Rolling Road resistance, does it really make that much difference to MPG?

AudA2Numpty

A2OC Donor
In my case a yes it effing does.

I've had 175 60 R15 81H Dunlop Sport 300's on my car since I bought it last year.
I have had a pretty good average so far of 69mpg I've had highs of 82 and only two lows of below 60mpg. Both of those times were dues to a fuel filter replacement and a fuel temp sensor being replaced, twice (that was because I stupidly fitted the wrong one)

Whenever I've had a 62-65mpg log (5 times) it's been because of a service or my better half has been town driving, a lot.

My car went in for a checkup recently and I ended up having to change two front tyres as they were needed doing.

As time was short (I had a long journey to make the next day) I had to make a quick decision (not really much of one) as there was only one set of tyres on the shelf.
Falkens 175 60 r15 81h Ziex ZE 914s
A like for like of my Dunlops were time uncertain so I went for the only option. The Falkens.

I filled up just before I left for our 350 mile round trip. Driving my usual way should have made a return of 75mpg.
I filled up at the same garage for the same price per litre and the mpg was 67mpg. Since I have not had a long motorway trip go less than 75mpg I was a bit upset.
(Though I probably need to spend time thinking about real life instead of obsessing about my MPG.)
The car has done a typical weeks worth of driving with short hops. Baring in mind the wife hasn't touched the car this time, I would normally get 68 mpg return.
As noted in my Fuelly log.

I filled up to day. 61mpg. WTF!!!! These tyres have to go.

I have now done some research on the tyres I've just bought and they are rating E for RR resistance. My Dunlops however are C rated.

Does RR resistance make a difference?
Yes I think so.
 
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MikeMarsUK

A2OC Donor
... Does RR resistance make a difference?
Yes I think so.
Yes, definitely, but there are two further things which will be affecting you.

* New tyres have a worse rolling resistance than old, worn tyres (even if you had put on A-rated RR tyres, their first few thousand miles would be worse, although obviously they'll catch up and overtake when they've had a chance to wear a bit). This is because a significant part of rolling resistance is due to the rubber blocks squirming - and new tyres will have bigger rubber blocks than the original ones.
* Additionally new tyres have a bigger circumference than old worn tyres.
 

AudA2Numpty

A2OC Donor
I'm going to replace these tyres and see if that makes a difference as I can't deal with seeing the MPG figures that low. I realise that's a small OCD issue I have, but there you go.
 

dan_b

A2OC Donor
Also, has the garage inflated them to the correct tyre pressure - that also makes a difference. In addition, the ambient temperature is starting to drop, which means longer warm-up times, especially bad for our little diesels, and if it's getting really nippy, the engine will be starting up the auxiliary heater, as will other ancillaries that haven't been used over the summer such as screen de-misters etc, all of which puts extra load on the car and reduces MPG...

According to the EU labelling scheme, the difference between an A rated and a G rated tyre for rolling resistance equates to a 7.5% increase in fuel consumption over the lifetime of the tyre.
 

AudA2Numpty

A2OC Donor
Also, has the garage inflated them to the correct tyre pressure - that also makes a difference. In addition, the ambient temperature is starting to drop, which means longer warm-up times, especially bad for our little diesels, and if it's getting really nippy, the engine will be starting up the auxiliary heater, as will other ancillaries that haven't been used over the summer such as screen de-misters etc, all of which puts extra load on the car and reduces MPG...

According to the EU labelling scheme, the difference between an A rated and a G rated tyre for rolling resistance equates to a 7.5% increase in fuel consumption over the lifetime of the tyre.
No they lowered it to the lowest pressure on the Fuel cap which is not where I had it set. Though I checked that first thing and set it to where I had it previously to keep continuity.
The tyres were fitted during last weeks really nice and warm weather, comparable to the previous recorded mpg logs.
To keep the Air con used I would periodically turn it on for about an hour, I would do this once a journey. The air con was used as much as it always has been.

It is only today that the weather has taken a turn for the worst so have not experienced a massive drop in temp from the last two fill ups since the new tyres.

Proof is in the pudding. If I can changes these tyres, drive the same way, even in bad weather and still get better MPG it's deffo the tyres.
 

Artsi_L

Member
Inflate to really high pressure. That will reduce rolling resistance.

One domestic 'hypermiler' uses 4,6bar pressure (i.e. 67PSI) on his 145/80-14 tyres.
On ecomodders forum I read on New York state hypermiler cpompetition winner to have used 50PSI tyre pressures.

Ride harshness will unavoidably increase with these elevated tyre pressures, that's for sure. Please find a happy medium.
 

spike

Well-Known Member
Something to consider. If tyre pressures are significantly higher than manufacturers recommendations and you are involved in a skidding related crash, would insurers have reasonable grounds to throw out your claim.

