Increase in wheel size does what to my odometer?

[warrenpfo]

I have changed tyres and put my BFG AT on for good now. I have found that this has put my speedo out by about 2m/h how do I translate that to my odometer?

Ie when my speedo says I am doing 50m/h my actual speed is 48.2m/h

I have just filled up and got a reading of 389miles on my trip but know that not the true reading how do I now calculate it?

Is it 48.2/50=0.964

then 389/0.964=403miles?

 

[piers_finlayson]

Almost, but wrong way around. If your car says you've been going faster than you really have then it'll also tell you you've gone further than you really have. So 389 * 0.964 = 375 miles.

Or, look at it like this:
real speed/clock speed = real distance/clock distance
Therefore real distance = clock distance * real speed/clock speed

 

[adrianr]

HI Piers

Not sure thats right.

Speedo always reed high so his new reading is closer to the true speed.

My understanding is that if the diameter of the tyre is larger then the truck will have covered more distance with each revolution so he has covered more distance than the odo will record?

 

[warrenpfo]

It may be my driving style but i have never got less that 400miles on a tank. Hence why I thought i had it right the way I calculated it at 403miles.

 

[chadr]

Isn't it as simple as working out the increase in the tyre size (i.e. when I went from 265/70R16 to 265/75R16 it was an increase of approx 3.5% in the rolling radius) - then that will be the difference between your odometer reading and actual mileage. So if the odo read 100 miles, you will have actually done 103.5.

Useful site : http://www.kouki.co.uk/utilities/visual ... calculator

Of course I could be wrong...................If you ask SWMBO, I usually am!

 

[piers_finlayson]

I'm pretty sure my maths is right. Your figures may of course not be .

If you car says you're going 50mph and you're actually going 48mph, then your odometer will say you've gone further than you are. (Cos, in one hour it'll say you've done 50 miles when you've actually done 48.)

Of course, as other posters say, you can figure out the impact based on the old/new tyre sizes. However, I'd assumed your 48.2mph came from a GPS which will be pretty accurate. (Although I don't think you can expect to read it more accurately than to the nearest mph.)

 

[adrianr]

Hi Piers on that understanding your maths is right -

and if the speedometer was 10% out as they all seem to be, and the odometer is accurate on standard size tyres then things change...

 

[piers_finlayson]

I myself have a much more accurate speedo and odometer my virtue of having bigger wheels, rather than the stock ones. Sadly it's still about 7% off!

(But then mine's an import and still has the kph clock - it has just been relabeled mph and obviously had some tweak to make it slightly more accurate than just reading kph!)

 

[chadr]

IMO we need to look at the speedometer and the odometer as two completely "independent" items, especially on the older LCs. (The newer models may have more integrated electronic items).

Now I'm not exactly sure how these things work but the odometer usually has some "widget" that counts revolutions at the gearbox or similar. Based on the standard tyre size a reduction gear is used to calibrate it, so that it shows the correct mileage fairly accurately. If you increase the tyre size then that calibration is thrown out and you will have covered a greater distance than the reading on the odometer.

A (mechanical) speedometer is also usually connected at the gearbox/transmission but will have a different mechanism and will usually be calibrated to always over read the speed by several %. Therefore when you increase your tyre size, you're reducing that built in margin of error and making the speedo more accurate. In some extreme cases, if you go large enough you might even get the speedo to under-read but that is very unusual.