KIA Rio Grades 1, 2 and 3 - Impact of tyre width/size/profile on fuel economy?

Honest John Back Room » Motoring discussion
What is life like with your car? Let us know and win £500 in John Lewis vouchers | No thanks
View Threaded
Wed 8 Jan 2014 17:54
KIA Rio Grades 1, 2 and 3 - Impact of tyre width/size/profile on fuel economy? - Firmbutfair

Can anyone explain why the impact of tyre choice can have such diverse impacts on fuel economy?

I have attempted to extract two tables from the KIA published data for the RIO - summarised below - they appear to show that KIA is 'admitting to' a fuel economy penalty going from standard 185/65 R15 tyres to a lower profile and wider 195/55 R16 tyre for three of the four engine choices but that unexpectedly on the 1.4 CRDI Manual ISG, going from the 195/55 R16 tyre on the Grade 2 Rio to a 205/45 R17 tyre on the Grade 3 RIO actually improves fuel economy?

Rio Exterior Features Rio '1' Rio '1 AIR' Rio '2' Rio '3' 15" Steel Wheels (185 / 65 R15) X X - - 16" Alloy wheels (195 / 55 R16) - - X - 17" Alloy Wheels (205 / 45 R17) - - - X Full Wheel Covers X X - -

FUEL CONSUMPTION MPG (l/100km) & EMISSIONS 1.25 Petrol Manual 1 & 1 Air 1.25 Petrol Manual 2 1.4 Petrol Manual 2 & 3 1.4 Petrol Automatic 2 1.1 CRDi Manual 1 ISG 1.1 CRDi Manual 1 Air ISG 1.1 CRDi Manual 2 ISG 1.4 CRDi Manual 2 ISG 1.4 CRDi Manual 3 ISG Urban 47.1 (60) 44.8 (6.3) 39.2 (7.2) 33.6 (8.4) 80.7 (3.5) 72.4 (3.9) 67.3 (4.2) 56.5 (5.0) 62.8 (4.5) Extra Urban 65.7 (4.3) 64.2 (4.4) 62.8 (4.5) 53.3 (5.3) 94.2 (3.0) 85.6 (3.3) 80.7 (3.5) 74.3 (3.8) 76.3 (3.7) Combined 56.5 (5.0) 55.4 (5.1) 51.4 (5.5) 44.1 (6.4) 88.3 (3.2) 78.5 (3.6) 74.3 (3.8) 65.7 (4.3) 70.6 (4.0) C02 Emissions (g/km) 114 119 128 150 85 94 99 105 105

Edited by Firmbutfair on 08/01/2014 at 17:59

Reply | Report as offensive | Link
Wed 8 Jan 2014 17:59
KIA Rio Grades 1, 2 and 3 - Impact of tyre width/size/profile on fuel economy? - Dabooka
We'll in the format it's presented there I'm sorry to say I can't be bothered to decipher it! Saying that is it just the tyre choice that differs? Do other spec changes not affect overall weight etc which may alter the calculated mpg?

Someone more wise and knowledgable will be along soon to clarify I'm sure, but I do believe the calculated mpg values are based on set formulas plus lab test results on "stock" engines
Reply | Report as offensive | Link
Wed 8 Jan 2014 18:07
KIA Rio Grades 1, 2 and 3 - Impact of tyre width/size/profile on fuel economy? - Firmbutfair
We'll in the format it's presented there I'm sorry to say I can't be bothered to decipher it! Saying that is it just the tyre choice that differs? Do other spec changes not affect overall weight etc which may alter the calculated mpg? Someone more wise and knowledgable will be along soon to clarify I'm sure, but I do believe the calculated mpg values are based on set formulas plus lab test results on "stock" engines

Yes in fact the 'real world' fuel economy penalty is much greater than the official govt figures suggest, as shown by the What Car Magazine True MPG Comparison Tool. The penalty is around 14% going from the 1.25 litre 1 Air 5dr to the 1.25 litre 2 5dr with the former (cheaper) 185/65 R15 shod model offering overall average mpg of 46.4 mpg - much better than the 39.9 mpg average calculated by the What Car road tests for the grade 2 Rio running on 195/55 R16 tyres.

