Speed

Page may contain affiliate links. Please see terms for details.
T

tanuie

Guest
Hi, i am about to fit 18" wheels to my car, on the original car it was fitted with 17" wheels.
I believe that there will be the speed difference, for example if my speedometer reads 30mph what will the actual speed be as I don't want to be caught out.

Many thanks
 
I know people take this topic quite seriously, but it doesn't make a huge amount of difference at the end of the day.

The Nissan came with 14" tyres from the factory, but by the time I bought it it was on 16" tyres; my speedometer is only 5% out. - I don't think that's too bad.
 
There are small falctuations in diameter even with the same tyre fitted - air pressure can make a difference, as well as tyre wear (the diameter of a tyre with 2mm thread will have 12mm of rubber less than a new tyre). So by definition car speedometers are not precise.... if you want it to be precise, you'll need to calibrate it regularly.
 
take a run at each speed with a GPS running, that will be as accurate as you need.
Yes... at constant speed :)

The GPS speed is slow to catch up when accelerating or braking.
 
You only need to do the constant speed once, then you will know iif your out, and by how much
Yes... at constant speed :)

The GPS speed is slow to catch up when accelerating or braking.
 
Ah... forgot to mention the effect of friction on actual speed (distance per wheel revolution).... different rubber compound, different road surface - different speed.....
 
50 indicated is more like 45 road speed, as above, so your one inch will likely just bring it closer to accurate. I doubt it’ll overshoot.

Set cruise using gps and give it a mo to stabilise and figure itself out and you’ll get an accurate reading.
 
As the others have said, as long as the overall diameter is unchanged (actually being pedantic, it's the rolling radius that needs to stay the same), then you'll not change the speedo calibration. Again as the others have said, tread wear (about 2% difference between new and legal minimum) and tyre pressure both have an effect.

All speedos read fast.....

MOST speedos read fast.

I've measured many many cars over the years using either "CORREVIT" or 20Hz GPS references on the road/test track or by rolling road (48-inch single roller machines. Twin roller machines are not as accurate).

They can be 10% out but rarely by more than that. Some that I've measured, for example cars from the USA, tended to be pretty well spot on. This included cars imported to the US from Europe and Japan as well as American cars. Some UK cars have been spot on too.

I've measured a few UK cars that were pessimistic by 5-10%. This is bad news in a world of zero tolerance for speed limit enforcement. We even a case of three identical brand new vehicles with sequential chassis numbers each with very different calibrations for their respective speedos.

These days checking via GPS or a smartphone APP is pretty straightforward. Our C250d is about 5% optimistic. Our Golf is more like 10% optimistic for speed, but distance travelled is pretty accurate, which is in line with other Golfs that I've measured
 
Hi, i am about to fit 18" wheels to my car, on the original car it was fitted with 17" wheels.
I believe that there will be the speed difference, for example if my speedometer reads 30mph what will the actual speed be as I don't want to be caught out.

Many thanks
All the above.

To be concise, when you change the diameter of the wheel you also change the size and profile of the tyres so that the diameter overall remains the same. Since you have not stated the size of tyres at any stage we cannot comment definitively.

So, tell us what tyres you have presently and we will tell you the tyres you need to keep everything as it should be.
 
All the above.

To be concise, when you change the diameter of the wheel you also change the size and profile of the tyres so that the diameter overall remains the same. Since you have not stated the size of tyres at any stage we cannot comment definitively.

So, tell us what tyres you have presently and we will tell you the tyres you need to keep everything as it should be.
The tyres are rear 255/35/18 and the front are 225/40/18.
Many thanks.
 
Ah... forgot to mention the effect of friction on actual speed (distance per wheel revolution).... different rubber compound, different road surface - different speed.....

:D:D:D:D
 
I think he has stated what the car will have on the 18s not what it has on the 17s.
Seems that way, so we still need to know the 17 sizes to be able to work out the speed difference.
 

Users who are viewing this thread

Back
Top Bottom