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Because of the recently fitted turbo I want to keep an eye on my fuel consumption...I think I have worked it all out...but I am pretty sure I have confused myself along the way. So any math wizz's out there could you run an eye over this.
HJ60 landcruiser by the way...working on an original tyre of 31x10.5x15...currently running 35x12.5x15.
according to my calculations the speedo is reading 13% too slow...meaning that when the speedo reads 88km/hr, I am actually doing 100km/hr.
But what I really want to know is will the odometer be out by the same amount. I think it would be, but like I said...I am confused. But if it was out by the same amount...If the odometer was reading 500km travelled, I would of actually travelled 568km.
Odometer and speedometer will be out by the same %age.
The way to test this is not to calculate it from tyre size, but to measure measured distance/speed against a known actual distance/speed, such as one of the defined 5km sections on a freeway or the overhead rader speed detectors on some major roads. This is because speedos aren't accurate in the first place. For example my SWB mav came stock with 29 inch tyres, but the speedo was MORE accurate after I switched to 32s.
But... leaving that to one side, your calculation would be roughly right . When your odometer says 500km, you have actually travelled 500 times 35 divided by 31, or 565km.
It is all simple ratios with this stuff. 35 inch tyres travel 35/31 times the distance of 31 inch tyres.
If you do the real world test, you might find that your speedo reads 100km when the radar tells you you were doing 106km. So then you know that when your speedo tells you you have travelled 500km, you have actually travelled 500 x 106 / 100 km, or 530km.
my 60 with 35's was exactly 10 % out according to the GPS and the distance markers. The 60's speedo and odo are pretty much spot on for 750/16's which area a 32" tyre, so 32 + 10% = 35.2, close enough. Get yourself a 10% box from Marks or similar (although Marks will tell you you need a 16% box which they sent me and it was way out !, I knew I'd done my maths right and the GPS couldn't be that far out)
