Fuel consumption with bigger wheels

Match · Post by **Match** » Sun Mar 23, 2014 9:22 pm

I've recently put 17x8.25 wheels on my polo and I've noticed that the fuel display is telling me that I'm only getting 400 miles to a tank. On the standard 15s I would get around the 500 mark. I didn't think it would change this much... plus the wheels are considerably lighter.

It's going in for a service this week so I think I'll ask them to check - or is this normal?

Mart!n-GTI-DK · Post by **Mart!n-GTI-DK** » Sun Mar 23, 2014 9:32 pm

Match wrote:I've recently put 17x8.25 wheels on my polo and I've noticed that the fuel display is telling me that I'm only getting 400 miles to a tank. On the standard 15s I would get around the 500 mark. I didn't think it would change this much... plus the wheels are considerably lighter.

It's going in for a service this week so I think I'll ask them to check - or is this normal?

Are you sure that wheels that size plus tires are lighter than 15" rims and tires? I'm not...

But it also comes down to width, thread on the road and so forth. Wheels and breaks are the worst places to add weight on a car. You will see it straight away in the consumption.

Post by **alexperkins** » Sun Mar 23, 2014 11:26 pm

Is the tyre size comparative? The speedo may be out a little.

ubiquitous-j · Post by **ubiquitous-j** » Mon Mar 24, 2014 7:12 pm

What engine is it? 400 miles from a tank doesn't sound unreasonable as that roughly equates to 40mpg.

Match · Post by **Match** » Tue Mar 25, 2014 4:53 pm

Mart!n-GTI-DK wrote:
Match wrote:I've recently put 17x8.25 wheels on my polo and I've noticed that the fuel display is telling me that I'm only getting 400 miles to a tank. On the standard 15s I would get around the 500 mark. I didn't think it would change this much... plus the wheels are considerably lighter.

It's going in for a service this week so I think I'll ask them to check - or is this normal?
Are you sure that wheels that size plus tires are lighter than 15" rims and tires? I'm not...

But it also comes down to width, thread on the road and so forth. Wheels and breaks are the worst places to add weight on a car. You will see it straight away in the consumption.

Possibly made a mistake with the weight. I didn't pick them up with tyres on, without they were much lighter though. Thanks for the info.

alexperkins wrote:Is the tyre size comparative? The speedo may be out a little.

165/55/15 compared to 205/40/17

ubiquitous-j wrote:What engine is it? 400 miles from a tank doesn't sound unreasonable as that roughly equates to 40mpg.

1.2 TDi. Usually gets closer to 500.

vc-10 · Post by **vc-10** » Fri Mar 28, 2014 4:16 pm

The width of the tyre makes a significant impact- larger contact patches are good for handling, not so good for drag (both aerodynamic and friction losses with the road). That's why eco versions of cars often have quite skinny tyres. When the Golf was available with optional Bluemotion Technologies in the Mk.6, they wouldn't let you spec the 'optional' alloys as that would have negatively effect the fuel consumption if you went for a BMT engine.

Also, larger diameter wheel/tyre combos effectively increase the gearing, and increase the speed vs what the car thinks is happening. If your speedo was bang on perfect before, it might now be reading slightly slow (and likewise for the mileage and MPG). The engine has to work harder to accelerate due to the higher effective gearing, which negatively effects MPG, and the aerodynamic drag when cruising at a set speed (according to the speedo) will now be higher, as the speedo is reading slower. When you're driving, the car knows it's used, say 1 gallon of diesel, and thinks it's gone 40 miles, but actually, due to the larger diameter wheels, it's gone 50 miles, due to the speedo being out of calibration. Obviously this is an exaggeration, but I wouldn't be surprised if a lot of the effects are purely due to the speedo being out of calibration.

Match · Post by **Match** » Sun Mar 30, 2014 2:50 pm

vc-10 wrote:The width of the tyre makes a significant impact- larger contact patches are good for handling, not so good for drag (both aerodynamic and friction losses with the road). That's why eco versions of cars often have quite skinny tyres. When the Golf was available with optional Bluemotion Technologies in the Mk.6, they wouldn't let you spec the 'optional' alloys as that would have negatively effect the fuel consumption if you went for a BMT engine.

Also, larger diameter wheel/tyre combos effectively increase the gearing, and increase the speed vs what the car thinks is happening. If your speedo was bang on perfect before, it might now be reading slightly slow (and likewise for the mileage and MPG). The engine has to work harder to accelerate due to the higher effective gearing, which negatively effects MPG, and the aerodynamic drag when cruising at a set speed (according to the speedo) will now be higher, as the speedo is reading slower. When you're driving, the car knows it's used, say 1 gallon of diesel, and thinks it's gone 40 miles, but actually, due to the larger diameter wheels, it's gone 50 miles, due to the speedo being out of calibration. Obviously this is an exaggeration, but I wouldn't be surprised if a lot of the effects are purely due to the speedo being out of calibration.

Very informative, thanks. I will be sure to slow down a bit extra for speed cameras then!

Post by **alexperkins** » Wed Apr 02, 2014 11:59 am

At 30 on the original wheels, youll be doing almost 32 mph now

At 70 on the original wheels, youll be doing almost 75 mph now

For reference

205/35/17 would be better for matching the speedo to the original wheels.

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