Trying to understand wheel sizes and effects on MPG and Speedo readings

[OllieGBR]

Morning all.
I've upgraded my wheels from the standard 215/50/17 Devonports to 275/40/20 on Super Metals.
Two immediate things stand out, apart from the improved looks

My MPG has decreased and the speedo reads pretty much exactly what the GPS shows. For me this is an increase in speed vs the speedo reading.

Now, according to 'will it fit', the speedo will be -5.25% and original vs new 20" speedo readings;
60mph is 56.85mph on the new 'bigger' wheels.


I clearly don't understand this correctly as I know my van is going faster at any indicated speed than it was on the original wheels.
Also, my mpg reading is lower - so I assume this is also set to the original wheels.
If the circumference of the wheel is bigger, then for every one revolution I will travel further????

I'm planning to get a VCDS cable and software as I need to program for a tow bar and clear my airbag error message. VW fit these wheels on some of their new vans, so assume they must make configurational changes to the ECU?

 

[Skyliner33]

The will they fit website assumes that on your old wheels the speedo is exactly right.
Therefore when you speedo reads 30mph you are travelling at 30mph.
your new wheels have a larger circumference.
Therefore when you are travelling at 28.43mph your speedo will read 30mph.
Now if your speedo is reading 30mph then your will be travelling (guestimate) at 31.5mph

Many factors affect mpg. You have put on heavier wheels and changed your final gear ratio.

 

[OllieGBR]

The will they fit website assumes that on your old wheels the speedo is exactly right.
Therefore when you speedo reads 30mph you are travelling at 30mph.
your new wheels have a larger circumference.
Therefore when you are travelling at 28.43mph your speedo will read 30mph.
Now if your speedo is reading 30mph then your will be travelling (guestimate) at 31.5mph

Many factors affect mpg. You have put on heavier wheels and changed your final gear ratio.

I only scraped a C in maths, so bare with me @Skyliner33 - but if the circumference is larger then for one gearbox revolution the tyre will cover a greater distance and therefore must travel faster to achieve the same distance.

Most cars are understating in their speedo readouts - we know this because if you compare against a GPS the van/ car is always a fraction slower.
When I ran my van on the standard wheels, this was the case.
According to the website on the new 275/40/20 wheels I will be going slower.
This is incorrect as I know from the GPS readout I'm going faster.

Maybe I should had this problem to one of my children studying maths

 

[Skyliner33]

I only scraped a C in maths, so bare with me @Skyliner33 - but if the circumference is larger then for one gearbox revolution the tyre will cover a greater distance and therefore must travel faster to achieve the same distance.

Just suppose your wheels are rotating at 1 rev per second (for arguments sake).
Suppose you had a tyre with a circumference of 1m. You would be travelling at 1m per second. So in a minute would cover 60m. ie 60m per minute

Now swap your tyre for a larger one (gearbox speed stays the same), wheel still rotates in 1 second.
Now if the larger tyre has a circumference of 1.1m then in 1min you will have travelled 66m. ie 66m per min. So travelling faster.

Most cars are understating in their speedo readouts - we know this because if you compare against a GPS the van/ car is always a fraction slower.
When I ran my van on the standard wheels, this was the case.
According to the website on the new 275/40/20 wheels I will be going slower.
This is incorrect as I know from the GPS readout I'm going faster.

Maybe I should had this problem to one of my children studying maths

Look at it way. Travelling at 30 km per hour. A wheel with a circumference of 1m will turn 30,000 times in that hour (30000 km divided by 1 m)
The slightly bigger wheel (1.1m) will turn 27,272 times. is its rotational speed will be less, for the same car speed.

 

[sparkzer]

That website appears to have its numbers around the wrong way.

A larger wheel will cover more distance given the same revolutions of the axle, in your example about 120mm per revolution.

So lets say with the old wheels at 2000rpm in 5th gear the speedo says 50mph, but as it over reads as is the standard you will actually be travelling at 47.25mph (these numbers are probably completely wrong, just using them as an example)

Assuming the circumference number quoted is correct (it looks about right, but I've not checked it) the ratio between the 2 is 2287 / 2167 = 1.055

Therefore given the same 2000rpm in the same 5th gear the speedo on the dash will show the same number, however with the new wheels fitted you will be now travelling 1.055 faster/further due to the larger circumference 1.055 x 47.25mph = 50mph.

Due to the slight change in gear ratio due to the larger wheel, the MPG will decrease however as you are now travelling further per RPM the numbers will likely cancel each other out give or take.

in summary, if you adjust the speedo cal to over read again by few MPH, your MPG will likely show close to what it used to, however where it is at the moment is actually the 'true MPG' (as much as a dash mpg can be true!).

