I have just bought the gorgeous Mini that runs on 10-inch Minilite wheels. The engine is a 1275 from a Metro, but the final spec is unknown other than a Stage 3 head, K&N filter, and a twinbox Maniflow exhaust system. It’s fun for occasional use, but it seems to have some very short legs and I’m hoping to use it for touring around the country a fair bit. On the motorway it’s pretty deafening, so would fitting 12 inch wheels make any appreciable difference, or must I go through the process of having of having a different differential fitted? I`m not so bothered about looks, I just want it to cruise better. Hamish
Although fitting 12-inch wheels and tyres seems like it should dramatically reduce the engine speed, it doesn’t. The overall rolling circumference is not a whole lot different because of the tyres’ construction. The 1275 engine/gearbox units fitted to Metros almost always had a 3.44:1 final drive fitted. So that along with the 10-inch wheel set-up means around 16.17mph per 1,000rpm. So to make 70mph you would be using around 4,325 rpm. By comparison on the same final drive, 12-inch wheels give 16.47mph per 1,000rpm, so not much change at all.
So, the only way to reduce the rpm required to cruise at motorway speeds you will need to change the final drive. Going to a 3.105:1 would give you around 17.92mph per 1,000rpm. Still not a massive change, but some. However, it’s likely a great deal of noise may well be coming from the induction noise where a replacement cone or pancake filter has been fitted in place of the standard plastic air filter casing. So before embarking on the major task of hauling the engine and ‘box to change the final drive, you should consider doing something about that. Also, make sure effective sound proofing has been used, if fitted at all!
10-inch or 12-inch - decision’s yours.