Hey all,

Have a question regarding my 1.0 Ecoboost Ford Focus 2016, regarding tyres and MPG, if this is a stupid question then my bad, but I’m just not sure so thought I’d ask in here!

So, I bought the car a few months ago and it has a set of 235/50R17s on it with the MPG is roughly 45. I was recommended to change all my tyres to 215/50R17s it will improve my MPG to 50/60+

Has anyone done this? Or if you have 215’s what is your rough MPG? I do a mix of motorway driving and city, but if the difference is big enough, might make the switch.

Cheers!

Most unlikely in my opinion. The narrower tyre may have a slightly lower rolling resistance but i doubt it would net you another 5mpg and definitely not 15mpg. Most of your energy losses are through aerodynamic drag and braking and affected a lot by your driving style.

I think 45mpg overall is pretty good for an Ecoboost Focus anyway. Whoever told you could get 60 mpg in a normal driving mix is pulling your leg. 

it also wont handle as well with a narrower tyre

And as it’s still a 50 profile but narrower tyre it’ll mess up the speedo readings, I think it’ll make it over-read the actual speed. 
 

Which might make the fuel computer give higher mpg figures but won’t be accurate. 

If using a different tyre gave you an extra 15 MPG, then I would expect we'd all be driving around an bicycle tyres 🤣

Iirc the standard size for this model was 215/50x17 anyway? Depending on tyre cost might be worth reverting to that size when a tyre change is due, but as already said, not for mpg reasons!

 

1.0L EcoBoost, 1.6L Duratec-16V Ti-VCT, 1.5L EcoBoost, 2.0L Duratec-HE - MI4, 1.5L Duratorq-TDCi, 1.6L Duratorq-TDCi

215/50 R 173

23 hours ago, troy45 said:

And as it’s still a 50 profile but narrower tyre it’ll mess up the speedo readings, I think it’ll make it over-read the actual speed. 
 

Which might make the fuel computer give higher mpg figures but won’t be accurate. 

Shouldn't that be the other way around. Profile would change the diameter wouldn't it, not the width?

Didn’t explain myself well enough 🙄

Narrower tyre with same profile spec means the tyre is not as tall (50% of 215) so yes the diameter of the wheel & tyre is affected

16 minutes ago, tomo2001 said:

Profile would change the diameter wouldn't it, not the width?

OP has tyres of width 235mm and is looking to change to tyres of width 215mm.

Both have a side wall of 50, that is 50% of the width. So a 215mm tyre will have both a smaller width tread and a smaller height side wall.

49 minutes ago, unofix said:

OP has tyres of width 235mm and is looking to change to tyres of width 215mm.

Both have a side wall of 50, that is 50% of the width. So a 215mm tyre will have both a smaller width tread and a smaller height side wall.

Thanks for that info, never knew it was a percentage of the width not a fixed dimension. Every day is a school day 👍

Tyres are a wonderful mix of units....

Diameter - Imperial - Inches

Width - Metric - mm

Aspect Ratio - Percentage - %

Load - no units, just a number.

and of course pressure, PSI or Bar........

 

Although IIRC, TRX had metric diameter sizes, but they never caught on.

 

 

 

3 minutes ago, Paulkp said:

Tyres are a wonderful mix of units....

You missed a couple !

Speed rating is just a Letter

and the sound level in dB

I'm always careful when ordering new tyres and get 'round' ones usually in black with a trendy pattern 🤣

It's late - past my normal bedtime !

You don't like cords then......

18 minutes ago, unofix said:

I'm always careful when ordering new tyres and get 'round' ones

I try to do that too, but by the time I get them home, they are sort of flattened at the bottom. Yet they looked OK when the tyre-fitter pulled them off the rack at the garage.

10 hours ago, Paulkp said:

Width - Metric - mm

To confuse things even more, rim width is measured in inches.  So working out which tyres are best suited to a rim needs a metric to imperial conversion.  Who decides these things?

9 hours ago, TomsFocus said:

Who decides these things?

The same person who designs equipment racks for professional electronic equipment......

Known as 19" racks, depth and height are metric, and equipment spacing is in something called "U".

A "U" is about 1.75", and equipment comes in multiples of "U", so you could fit five 2U boxes in a 10U rack....

Blimey! - that takes me back

235/50 R17 has never been fitted to a Focus, that's the wrong size to be on the car and it will be throwing the speedo out.

You might gain some tiny percentage of MPG, but more importantly, 215/50 R17 is the size it should be on.

The tyres can't even fit on the rims properly surely, they are too wide for the standard 17" rim?

1 hour ago, alexp999 said:

The tyres can't even fit on the rims properly surely, they are too wide for the standard 17" rim?

We don't know for sure if it's the standard 17" rim. Might not be original rims, so the OP should check rim width (in inches!) before changing tyre width.

Probably on a set of Mondeo rims.  They were 235/50/17 as standard.

No need to measure the rim.  Just find the number marked 'J' on the back of the spokes.  Will either be 7 or 7.5J in this case.

Hadn't thought of them being different rims. Just assumed previous owner stuck wrong tyre size on.