Mpg On Bigger Tyres

[MitchR]

Hi Gang,

I've took the plunge and have 19" wheels fitted to my 2009 IS 220d. The tank avg dropped from 45 to 35 (motorway cruising at 70mph) instantly. The tyre sizes are 275 x 35 x 19 rear and 235 x 35 x 19 front, pressures are 32 front and 36 rear. My driving style has not changed and nothing else has been changed on the car. Could the loss in MPG be due to the stickiness/larger contact area of the larger tyres or is it because of the effective increase in outside diameter of the new tyres kidding the on board electronics that calculate the MPG?

Original wheels and tyres were the standard LEXUS 16"

As another thought is this why some of us get better MPG; - smaller wheel size????

Many thanks Mitch

[ColinBarber]

Hi Mitch

There are three main reason why this is occurring

1. Your wheel circumference is different to standard. Ideally this shouldn't have occurred because tyre size choice should have matched the stock circumference. If this is out then your speedo, odometer and mpg figures will all be out.
2. Total weight of the wheel/tyre has increased.
3. Increased rolling resistance due to different tyre performance and a larger contact area resulting from the wider tyres

What were the original size tyres, 205/55/16? If so your new front tyres aren't too bad - maybe 2.5% out. Your rears should be 275 x 30 x 19 if you really want the 275 width. At the moment the rear wheels/tyres will be higher than the front and almost 7% out.

[johnatg]

Well, if your originals were 205/55/16, the new ones (rears only significant) are about 43mm bigger diameter - that's about 6.8% and accounts directly for about 3mpg of your apparent loss (and incidentally your speedo will read 6.8% slower than it did for a given speed). (But you will have travelled 6.8% further than your odometer reads)

The rest I guess is down to increased rolling resistance - wider and bigger tyres wear more quickly so they must consume (waste!) more energy - maybe! Possibly some effect too from the fact that your gearing is higher, so the engine will need to work harder/consume more fuel during acceleration. Conversly, the engine will be running a bit slower at like for like cruising speed and should be using less fuel - but if you were going by your speedo you will have been going that bit faster, so using more fuel (and it's significant at 70 mph!)