Bigger Tyres...low Mpg

[Bobby4Focus]

Hello guys...i fitted some 225/40/18 wheels with some kenda tyres on my 56 plate focus mk2 1.6 tdci 90 ps..and i noticed something very weird...on the motorway in fifth gear i get about 42mpg ( i reset the average mpg ) and with the stock wheels i was getting like 60 . When i drive in forth gear it goes up...but in fifth gear goes down...its weird to get better mpg in forth gear :( any ideas? Maybe the new wheels are too heavy...or too wide? In town ots not such a big difference...its hard to calculate anyway...but when im on motorway and i check the average mpg i get 42 instead of 60 :(

[Vendee]

Your new tyres have a 5% larger rolling radius than your original ones (I assume 205/55/16) which means that your speedo accuracy will have changed and your MPG readout will also have changed. Also, having wider tyres will increase your fuel consumption (more rolling resistance).

[Bobby4Focus]

I will go to Bristol next week and i will see how is the mpg on a longer run...but from 60 to 42 its a big drop :(

[iantt]

Theres only a 3 mm difference in rolling radius, circumference is 16mm bigger, so very small difference you wont notice on the road, even the speedo will only 0.5mph out at 70mph (70.5mph)

only difference will be the resistance of tyre on road due to higher surface arwa in contact with road, even then its not that much more.

Nothing like the fuel consumption figures your saying

[Vendee]

Theres only a 3 mm difference in rolling radius, circumference is 16mm bigger, so very small difference you wont notice on the road, even the speedo will only 0.5mph out at 70mph (70.5mph)

My bad. for some reason I typed 185/55/16 into the calculator for his original tyres.

[IINexusII]

what brand were the old tyres and what are they now on the new tyres? that will also make a difference (rubber)

[Bobby4Focus]

Ive just had a 10 miles run on the motorway ... I reset the mileage and average fuel....and i drove 70-75 mph and ive got a 38.5 average...its ridiculous...before getting the new tyres and wheels i drove the car like 3 weeks with the old ones...i didnt pay too much attention till now cause i mostly drive in town...but now im worried and !Removed! off cause i cannot change the wheels every time i go on a longer trip on the motorway...i have a 1.6 tdci engine...what mpg do u get on that engine? Not a dpf....56 plate

[iantt]

you need to do more than 10 miles to get a good average for fuel consumption, i wouldnt take any average less than a tank full at a time, if you think its a wheel/tyre combination, then fill it up, use it like you would normally and dont look at avg mpg until your nearly empty, do the same on the 16in wheels full tank to empty, assuming you do simular journeys most of the time.

and see what the differnce is. i seriously cant see tyres making hardly any difference.

none of the customers cars ive put 225/40/18 on replacing 205/55/16 focus have ever come back complaining of nearly double fuel consumption