Cheers Spike
 

dan_b

A2OC Donor
My understanding is no - so long as they weren't in excess of the maximum permitted pressure as shown on the tyre sidewall.


Something to consider. If tyre pressures are significantly higher than manufacturers recommendations and you are involved in a skidding related crash, would insurers have reasonable grounds to throw out your claim.

Cheers Spike
 

dan_b

A2OC Donor
MikeMars is our resident hypermiler - he runs his at 46PSI I believe on the 15" wheel.
I tried running my car at 38PSI on my 16s for a short while and it did give a small, but measureable, increase in MPG over stock pressure, but I found the loss of compliance in the ride too much to bear long-term.
 

Birchall

Dick Chown Award 2016
Something to consider. If tyre pressures are significantly higher than manufacturers recommendations and you are involved in a skidding related crash, would insurers have reasonable grounds to throw out your claim.

Cheers Spike
I couldn't agree more.

Remember you WANT resistance from the tyres when cornering hard and braking hard, so reducing that by over inflating the tyres is NOT recommended.

There is a maximum pressure that you should use and rely on Audi's recommendations because they relate to the car that the tyres are fitted to. Don't think that you can inflate tyres to the maximum suggested by the tyre manufacturer because the weight of the car influences that maximum too.

PLEASE do not inflate your tyres above the Audi recommendations for your A2. What do you want? high MPG, Safety or both. I would opt for both and inflate the tyres to the maximum recommended by AUDI.

67 PSI sounds scary, not just because of the grip problems but also the thought of having a puncture at those pressures is WAY too scary - BANG!!!!

Steve B
 
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AudA2Numpty

A2OC Donor
I'm just about to add my Fuelly log ins as I have not had a chance since Oct.
Well things have certainly been interesting since Oct 6th. I reported a weird rattling sound in the engine compartment to my favourite A2 Garage - A2 Cars - MK.
I took it in as explained in my first post on the 13th only to find I needed two new front tyres. This was an over riding immediate fix, the rattle came second.
After a lot of prodding a poking we thought we'd found the problem to the rattle as well but it needed a little clip to fix a metal brake pipe which seemed to be vibrating.
The other thing i noticed on my next fuel up on the 13th of October was a massive 6 MPG drop. Which is why I reported that the tyres must have been the culprit.
Well whining and winging to Tony at A2 Cars about my mpg made him feel sorry for me and he agreed to change the Falkans I bought with the Dunlops.
Things got in the way and I could't get to Tony's for a couple of months. All the while the rattle was still there and weird electrical things started happening.
Finally one early frosty morning early December the car died while driving. Everything stopped working, no electrics no engine, no steering. Arrrgh.
I called superman Tony at A2 cars and he kindly dropped everything to help and told me to get the car to him stat.
As I looked under the bonnet to see if I could see the issue while I waited for the AA I noticed that the alternator Pulley was missing. OOoooohhhhhhh, that's what the rattling was ooohhhhhhhhh!
I managed to get the car to Tony but had royally messed up the pulley so needed a new alternator and belt. While it was there Tony kindly changed out the Falkans for the Dunlops ordered in October.
The car is now working, touch wood, Tony did an amazing job and service, thank you sir.
My MPG is not back where it was but there is a service due now and I reckon the EGR valve is clogged but I have no cash at the mo to get the next bits done so it will have to wait.
All in all has the new tyres made a difference since they've been on?
Typically I won't know now as the alternator and tyres were done at the same time, there has been a minor 4mg improvement since the alternator and tyres were changed but that could just be the car settling back in to the new components.
Happy new year everyone. :)
 

AudA2Numpty

A2OC Donor
Update.

Well since my last post February to April my MPG has only been (at best) 57mpg average. Not good.
Having a slow puncture was one of the issues that took most of February to detect. Two fills ups early March were awful due to short trips and very cold weather.
On the 31st I had to have two new tyres fitted to my car and in early April the weather picked up so I managed to break the 50mpg barrier and reached a 61mpg.
That was a mid/end of the late March tank of fuel.
Aprils tank with checked tyre pressures and a nice long journey down to Bristol brought it up to 65mpg (probably because the Air cons was accidentally on for 80% of the journey).
Still not where I would expect it to be based on previous performance.

There is an odd rattle which is still undiagnosed but I suspect it is a failing drop link (Conn link) which means a potential tracking issue, this may also explain the uneven tyre wear and 'hunting' on the road it does if I try to drive on a straight road.
Once those are sorted I hope to get better mpg logs.
Fingers crossed.
 
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