Changing the wheeles or tyres

by putting bigger tyres \ wheeles on how does it affect the speedometer!
as iv got road tyre on 16" steels and a set off road on 15" but the off road set are bigger?

measure the rolling radius of both, then work out what the % is of the difference between the two. then add this % to your speed on the speedo

..or if you're idle like me take a look at the Tyre Bible.

About halfway down you will find a tyre size calculator which you can put in the size of the tyres you are looking at, and your current ones, and it will calculate the difference and the indicated road speed.

Much easier!

its usually upto about 5mph... you could always take along a handheld GPS. im about 5mph out at 65mph on 265/75r16. it doesnt seem to much different at slower speeds.

G

Excellent link Jumpjim, very handy.

so does bigger wheele mean your goin faster then the speedo? and what does it do to the mpg?

did ypou do physics at school

or English for that matter... no the speedo reads higher than your actually going, but if you can read the needle jumping to any degree of accuracy you deserve a medal. your milage will also turn over slightly slower than the "actual" miles your covering.

I found this out when trying to follow the TOR roadbooks for the 1st time

G

If your new rolling radius is bigger than standard you'll be moving faster than your speedo says you are if the speedo is dead accurate with standard wheels and tyres. Likewise, you will actually cover slightly more miles on a journey than your odometer says you travelled.

Example:
Changing stock LR defender wheels and tyres;
205/80/16, rolling radius = 367.2mm
To;
235/85/16, rolling radius = 402.95mm
You end up with a new rolling radius 9.74% bigger.

Put another way, for every revolution of the wheel you travel 9.74% further than before. Only way to travel further in the same amount of time is to go a little bit faster but as your speedo is calibrated for the smaller radius it under-reads by 9.74% (i.e. when your speedo says 70mph you're doing 76ish)

Trick is, very very few speedos are accurate at speeds above 30mph anyway and usually have between a 5% to 10% over-read. Only way to be absolutely sure how fast you're going is with a GPS unit in the car as Griff said.

Since relative economy isn't affected by such dubious things as mechanical speedos, you can expect a mathematical 9.74% increase in economy with the bigger wheels, but this being the real world I'd guess you're only likely to see up to about 5%.

Cheers,

doh,

yes i got it the wrong way around. faster than speedo...

we toke the handheld in my 90 and it read about the same as the 90 throughout the range, showing that when standard they must set them a little higher than actual recordings.

we also toke it in my g/f pug 306 and the gps was showing a slower speed than the speedo. therefore proving that speedo's are usually set higher than accurate mph.

so bigger wheels will prob calibrate your speedo more accurately! and you car will show less miles

G