Reply | Report as offensive | Link
Wed 8 Jan 2014 18:15
KIA Rio Grades 1, 2 and 3 - Impact of tyre width/size/profile on fuel economy? - Dabooka
Yes, but was the tyre size difference the only change?

I'd assume the actual difference would be forces acting against the larger circumference wheel and the transmission. But my physics was school boy level and rather long ago, but there a couple in the know on here
Reply | Report as offensive | Link
Wed 8 Jan 2014 18:27
KIA Rio Grades 1, 2 and 3 - Impact of tyre width/size/profile on fuel economy? - Firmbutfair
Yes, but was the tyre size difference the only change? I'd assume the actual difference would be forces acting against the larger circumference wheel and the transmission. But my physics was school boy level and rather long ago, but there a couple in the know on here

Apart from maybe a minor (less than 1 %) increase in kerb weight going from the Grade 1 to the fully loaded Grade 2 car, all other factors such as gear box ratios, engine specs, 0 to 60 mph , top speed etc appear to be identical. But then conversely, the Grade 3 1.4 litre CRDI on its 17 inch 205/45 alloys claims better fuel economy across the board than its arguably lighter Grade 2 cousin on narrower 16 inch 195/55 alloys !

Edited by Firmbutfair on 08/01/2014 at 18:33

Reply | Report as offensive | Link
Wed 8 Jan 2014 18:39
KIA Rio Grades 1, 2 and 3 - Impact of tyre width/size/profile on fuel economy? - Firmbutfair
Yes, but was the tyre size difference the only change? I'd assume the actual difference would be forces acting against the larger circumference wheel and the transmission. But my physics was school boy level and rather long ago, but there a couple in the know on here

Apart from maybe a minor (less than 1 %) increase in kerb weight going from the Grade 1 to the fully loaded Grade 2 car, all other factors such as gear box ratios, engine specs, 0 to 60 mph , top speed etc appear to be identical. But then conversely, the Grade 3 1.4 litre CRDI on its 17 inch 205/45 alloys claims better fuel economy across the board than its arguably lighter Grade 2 cousin on narrower 16 inch 195/55 alloys !

For the ease of reference the link to the Kia Data is here:

www.kia.co.uk/new-cars/range/compact-cars/new-rio/...x

Re your reference to 'larger circumference wheel' - my general assumption is that the overall diameter stays about the same because whilst the rim diameter goes up from 15 to 16 and 17 inches the profile of the tyre reduces from 65% to 55% and then 45%.

Good luck !

Edited by Firmbutfair on 08/01/2014 at 18:44

Reply | Report as offensive | Link
Wed 8 Jan 2014 18:40
KIA Rio Grades 1, 2 and 3 - Impact of tyre width/size/profile on fuel economy? - gordonbennet

Probably find those 45 aspect tyres are running about 8psi higher pressures than the 55's.

You'll need that fuel saving to pay for the fillings that shake loose as you negotiate Britains third world cart tracks on 45's.

Reply | Report as offensive | Link
Wed 8 Jan 2014 18:46
KIA Rio Grades 1, 2 and 3 - Impact of tyre width/size/profile on fuel economy? - Firmbutfair

Probably find those 45 aspect tyres are running about 8psi higher pressures than the 55's.

You'll need that fuel saving to pay for the fillings that shake loose as you negotiate Britains third world cart tracks on 45's.

Thanks for that Gordon - I quite agree, I have heard that the closer the rim gets to the road, the less comfortable the ride is!

Reply | Report as offensive | Link

From choosing the right tyre to what to do when it goes wrong - we have the answers.

Tyre News And Advice