MPG Loss

[DazzerVTR]

Since I've put my new VTR alloys on the MPG has shot down, I used to get 90 miles to £10 but now I struggle to hit 80.

I basically replaced standard VTR wheels with borderline legal tyres with sprayed Matt Black VTR wheels with four good tyres.

The new wheels do need balancing as it shakes like a bitch above 60 but surely that wouldn't loose me 10 miles to a tenner as I haven't gone above 30 really since I last filled up.

Any suggestions?

Tyre pressures are fine and what I normally have them 32f 30r.

[Barry123]

ok it's gonna have to be geek mode time....
This is going to be a long answer but it does make sense, so hear me out

the difference in tread between a new tyre and a barely legal tyre is around 8mm, whichs means that your rolling circumference will be bigger on the new tyre, ok with that so far?

So for a new VTR tyre 185/55
the radius (from centre to the top of the tyre) will be 177.8mm

now imagine your barely legal tyre
the radius (from centre to the top of the tyre) will be 169.8mm

so you're looking at a difference of 4.5% in rolling radius... not only does this effect speedo reading, it will affect acceleration, trip distance and torque to the road.

You will find with a smaller wheel the engine doesnt have to work as hard to move the car, this is due to the increased torque to the road (this means better acceleration too). hence you tend to see a slight improvement in fuel figures if you drive moderately.

The other thing is that you'll find that the distance between places seems to 'go up'. This is due to the wheel having to rotate more thus the trip distance will appear larger.

So you get slightly better fuel consumption and more (apparent) distance on smaller tyres (ie. old barely legal tyres).

So with larger tyres (ie. new ones) the opposite applies - which is why you might suddenly be experiencing a prob with fuel economy.

There are other factors which are specifically to do with the type of tyre i.e. quality and performance. for instance... Michelins Eco tyres have been constructed to wear less (but this also limits grip) which gives less rolling resistance (again improves fuel economy).

Performance tyres will have a high grip factor which means that rolling resistance is increased (which will have a negative effect on fuel economy)

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if that doesnt deserve rep i dont know what will lol

[dannyjay]

I'll even give you rep for that! lol :

[/Correction] I can't, lol: "You must spread some Reputation around before giving it to adsayer again."

[DazzerVTR]

Cheers for that mate, rep given

[b0t13]

lol, nice answer

[Barry123]

i know its a bit of a bastard answer but I hope it helps... My dad had the same issue a few years ago and we managed to sus out why... all of the above ^^^

Cheers peeps
Share the knowledge

[Dave86]

Fantastic knowledge that! I actually feel like i have learnt something new!!!

[Lofty86]

Only other thing it could be is, if your usein softer rubber tyeres then you will atcually have more drag than a hard rubber. This is 'cos hard rubber rolls over the gaps in tarmac (between stones) and soft rubber goes in the holes... When the soft rubber rolls out the hole, it creates a small vacuum which is wheree you get the resistance from. Dave

[Imfromwales]

Lol ad you know it all like! not beeing funny or cheecky there but you do seem to kow a lot!!!!

[Barry123]

Originally Posted by Lofty86

Only other thing it could be is, if your usein softer rubber tyeres then you will atcually have more drag than a hard rubber. This is 'cos hard rubber rolls over the gaps in tarmac (between stones) and soft rubber goes in the holes... When the soft rubber rolls out the hole, it creates a small vacuum which is wheree you get the resistance from. Dave

crap thats deep thinking there :Y: tryin' to take my job are you

Originally Posted by Imfromwales

Lol ad you know it all like! not beeing funny or cheecky there but you do seem to kow a lot!!!!

I get bored too easily, need do geeky things and learn about geeky things to simulate my mind

[DazzerVTR]

I've done some thinking of my own... don't laugh.

I was thinking going by what you said would it affect it as much as 10 miles to a tenner maybe 1-3 miles I could live with but we're talking about 40 miles to a tank diffrence.

[Barry123]

Originally Posted by DazzerVTR

I've done some thinking of my own... don't laugh.

I was thinking going by what you said would it affect it as much as 10 miles to a tenner maybe 1-3 miles I could live with but we're talking about 40 miles to a tank diffrence.

Yeah thats a fair point dude... first of all you're talking about a fuel consumption difference of 10%.
We reckon that the tyre size counts for 5% of that (due to the change in rolling circumference). The type of tyre used is also VERY critical... what were your old tyres and what are your new ones?

Remember that technically your not actualyl losing that many miles to a tank... your trip distance will be reading more miles than your are actually doing on barely legal tyres... its best to judge it by say, filling the car up making a trip to another location fill it up again and log the number of litres put in then repeating the process with the new tyres... you'll find they'll not too much difference.

a

[Ramigojag]

i miss the days when you could even get 80 miles to £10

[Stevo123]

You also mentioned tyre pressures as 32 front and 30 rear. Std VTR pressures are a bit higher than that IIRC. Check sticker on A post (1 Bar = 14.7 PSI) btw. Cold weather is bad for mpg as well as the engine spends more time cold hence fuel mixture is richer more of the time as well.
I must admit i noticed a difference when i fitted Toyo's to the front instead of the plastic chinese tyres I had previously (dangerous they were!).