Determining the best tire size to buy for your MGB is a popular discussion among owners. Leaving aside the question of brand preference, it of course depends a great deal on what size rims you have, wire or steel, and whether they are original equipment or not. I'll try to cover some of these issues in this article, and hopefully answer your questions.
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Replacements for Stock Tires
The first and most common question is what is the modern equivalent for the original tire size, specified as 155R14 or the old replacement 165R14. This translates as a tread 165mm wide, which fits on a 14in rim - mixing imperial and metric. You can find tires in the size specified as this, but they are often expensive "period reproduction" tires from specialist suppliers. For quite a few years, tire sizes have been expressed in a standard way with an extra set of numbers to the above, namely, the sidewall ratio.
Why do you want to maintain the stock tire diameter? Well, the main reason is that if you change the tire diameter, your speedometer will read either faster than actual speed (shorter tires) or slower than actual speed (taller tires). This may be desirable; shorter tires will increase your 0-60 times, but reduce your top speed, and taller tires will do the opposite.
I've seen articles that state that the BGT used 165 and the B used 155 tires, but I don't see any proof of this, as the part number charts I have show the same speedometer fitted to the B and BGT. If they had different sized tires, they would require different speedometer gearing ratios.
Thanks to some clever math (not by me), the "implied" sidewall ratio for the orignal tires was calculated to be 78 (height = .78 * width). Anyone that has been tire shopping in the last 10 years will know that finding tires with sidewall ratios greater than 70 is tough, especially with the trend towards lower profile tires on ever-larger rims. The first task then, is to find the original diameter of the original tires, so that we can calculate some possible modern replacements. Assuming the original sidewall ratio of 78 (if this is inaccurate, someone please tell me!) we get the following:
So, calculating diameters for some common metric tire sizes, we get:
Now we can introduce another constraint - the width of the rim you're mounting the tire onto. The rim is not supposed to be less than 7/10ths as wide as the tire's section width. For the stock 4.5" rims, this works out to 163mm, rounded to 165 - but we're not going to find any sidewall ratios of 80. 175 tires are supposed to go on 5" rims, but some manufacturers say 4.5" rims are OK. 185 tires are certainly pushing the limit, as they are designed for a 5.5" rim, and you risk sidewall roll or debeading on extreme cornering, which could be very dangerous if you're trying to execute and emergency manoeuver.
So, since the 175/65R14 is way too short, we're left with the 175/70R14 as the best stock replacement. This size is fitted to some Saturns, but your brand selection in this size may be limited. The good news is they will probably be cheap, as tire companies make their margins on selling trendy low profile tires.
Wide Tires - Pushing the Envelope
If you have aftermarket wheels on your MGB, and specifically if the rim is wider than stock, then you have quite a bit more selection of tires. I have a set of "Dan Gurney Victory Mags" on my MGB which are 6" wide, and therefore I could fit Dunlop D60 A2 JLBs in 195/60R14. These are of course much wider than stock, and therefore heavier, and that does hurt acceleration a bit, but the cornering is much improved with the extra 80mm of rubber gripping laterally.
The problem you get into when dealing with non-stock wheels is avoiding rubbing or other interference. My wheels cause the outside of the rear tires to rub on the inside of the fender, and I'm not sure how I'm going to fix it, perhaps "roll" the fender or install flares. When trying to figure out why, I made the following calculations to find the "offset" for the rims. Offset is the current standard way of expressing how far in or out of the wheel well the wheel sits (ET) in mm, and can also be expressed in the older "backspacing" method. I calculated that my 6" (measured bead seat) rims should be about ET15 to ET30 to fit right.
Essentially, zero offset means the hub mounting flange is dead center in the wheel. Positive offset moves the wheel "into" the well, while negative offset moves the wheel out from the car. The following diagram explains the various terms pretty well:
I called Moss Motors, and the Minilite replica they sell (14x5.5 PN 455-386) has 23 mm positive offset. Main Street Motorsport reports that their Minilites (the real ones, not replicas) are available in offset ranges from 15-30mm (backset 105-120mm) and they have a very nice but spendy 14x6 alloy rim. This confirms my calculations above.
Too much offset can be fixed by using wheel spacers to push the wheel out; most front wheel drive cars have around ET40 offset wheels, but these can be corrected with spacers, and if necessary, longer wheel studs. Not enough offset is basically impossible to rectify, but if its close, you can fit skinnier tires. Wire wheel vs steel wheel axles are also different lengths on the MGB, I need to research this information more.
With all non-stock wheels, test fit and make sure there is no rubbing on the outside of the tires, and no interference on inside with the tie-rod ends, brakes or other suspension parts, on full lock on both sides.
A list of possible junkyard replacement wheels from other cars, and calculators for tire sizes and diameters, can be found in The MG Experience's Tech Library.