 

[Skyliner33]

That website appears to have its numbers around the wrong way.

A larger wheel will cover more distance given the same number of revolutions, in your example about 120mm per revolution.

So lets say with the old wheels at 2000rpm in 5th gear the speedo says 50mph, but as it over reads as is the standard you will actually be travelling at 47.25mph (these numbers are probably completely wrong, just using them as an example)

Assuming the circumference number quoted is correct (it looks about right, but I've not checked it) the ratio between the 2 is 2287 / 2167 = 1.055

Therefore given the same 2000rpm in the same 5th gear the speedo on the dash will show the same number, however with the new wheels fitted you will be now travelling 1.055 faster due to the larger circumference 1.055 x 47.25mph = 50mph.

Due to the slight change in gear ratio due to the larger wheel, the MPG will decrease however as you are now travelling further per RPM the numbers will likely cancel each other out give or take.

in summary, if you adjust the speedo cal to over read again by few MPH, your MPG will likely show close to what it used to, however where it is at the moment is actually the 'true MPG' (as much as a dash mpg can be true!).

No you are incorrect.
The first column is the true actual speed of the car.
In the third column the speedo reading is 28.43mph, but this is an under-reading due to the larger circumference of the tyre, giving the true speed of 30mph.

 

[sparkzer]

ok..ill revise my comment to 'that website appears to be making something quite simple seem quote complicated' the implication without reading column 1 (which i didn't ) is you will be travelling slower on larger rubber. They are quoting the speedo reading when travelling at a fixed speed (30/60mph)

I find it much easier to explain by assuming fixed RPM and speedo readings which then relate to the actual speed the vehicle is travelling at...i just found this website that explains things a bit easier, clearly showing a larger tyre will make the vehicle travel faster/further! - Link

 

[OllieGBR]

Great website link @sparkzer - my mind is blown!
I needed an easy visual

 

[OllieGBR]

Sooooo my new wheel/ tyre combination is 1.8% faster;

Less than I thought but in the right direction.

 

[Skyliner33]

ok..ill revise my comment to 'that website appears to be making something quite simple seem quote complicated' the implication without reading column 1 (which i didn't ) is you will be travelling slower on larger rubber. They are quoting the speedo reading when travelling at a fixed speed (30/60mph)

I agree, its not the clearest of websites

 

[j4ckal]

Sooooo my new wheel/ tyre combination is 1.8% faster;
View attachment 72576
Less than I thought but in the right direction.

215/60 R18???

 

[OllieGBR]

215/60 R18???

Good spot and this now makes more sense!!!!!!

 

[Skyliner33]

Sooooo my new wheel/ tyre combination is 1.8% faster;
View attachment 72576
Less than I thought but in the right direction.

In the original post you put starting tyre size as 215/50/17 Devonports, in the link above you used 215/60R18?This cant be on Davonports.
Which is the correct one as they are a lot different?

 

[j4ckal]

In the original post you put starting tyre size as 215/50/17 Devonports, in the link above you used 215/60R18?This cant be on Davonports.
Which is the correct one as they are a lot different?

I spotted that too, but he has put 215/60r17 in the original calculator so correct size.

 

[OllieGBR]

In the original post you put starting tyre size as 215/50/17 Devonports, in the link above you used 215/60R18?This cant be on Davonports.
Which is the correct one as they are a lot different?

My bad, apologies for the original incorrect data.

 

[.50]

Its pretty simple.

Bigger wheels will record less distance on the odometer than you actually travel.

If you have wheels and tyres that are 5% bigger than standard, for every 10,000 miles the odometer records, the van would have actually covered 10,500 miles.

This is also why when your speedo is reading, say 50mph, you will actually be travelling at 52.5mph.

And, technically, your mpg will also decrease by 5%, however its usually more because extra wide tyres.

 

[Vinci]

Just changed wheels and tyres from 205/65 16, to 235/16 17, not sure if these photos of measurements muddy the waters, but gps 40 = 42 on speedo, 60 = 63.

 

[JOG]

I only scraped a C in maths

Swot! Bloomin’ hate it when someone clever pops up!!!

*I’m just jealous because I failed

 

[Texxaco]

Doesn’t helps if you wagged Maths @JOG

I did yet Still got a B.

 

[rolling_stone]

I have 18" wheels and run Nokian 235/55/18s and comparing GPS speedo to van speedo, mine's pretty much bang on now