adjustment for tyre size - fuel efficiency got worse

[ChaybobbTidbit]

hi, about a year ago, I replaced the original tyres on a Superb L&K (I think they were PZeros?) with all season SF2.

Rims are 19".

These have a larger circumference than the original tyres.

Indicated fuel economy has been quite poor since then.

I'm wondering if some adjustment needs to take place to account for the larger circumference tyres? 

Any ideas why efficiency would show as worse than before despite the EU Tyre Label Info indicating it should be better?

Thanks.

[Ootohere]

?

How much a change of circumference / total diameter is there and how many miles do you actually do now compared to what the mileometer tells you.

 

Really you did not change the circumference by that much did you?

?

Are they the same width of tyres. (What is on the road surface.) 

 

Tyre labels really mean nothing.   Give a clue only.    LIke the noise they give. nonsense often.

Then there is the tyre pressures you are running.

 

All Season Tyres are sometime not as economical as 'All Year / Summer bias'   Tyres, and some are actually better, until warmer weather come, 

 

Edited 26 April by Ootohere

[Stonekeeper]

As above ^^

 

All season tyres are not as economical on Fuel but may last longer.

 

https://www.justtyres.co.uk/tyre-tips/all-season-tyres-vs-summer-tyres

[Carlston]

4 hours ago, ChaybobbTidbit said:

hi, about a year ago, I replaced the original tyres on a Superb L&K (I think they were PZeros?) with all season SF2.

Rims are 19".

These have a larger circumference than the original tyres.

Indicated fuel economy has been quite poor since then.

I'm wondering if some adjustment needs to take place to account for the larger circumference tyres? 

Any ideas why efficiency would show as worse than before despite the EU Tyre Label Info indicating it should be better?

Thanks.

 

See the chart below.

 

If you changed from 235/40R19 to 245/40R19, then you need to multiply your odometer by 1.012 to take into account that you are now travelling 1.2% further than your odometer indicates...assuming that the odometer is 100% accurate (unlikely) when on new 235/40R19 tyres.

 

However, if you were basing your old fuel consumption figures on worn tyres, then the difference will be even more...perhaps you need to add about 3% to account for 6mm of tread wear on the old 235/40R19 when comparing to new 235/40R19.

 

Outside diameter of tyres

Old 235/40R19 with 6mm of tread wear 658.6mm

New 235/40R19 670.6mm

New 245/40R19 678.6mm (1.2% bigger outside diameter compared to new 235/40R19)

New 245/40R19 678.6mm (3.0% bigger outside diameter compared to old 235/40R19 with 6mm of tread wear)

 

Edited 26 April by Carlston

[Carlston]

1 hour ago, Stonekeeper said:

All season tyres are not as economical on fuel...

 

All-season tyres sometimes have better rolling resistance than summer tyres, as shown in the example below.

 

Continental AllSeasonContact 2 215/55R17 94V (Euro label B B 70dB)

https://www.camskill.co.uk/m140b0s8825p220589/Continental_Tyres_All_Season_Car_Continental_Conti_All_Season_Contact_2_215_55_R17_94V_TL_Fuel_Eff_%3A_B_Wet_Grip%3A_B_NoiseClass%3A_B_Noise%3A_70dB

 

Michelin Primacy 4+ 215/55R17 94V (Euro label C A 69dB)

https://www.camskill.co.uk/m54b0s1714p210234/Michelin_Tyres_Car_Michelin_Primacy_4%2B_215_55_R17_94V_TL_Fuel_Eff_%3A_C_Wet_Grip%3A_A_NoiseClass%3A_B_Noise%3A_69dB

 

Edited 26 April by Carlston

[Stonekeeper]

The same tyres in different sizes can have different ratings too

 

 

[Ootohere]

My MINI came on near smooth Goodyear Eagle F1,s and Vredstein Quatrac Pro all seasons i fitted  in the same size were actually more efficient and that is very easy to check in an EV.

Now i have on narrower Maxxis AP3,s on smaller rims and the same circumference and they are more efficient again and not ditch finders like the Goodyears. 

 

(Same with Corsa Electric which came on Michelin Primacy 4,s and there was not ill affect putting on Michelin Alpin 6,s.

or the Riken Snow tyres.)

 

Over the years i have always put on All Season or winter tyres on every vehicle i run and they are no less efficent for fuel than what ever the car comes on New or used.

It is how things are that matters and not what someone thinks.  

& then actually knowing what you get in the way of miles out of 45 litres, or 55 or whatever. Not what you are seeing on a Dash Reading.

Then there is winter, spring, summer or autumn and the possibility of differences.

 

& setting tyre pressures as the seasons change or temperatures, and know of that makes a difference.

Edited 27 April by Ootohere

[Stonechip]

I changed my  19 inch tyres from OEM Bridgestone Potenza  to Michelin Cross Climate 2  18 months ago,

 

 My average was 32.5 mpg under Bridgestone and interestingly  has risen to 34.0 mpg  with the Cross Climates. Another benefit was a quieter ride.

 

Tyres were both standard  235/40  R19  and pressures were 36psi.

 

Hope the above helps

 

[numskull]

On 26/04/2024 at 20:31, Stonekeeper said:

All season tyres are not as economical on Fuel but may last longer.

Don’t have any difference in MPG, moving from P7’s to Conti All Seasons. 

[ChaybobbTidbit]

On 26/04/2024 at 18:07, Ootohere said:

?

How much a change of circumference / total diameter is there and how many miles do you actually do now compared to what the mileometer tells you.

