I bought a new Terracan 2,9 CRDi....what a lovely car to drive both on and off road, magic !!

She was fitted with 255/65/16 standard radial tyres for town & highway driving. I had them replaced with my Goodyear Wrangler AT 255/70/16 which was on my previous vehicle and had only done 10000 km's.

I know that this will have a definite influence of the fuel consumption, but as this is a new vehicle I do not have any history data for comparison purposes.

Question: Why is it that this slightly different tyre, 65 to 70 ratio. causes the speed on the speedo vs my GPS to indicate 120 vs 115 and the trip counter (electronic) 224km vs 232km
travelled. A detailed technical answer will be appreciated.

Gerhard

Gerhard, your rim has a radius of 8 inches (203,2 mm). With the 65x255 tyres the height of the tyre (i.e. the distance between the rim and the road) is 0,65x255 mm = 165,75 mm. This gives a rolling radius of 203,2 + 165,75 mm = 368,95 mm. Similarly, with the 70x255 tyre your rolling radius is 381,70 mm. It is not necessary to bring in gear ratios or anything else, nor is it necessary to work out the km/h per 1000 rpm because all other factors remain unchanged.

So your GPS readings will change by the factor 381,70/368,95 if you swop tyres. This assumes that both tyres were new or fairly new - or at least worn more or less the same amount; if one set is new and the other smooth one would have to factor in the difference in thread depth as well.

Anyway, 120 x 368,95/381,70 = 116,0 (almost what you got - the difference could be due to tyre wear or inaccurate reading of the analogue speedometer). Simlarly, 232 x 368,95/381,70 = 224,3, which is close to your 224.

Please respond if my explanation is unclear.

Regards

Kobus Vorster

Now that's detailed.

Thank's Kobus it make a lot of sense to me, now I can determine the factors on my SFA


CJ

Hi Gerhard,
Use this link, Here you can work out what the
speedo diffrence is
http://www.exploroz.com/Vehicle/Tyres/SizeCalc.aspx