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NeuroBeaker · Jan 6th, 2008, 01:12 PM

Here's a link so you can calculate tyre size alternatives. I got it from someone else on MINI2, but I can't remember who now.

Anyway, here you go:

Tyre Size Calculator

From this, we can calculate the relative differences in in the speedometer estimation of your velocity and therefore the percentage change to gearing ratios as well. I've ignored any tire size above 205mm in width, as for the 5.5J rim (the 7-hole MINI 15" alloys) any tire wider than this is unacceptable to fit due to safety concerns (i.e. the tyre could roll off the rim) - according to an e-mail I have from the European Tyre and Rim Technical Organisation (or ETRTO for short) where they advised against a 215mm width.

The following wider alternatives to 175/65/15 will shorten your gearing (aid acceleration / reduce top speed), your speedometer will overestimate your speed, and your mileage on the clock will increase disproportionately to distance actually covered:
205/55/15: speedometer will overestimate speed: +0.33%

185/60/15: speedometer will overestimate speed: +0.90%

195/55/15: speedometer will overestimate speed: +2.14%

205/50/15: speedometer will overestimate speed: +3.70%

185/55/15: speedometer will overestimate speed: +3.94%

195/50/15: speedometer will overestimate speed: +5.34%

The following wider alternatives to 175/65/15 will lengthen your gearing (harm acceleration / increase top speed), your speedometer will underestimate your speed, and your mileage on the clock will be under-reported for distance actually covered:
195/60/15: speedometer will underestimate speed: -1.07%

185/65/15: speedometer will underestimate speed: -2.14%

205/60/15: speedometer will underestimate speed: -3.04%

195/65/15: speedometer will underestimate speed: -4.27%

205/65/15: speedometer will underestimate speed: -6.41%

Although, do bear in mind that the speedometer currently overestimates your speed to start with, but not sure what the OEM overestimate actually is though - you'll have to ask someone else for that.

As an example, let's assume that your speedometer currently overestimates your speed by 3%. If you fit a 205/55/15 tyre to your 15" alloys (my preferred option for once I change insurers), this will further increase the overestimation by the speedometer by the above listed percentage:

OEM overestimation:
60mph actual speed + 3% overestimation = 61.8mph reported

OEM overestimation + overestimation by tyre fitments:
60mph actual speed + 3% overestimation + 0.33% overestimation = 62.00394mph

Note that is is more than a 3.33% overestimation, which would have come out as:

60mph actual speed + 3.33% overestimation = 61.998mph

While that's not such a big difference for this tyre selection, a tyre which introduces a larger percentage change would induce a larger difference if not taken into account in this way.

So hopefully that's useful. Let us know how you get on.

All the best,
Andrew.

Jan 6th, 2008, 01:12 PM	#10
NeuroBeaker Where's my navigator?! Join Date: Feb 2007 Location: Behind the wheel Local Time: 03:49 PM Posts: 5,763 Offline	Here's a link so you can calculate tyre size alternatives. I got it from someone else on MINI2, but I can't remember who now. Anyway, here you go: Tyre Size Calculator From this, we can calculate the relative differences in in the speedometer estimation of your velocity and therefore the percentage change to gearing ratios as well. I've ignored any tire size above 205mm in width, as for the 5.5J rim (the 7-hole MINI 15" alloys) any tire wider than this is unacceptable to fit due to safety concerns (i.e. the tyre could roll off the rim) - according to an e-mail I have from the European Tyre and Rim Technical Organisation (or ETRTO for short) where they advised against a 215mm width. The following wider alternatives to 175/65/15 will shorten your gearing (aid acceleration / reduce top speed), your speedometer will overestimate your speed, and your mileage on the clock will increase disproportionately to distance actually covered: 205/55/15: speedometer will overestimate speed: +0.33% 185/60/15: speedometer will overestimate speed: +0.90% 195/55/15: speedometer will overestimate speed: +2.14% 205/50/15: speedometer will overestimate speed: +3.70% 185/55/15: speedometer will overestimate speed: +3.94% 195/50/15: speedometer will overestimate speed: +5.34% The following wider alternatives to 175/65/15 will lengthen your gearing (harm acceleration / increase top speed), your speedometer will underestimate your speed, and your mileage on the clock will be under-reported for distance actually covered: 195/60/15: speedometer will underestimate speed: -1.07% 185/65/15: speedometer will underestimate speed: -2.14% 205/60/15: speedometer will underestimate speed: -3.04% 195/65/15: speedometer will underestimate speed: -4.27% 205/65/15: speedometer will underestimate speed: -6.41% Although, do bear in mind that the speedometer currently overestimates your speed to start with, but not sure what the OEM overestimate actually is though - you'll have to ask someone else for that. As an example, let's assume that your speedometer currently overestimates your speed by 3%. If you fit a 205/55/15 tyre to your 15" alloys (my preferred option for once I change insurers), this will further increase the overestimation by the speedometer by the above listed percentage: OEM overestimation: 60mph actual speed + 3% overestimation = 61.8mph reported OEM overestimation + overestimation by tyre fitments: 60mph actual speed + 3% overestimation + 0.33% overestimation = 62.00394mph Note that is is more than a 3.33% overestimation, which would have come out as: 60mph actual speed + 3.33% overestimation = 61.998mph While that's not such a big difference for this tyre selection, a tyre which introduces a larger percentage change would induce a larger difference if not taken into account in this way. So hopefully that's useful. Let us know how you get on. All the best, Andrew. * * * * * NeuroBeaker Proud owner of "Zeus" (a 'slightly modified' 2004 Mini ONE).