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Old 02-02-2015   #1
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+3 wheels/tyres increasing fuel cons/poor performance?

A few months ago I fitted 17x7 wheels and 205/40R17 tyres in place of the 14in steel rims on my Easy TA manual.

They fit perfectly and ride quality isn't bad. I admit I cheated a bit by setting tyre pressures at 28psi in the hope that made the tyres more compliant. Yesterday I stopped after an hour of 110km/h driving to inflate them to 38psi. The tyres were not hot to touch and were showing 31psi, so that's only +3psi over cold pressure.

First thing that I noticed that seemed strange since fitting the new wheels a few months ago was the average fuel consumption on the trip computer was showing 16.2L/100km around town. This was with a number of short trips (2km or less) over about 80km total. The worst I've seen before under such conditions was about 9-10L/110km.

Then I went on a 600km highway round trip yesterday. Over 250km the car used 19.8 litres of fuel. The trip computer showed 8.3L/100km. Actual was this probably more like 7.9L/100km. On the way back, I achieved 6.8L/100km indicated.

Performance seemed blunted too. Before on the highway I could squeeze the throttle in 5th and speed would increase quickly from 110km/h to 130km/h - up a hill. Yesterday it was struggling up some of the long hills to maintain 115km/h, foot planted to the floor. It has never been like that before.

Okay, so the tyres are not an exactly replacement in terms of rolling radius; they are a few percent taller according to the tyre size charts I've looked at. I bought them as they are a much more common tyre size than closer size replacements.

The GPS shows 107km/h at speedo reading of 110km/h. This is +3km/h over the original wheels (104km/h at 110km/h indicated).

I checked the trip meter against roadside kilometre makings yesterday and it is spot-on.

I suspect the heavier, wider and slightly taller wheels/tyres are playing havoc with this little car. It could well be something else, but I am going to change wheels back to the originals for a while to see what happens. A bugger if it is the new wheels, because it looks so much better and lateral grip is really impressive...
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Old 03-02-2015   #2
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Re: +3 wheels/tyres increasing fuel cons/poor performance?

Quote Originally Posted by AusPanda View Post

I suspect the heavier, wider and slightly taller wheels/tyres are playing havoc with this little car. It could well be something else, but I am going to change wheels back to the originals for a while to see what happens. A bugger if it is the new wheels, because it looks so much better and lateral grip is really impressive...
You've got it there, the lower psi won't help either. A wider tyre has a greater contact patch & therefore more drag too. It might also be worth weighing a wheel against your originals to see the difference but remember the centrifugal force effect on wheel weight will increase considerably when the wheel is spinning.

I used to race a Pug 106 rally, it came with 14" steels but I swapped to 13" for the track & remember once trying some 15" alloys on it, the car was slower on the track and handled like a bag of washing, the alloys were heavier.

You are paying the price for looking good
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Old 04-02-2015   #3
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Re: +3 wheels/tyres increasing fuel cons/poor performance?

Quote Originally Posted by samol View Post
You've got it there, the lower psi won't help either. A wider tyre has a greater contact patch & therefore more drag too. It might also be worth weighing a wheel against your originals to see the difference but remember the centrifugal force effect on wheel weight will increase considerably when the wheel is spinning.

I used to race a Pug 106 rally, it came with 14" steels but I swapped to 13" for the track & remember once trying some 15" alloys on it, the car was slower on the track and handled like a bag of washing, the alloys were heavier.

You are paying the price for looking good
Yes I think you might be right. I just changes the wheels, the standard 14in wheels weigh 12.2kg and the 17in wheels weigh 20.3kg. That's a big difference. I'll wait and see how it drives and consumes fuel over the next tankful now it's back on the std wheels.
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Old 04-02-2015   #4
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Re: +3 wheels/tyres increasing fuel cons/poor performance?

it's the width of the tyre that's the bigger issue / surface contact with the road..,

why all little OLD FIAT cars had 135 width ; - ),
Charlie
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Old 04-02-2015   #5
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Re: +3 wheels/tyres increasing fuel cons/poor performance?

You've also destroyed the sprung-to-unsprung weight ratio by adding two thirds to the weight of each wheel; I'd imagine the handling on bumpy or ripply surfaces has been adversely affected, plus all the additional kinetic energy you need to disperse under braking.
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Old 04-02-2015   #6
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Re: +3 wheels/tyres increasing fuel cons/poor performance?

Yet there are heaps of Fiat 500s rolling on 17in wheels, yet the base 500 is on 14in. I guess they are often the performance 1.4T models so you don't notice the hit to performance or economy.
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Old 15-01-2017   #7
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Re: +3 wheels/tyres increasing fuel cons/poor performance?

You'll grow out of this one day. It took me years to stop meddling and stick with the standard kit. Or maybe its to help ride the rough roads? I love your outfit in your headline pic!
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Old 15-01-2017   #8
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Re: +3 wheels/tyres increasing fuel cons/poor performance?

205/40R17" is more then 2% oversized compare to standard, you will notice that in fuel consumption and performance.
You need a 215/35R17" tire for the right circumference compare to stock.
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Old 15-01-2017   #9
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Re: +3 wheels/tyres increasing fuel cons/poor performance?

sorry to hijack the thread a little but if I was to swap the 15inch alloys on my lounge for 14inch steels would it mess around with correct speed etc???? Or being barely different sizes would it not do anything .... I'm just a sucker for small wheels.....
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Old 15-01-2017   #10
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Re: +3 wheels/tyres increasing fuel cons/poor performance?

Quote Originally Posted by quotethepigeon View Post
sorry to hijack the thread a little but if I was to swap the 15inch alloys on my lounge for 14inch steels would it mess around with correct speed etc???? Or being barely different sizes would it not do anything .... I'm just a sucker for small wheels.....

1. You'd be best advising your insurer, just to be clear. Same width or a tad wider would probably suit them best


2. Ideally you'd go for the same rolling circumferences


https://tiresize.com/converter/
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Old 16-01-2017   #11
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Re: +3 wheels/tyres increasing fuel cons/poor performance?

Speedo readings aren't what you call acurate.
By law, from the factory they can over read (faster than you are going) but cannot under read.

If you look at the options on Panda wheels, they fit 14" to the Pop and Easy (with 15" option) and 15" on the Lounge and 4x4's.

Fiat won't adjust any speedo gearing as the rolling radius of each is very similar.

The difference between 185/55 15's and 175/65 14's is around 0.24%.
So 60mph on the 14's will be 60.14mph, a whopping 225 metres further per hour!
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Old 16-01-2017   #12
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Re: +3 wheels/tyres increasing fuel cons/poor performance?

Quote Originally Posted by Goudrons View Post
Speedo readings aren't what you call acurate.
By law, from the factory they can over read (faster than you are going) but cannot under read.

If you look at the options on Panda wheels, they fit 14" to the Pop and Easy (with 15" option) and 15" on the Lounge and 4x4's.

Fiat won't adjust any speedo gearing as the rolling radius of each is very similar.

The difference between 185/55 15's and 175/65 14's is around 0.24%.
So 60mph on the 14's will be 60.14mph, a whopping 225 metres further per hour!
Exactly, and way less than the change in circumference between a new tyre and the same that's down to the 1.6mm legal minimum. Sleep easily I'd suggest
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