Upgrading the wheels and tyres

Frequently Asked Questions about the Cappuccino.

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Upgrading the wheels and tyres

Postby Douglas Rogers » Sun Jul 07, 2002 18:36

The Cappauccino is fitted with 165x65x14 tyres as standard, that is about 165mm wide with 65mm sidewalls fitted on 14inch rims. These are too tall and narrow to extract the best from this car. By fitting 175x60x14 tyres to the existing rims, not only is the height to width ratio improved but the tyre will extend beyond the rim protecting them against kerbing damage.

By buying new alloys, 165x45x15 is the largest and lowest profile tyres you can fit without flaring the wheel arches. Being smaller in diameter than the originals will marginally change the speedometer readings. A slightly taller tyre, at 165x50x15 tyre can be fitted with less impact on the final drive gear ratio. Both of these tyres require fitting onto 15inch 6.5J rims. The following table gies the errors for each of these suitable tyres for the capo. Try the website http://www.miata.net/garage/tirecalc.html to find the change for other combinations.
    Tyre size Error Description
    165x65x14 0% Original equipment tyre
    175x60x14 +0.8% Best tyre for fitting on the standard rims
    195x45x15 +2.4% Possers perfection!
    195x50x15 -1.0% Compromise tyre on the larger rims

A positive error means your speedo tells you that you are going faster than you actually are. To put this into perspective, when Robert Austin tested his speedometer against his GPS and found a consistent error of +7%. Note that a with a +2.4% error when we think we are driving at 71.7mph Mr Plod would only record 70mph on his radar gun. We shouldn't though rely, as Robert can do, that driving down a motorway at an indicated speed of 75mph is still legal!

As important as the tyre profile is the rubber it is made of. The capo has an enjoyable tendancy to step out at the back under hard acceleration when the turbo is on song in the lower gears, especially in the wet. I would therefore buy a softer compound tyre like one of the Yokohama offerings before I forked out for some new rims.
Douglas Rogers

Postby William McDougall » Sun Mar 02, 2003 12:13

My Cap has 16" wheels - 6 1/2 J, fitting fine, just needed the rear arches rolled for extra body clearance.

The tyres are 205/40R16 = 0.1% slow according to the Miata tyre calculator!

The wheels I chose are the same weight (including tyres) as the ones that were on the car before. Steering feels slightly heavier, but the grip is phenomenal with the Falken's I'm using.

The larger wheels look great on the car, IMHO. You'll see it in the caption competition ("Sporty silver").

William :)
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