" wheels on. These usually fit with no problems with the correct offset and diameter. If you decide to fit 17" wheels then be prepared to have virtually no suspension travel (very harsh ride), speedo reading lies and the gearing is thrown out. Plus you will need to do some modification to your rear arches.

Then there is the width of the wheel, which is usually between 5" and 7". The thinner the wheel, the more acceleration and better handling you have in the snow. The wider the wheel and the more grip you have, but less acceleration plus it will feel 'heavier' when turning the steering wheel. The best option is to go for a 6.5" width.

So the best size to go for is a 16x6.5" wheel, but what size tyres do you get?

TYRES

First let me explain what code on the tyres mean, let's take for example the tyre marked 225/40VR17. For a start these will be very difficult to fit under a Nova's arches (but not impossible). The units are both metric and imperial. The 17 is the inner diameter of the tyre in inches, so this will fit onto a 17" wheel. The first number, 225, is the width of the tyre in millimetres. In this case the widest part of the tyre is 225mm, which is 8.8" wide (25.4mm = 1 inch, so 225mm divide by 25.4mm = 8.8").

The 40 represents the percentage of the profile, also known as the aspect ratio. In other words, the smaller the number, the more the tyre looks like a rubber band on the wheel. As its 40% of the height of the tyre, then the profile in mm is 0.40 x 225mm = 90mm. If there isn't a aspect ratio shown on the tyre then it has a default ratio of 82% (looks like tractor tyres).

Diagram of aspect ratios.
Diagram showing the aspect ratios for same size of wheel.

After 225/40, we have the letters VR. This is the speed rating of the tyre and the type of construction. The 'R' stands for Radial and the 'V' is the speed code, in this case 149mph MAX. You should choose a tyre that will be the appropriate speed rating for all different types of road. NOTE that remoulds do not reach high in the speed ratings, so be careful on the motorways.


Symbol Max
KPH
Max
MPH
S
T
U
H
V
W
Y
ZR
180
190
200
210
240
270
300
240+
112
118
124
130
149
168
186
150+

14" Wheels

Remember that different size wheels and tyres from that of the standard ones will upset the gearing and speedo readings. On 14" rims use a 185/60 tyre, this should keep the speedo OK and give good traction. For a better look try a tyre with a lower aspect ratio, ie 185/50, which gives better acceleration. So for 14" wheels go for 185/50's.

15" Wheels

For 15" wheels use 185/55, as these will give greater wheel arch clearance. But I would consider trying 195's. The 195/50 is widely used on 15" rims, is ideal for widths up to 6.5". For larger widths, like 7", try 195/45's. But these probably will require work on the rear arches, but it's worth it because they do look very nice, especially if the Nova is lowered. So rule of thumb, on 15" wheels get 195/45's.

16" Wheels

The lowest profile tyre for 16" wheels are 195/45's, so get them. The negative aspect of them is that they WILL give inaccurate speedo readings plus it will have slower acceleration, plus a harsher ride, but it will have good top speed. Again this will almost defiantly need modifications to the rear arches. The front ones should be OK, just be careful of the callipers (especially if you replaced the standard ones). Conclusion, for 16" wheels, get 195/45's.

Gearing/Speedo Readings

"So what's all this about upsetting the gearing?" you may ask. To calculate the effect of gearing you need to know the OVERALL DIAMETER of the wheel in millimetres. To do this, add the tyre inner diameter (17") to two sidewall heights (90mm). So for our example (the 225/40VR17), the calculation looks like this:

(17 x 25.4mm) + (2 x 90mm) = 611.8mm or 24.1 inches is the overall diameter of the wheel.

Great, so what does this mean? We'll do the same calculation to that of the standard size wheel/tyre you had on your Nova. The closer the two answers are the better, look for something within 2 to 3%, preferably the new wheels should be bigger than the originals. The original tyre should give (in theory) the best speedo reading, but on average all speedos are faster by 5%.

Lets take for example a standard Nova SR (with 175/70SR13) and we want to put some 16x6.5 wheels, with 195/45 tyres on.

175/70SR13 = (13 x 25.4) + (2 x 122.5) = 575.2mm or 22.65 inches
195/45??16 = (16 x 25.4) + (2 x 87.75) = 581.9mm or 22.9 inches

The difference between the two is 11mm or 1.16%. To work out the percentage you need to take the smallest diameter from the largest diameter then divide the answer with the smallest diameter. Then finally to get the answer as a percentage, multiply it by 100 (confused?). The calculation is shown below:

Let A = 581.9mm and B = 575.2mm

((A - B) / B ) x 100 = 1.16481% or rounded down 1.16%

In other words this choice of wheel and tyre size is good for the gearing and speedo. The speedo reading is only an extra 1.16% extra FASTER. If the speedo is 5% (worst case) FASTER originally, plus the extra 1.16%, the speedo reads 6.16% faster than what you are actually doing in your car. So if your speedo reads 40mph, you are actually travelling at 42.5mph.


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