 

Really you did not change the circumference by that much did you?

?

Are they the same width of tyres. (What is on the road surface.) 

 

Tyre labels really mean nothing.   Give a clue only.    LIke the noise they give. nonsense often.

Then there is the tyre pressures you are running.

 

All Season Tyres are sometime not as economical as 'All Year / Summer bias'   Tyres, and some are actually better, until warmer weather come, 

 

Thanks for your reply. These are all very good questions and I can't answer any of them properly.

The new tyres are Pirelli Cinturato All Season SF 2. Tyre Size 235/40Y19

I don't think the "marketing size" is any different. 

I find these quieter than the previous ones and I'm relying on the odometer, I'm not measuring in any other way. I don't do much mileage (~3-4k a year)

I tend to keep the pressure on the higher side of the limit indicated on the fuel flap and similar before and after.

I had them rebalanced and checked after fitting.

It could all be a false impression.

 

 

On 26/04/2024 at 21:55, Carlston said:

 

See the chart below.

 

If you changed from 235/40R19 to 245/40R19, then you need to multiply your odometer by 1.012 to take into account that you are now travelling 1.2% further than your odometer indicates...assuming that the odometer is 100% accurate (unlikely) when on new 235/40R19 tyres.

 

However, if you were basing your old fuel consumption figures on worn tyres, then the difference will be even more...perhaps you need to add about 3% to account for 6mm of tread wear on the old 235/40R19 when comparing to new 235/40R19.

 

Outside diameter of tyres

Old 235/40R19 with 6mm of tread wear 658.6mm

New 235/40R19 670.6mm

New 245/40R19 678.6mm (1.2% bigger outside diameter compared to new 235/40R19)

New 245/40R19 678.6mm (3.0% bigger outside diameter compared to old 235/40R19 with 6mm of tread wear)

 

Thanks, good to see those numbers. So the difference would be minimal in any case.

I think the "tyre numbers" were the same before and after, the new ones are Pirelli Cinturato All Season SF 2 Tyre Size 235/40Y19

 

 

On 26/04/2024 at 22:39, Ootohere said:

My MINI came on near smooth Goodyear Eagle F1,s and Vredstein Quatrac Pro all seasons i fitted  in the same size were actually more efficient and that is very easy to check in an EV.

Now i have on narrower Maxxis AP3,s on smaller rims and the same circumference and they are more efficient again and not ditch finders like the Goodyears. 

 

(Same with Corsa Electric which came on Michelin Primacy 4,s and there was not ill affect putting on Michelin Alpin 6,s.

or the Riken Snow tyres.)

 

Over the years i have always put on All Season or winter tyres on every vehicle i run and they are no less efficent for fuel than what ever the car comes on New or used.

It is how things are that matters and not what someone thinks.  

& then actually knowing what you get in the way of miles out of 45 litres, or 55 or whatever. Not what you are seeing on a Dash Reading.

Then there is winter, spring, summer or autumn and the possibility of differences.

 

& setting tyre pressures as the seasons change or temperatures, and know of that makes a difference.

What's your decision mechanism for setting the pressures vs seasons and temperatures?

 

21 hours ago, Stonechip said:

I changed my  19 inch tyres from OEM Bridgestone Potenza  to Michelin Cross Climate 2  18 months ago,

 

 My average was 32.5 mpg under Bridgestone and interestingly  has risen to 34.0 mpg  with the Cross Climates. Another benefit was a quieter ride.

 

Tyres were both standard  235/40  R19  and pressures were 36psi.

 

Hope the above helps

 

Thanks, good intel.

 

But it sounds like there's nothing to "adjust" in the car settings in any case?

Edited 28 April by ChaybobbTidbit

[Bap33]

Fuel consumption with tyres has to be compared over a year or at the same period of the year, since it’s more sensitive to ambient temperature.

[numskull]

38 minutes ago, numskull said:

Don’t have any difference in MPG, moving from P7’s to Conti All Seasons. 

And the P7’s they replaced lasted 37,000 and still had 2 - 2.5mm remaining.
My son rips through tyres at an alarming rate; 10-12k at most because he brakes late and screams round corners. What I’m saying is driving style is the paramount reason behind early or late tyre wear, nothing else. These tyre-test sites do not represent the real-world experiences; the Michelins on my first Superb covered 63k before I replaced them, albeit 90% of driving was motorway at that time. 

[Ootohere]

@ChaybobbTidbit  Decision on pressures is from driving vehicles of all types but pretty much automatics over the past 5 decades in all seasons on many tyre types and sizes so 50 winters and springs, summers and autumns.

 

Grip / Traction (Friction) and safe driving in the wet, cold. snow, sleet & ice being more than important than the MPG really.

Summer time i might run tyres a bit higher pressure in quite warm weather if not doing 'Spirited driving'. 

But then it can be horribly wet roads you encounter even in the summer.   But then simple enough to adjust tyre pressures. 

 

A good tyre pressure gauge and a way to inflate tyres is 'Simply Clever' to have now with the price of going someplace to inflate your tyres. 

[numskull]

25 minutes ago, Ootohere said:

A good tyre pressure gauge and a way to inflate tyres is 'Simply Clever' to have now with the price of going someplace to inflate your tyres. 

Totally agree. I have a good pressure gauge and a handheld Aldi battery pump (which is brilliant btw) in the boot at all times. The gauge on the pump reads 1psi low, so I set that to +1psi to achieve the correct pressure.