Does tyre diameter change you speed or you speedo

maybe this will help, if you have 13" wheels and then replace with 14" wheels it will make the speedo read 10 miles per hour slower than the car is traveling.
Speedo will say 40 MPH but the car will be doing 50MPH
Hope that helps

Steve,

Hope you are joking!

When you change something real - what would you expect to change. Reality or the indicated reality?

Muddy

My mate has an OKA`and the other day I drove it with the larger tyres on it it did 90kph flat out.

He put a different set on it after smaller diameter and now seems to have no torque and speed seems to be slower 75/80 kph

Cheers Steve.

80kph is 80kph, regardless of tyre size. If two otherwise identical vehicles with different sized tyres are doing the same speed, the wheels with the smaller diameter will be turning more rapidly than the larger ones. The percentage by which wheel B will be going faster (or slower) than wheel A will be (diameter difference) divided by (wheel A diameter) x 100. The speeds shown on the speedos should also differ by this percentage.

Here's a related problem, and I'll post the (very surprising) answer tomorrow:
Imagine a string that just fits exactly around the equator of the earth (ignoring mountains etc.). Cut the string and insert a bit of string 1 metre long. Now somehow raise the 1-metre longer string so it is the same height above the equator all the way around the earth. How high above the equator will the string now be?
(That's the same as asking if the earth's equator were 1 metre longer, how much bigger would the earth's radius be?)
The answer is scarcely believable, but it's correct!

Please disregard this thread, after double checking the diameter difference is not worth worrying about, it must be something else.

Did I mention that we were discussing this over a beer or 2 when I asked.

Cheers Steve.

Steve,

Your speed does not change, however your speedo shows you the incorrect speed. I recently upsized my Prado 265/65R17 oem tyres to 265/70R17. The result was when my speedo says 100kph, I was actually travelling at around 103kph. Depending on how the cops feel when they get you on the radar, it may be enough to be booked. The higher your speed, the higher your speedo variance is. On a recent trip to Fraser Island, on 110kph freeways, I kept my speedo to around 105kph. I checked my speed using those 5km test on the side of the road, and got the same result. 105kph was around 109kph.

I recently had the speedo adjusted by a mob recommended by Toyota. They did a GPS check of the speedo and confirmed the difference was around 4%. They adjusted the speedo and the Prado is now more accurate. When my speedo says 100kph, the GPS showed 99kph. Godd enough for me!

It changes both..........
If you used exactly the same amount of accellarator pedal for both tyre sizes, the larger diameter one would most likely be slower & the speedo would read lower than actual.(depending on gearing & power of the vehicle)

Cheers Craig............

How accurate are them GPS....?
Mine always says 5 kmh less than the speed of the car, it's a handheld and I take it with me in everyones car.. Garmin etrex vista c

I have used this page to check before.
If you know your original diameter size in inches
you type your new size in and the speed you are going.

It then gives you a new speed reading

Site Link

allan

They are both related.

If you keep your speed constant, your speedo will change
If you keep your speedo constant, your speed will change.

eg
On original tyres you travel 100km/h and your speedo shows 100km/h.

On new tyres you travel 90km/h your speedo shows 100km/h
OR
You travel 100km/h and your speedo shows 110km/h

of course your speedwill change your wheels are gears in the total configuration of the drive train, change any gear in the system ie diff ratio or gearbox your speed will change, look at the gears on a push bike. Col