mpg down with new wheels

[RMDrive]

I'd been getting around 57mpg (per the trip computer) since I bought the car however since I changed the wheels about 2 months ago, it has dropped off to around 51. The new wheels are 17" Solids (225/50 I think??). The only reason that I can think of for the change in mpg is the new wheels. Has anyone experienced similar?

Cheers.

[cav]

Could the colder weather have any affect on the mpg?

[allclownsareevi]

I doubt it, I run 17'' alloys and get good mpg figures. Have gotten 63 mpg (picture in my sig I think) on a run but usually get around 52-58mpg on a motorway journey.

Would more likely to be down to tyre pressures, tracking or tread pattern perhaps (if you've got some weird and wondeful tyre treads)? You getting even wear on those tyres?

[Brimma]

It's more likely to be the tyre make than the wheels I would imagine - are they different on the new wheels ?

[magic62]

Might be a stupid question, but what wheels (size and offset) were on before these? And what tyre size? And than off course, tyre pressure?

[RMDrive]

Thanks for the replies. Previously there were 16" 205/55's on her with 2x Michelin and 2x Pirrelli tyres. The new tyres are Avons and I've been keeping a good eye on the pressures (35 front, 32 back). The tracking seems fine as well with all wear being even. It really was a step change in mpg, not a gradual change which is why I suspected the new wheels. I suppose the different thread pattern could be causing it but I wouldn't have expected such a big change.

[Dr Zoidberg]

Could the colder weather have any affect on the mpg?

Absolutely.

For a start your car will take a lot longer to warm up , and you'll also have the heating and lighting on which won't help.

Also in good weather traffic tends to flow better with less random breaking in poor visibilty - each time that happens you need to get up to speed again using more fuel.

Larger wheels won't help either - more weight to get up to speed.

[allclownsareevi]

Depends on what type of mileage you're doing with larger wheels. Theoretically on motorways they should give better economy at sustained speeds, but as Zoidberg says getting up to speed will require more fuel.

Wouldn't have expected tread patterns to have such an effect on mpg figures but perhaps?

[RMDrive]

If it was the cold weather kicking in then I'd have expected a variation in mpg depending on the conditions on that day/week. Instead it's been a very defined change. It's not the end of the world but it's still a little annoying.

[SamsterXX]

Colder weather will have an affect yes, but I can't believe a change of wheels would cause a jump like that. What about a dodgy tank of derv?

I've also changed from 16" alloys to 17" Solids this week, (thanks to Andrew on here) and there's very little variance in consumption if any. 56ish MPG on the way there (65miles) and 56ish on the way back - with a reset

Car's a P-Torqued 1.9TDi - 170 horses, 300 torques

[rotodiesel]

The colder weather and change to winter diesel won't help, but in my experience, fitting Avon tyres as opposed to well developed products from Michelin, Continental or even Pirelli will wreck your economy.

The better mainstream makers are streets ahead of Avon in producing tyres with low hysteresis rubber which gives a low rolling resistance.

rotodiesel.

[cheezemonkhai]

Question I'd be asking is how much do the old wheels weigh each with a tyre and how much does a solid weigh with a tyre.

I bet the solids give a rather large increase in unsprung mass.

[rotodiesel]

Unsprung mass will have only a tiny effect on economy during acceleration, and no effect whatever during steady state running.

Rubber hysteresis is the major component of the rolling resistance losses (as opposed to windage) and varies a great deal according to the tyres fitted.

rotodiesel.

[RMDrive]

The colder weather and change to winter diesel won't help, but in my experience, fitting Avon tyres as opposed to well developed products from Michelin, Continental or even Pirelli will wreck your economy.

The better mainstream makers are streets ahead of Avon in producing tyres with low hysteresis rubber which gives a low rolling resistance.

rotodiesel.

Yeah, I suppose I'll have to see if there's any impact when I change the tyres but that won't be until around the middle of next year. The avons were on the wheels when I got them so I had no say in choosing a tyre. Would there be a ~12% difference due to the rolling resistance? (+5mpg/51mpg)

[sidewaze samm]

Rubber hysteresis is the major component of the rolling resistance losses (as opposed to windage) and varies a great deal according to the tyres fitted.

is that to do with the silica content?

[Supurbia]

I'm not 100% sure but I think you will find it's gear ratio's in the box doing it, your car was setup with one size in mind, and the larger wheels will have their own set of ratios slightly different to yours, I had a similar thing in an Escort I mashed the box up in doing Starsky's a long time ago, it was a 1.3 and I got a s/h box for it, iirc it either came out of a 1.1 or a van, afterwards she wouldn't top 60 mph.

[SamsterXX]

But isn't his rolling circumference exactly the same as before?

[Supurbia]

Not sure tbh.

[allclownsareevi]

But isn't his rolling circumference exactly the same as before?

I don't think it is man. I know when folk fit larger alloys the speedo often misreports by a little. If I remember rightly the speedo runs off of a spindle on the driveshaft so doesn't take into account the circumference of the wheels ... hence the speedo can misreport the speed.

I'm going to sit back smugly and wait for about 15 people to contradict me now.

[magic62]

That is why I asked

There is a small difference between 205/55 16 and 225/45 17. The 17s are a bit bigger. If they are 225/50 as written in the first post, than the circumference will be bigger.

If this is between 5-10% than we already have most of the difference. The difference between worn tyres (16) and newer (17) also gives a difference.

Larger wheel will read less tachospeed, as written before, speed signal is picked up somewhere in the gearbox and only counts these revolutions. If the wheels become bigger, a longer distance will be driven per revolution.

And this means the tripmeter will read less over the same distance as before. Also influences the average economy readings from the BC.

And than there is a difference in frontal area if the wheels are wider and the offset is different (this influences the aerodynamics more than the Cw).

Best way to compare is to make a brim to brim drive and calculate yourself as the readings from the BC can differ.

[Supurbia]

That is why I asked

There is a small difference between 205/55 16 and 225/45 17. The 17s are a bit bigger. If they are 225/50 as written in the first post, than the circumference will be bigger.

If this is between 5-10% than we already have most of the difference. The difference between worn tyres (16) and newer (17) also gives a difference.

Larger wheel will read less tachospeed, as written before, speed signal is picked up somewhere in the gearbox and only counts these revolutions. If the wheels become bigger, a longer distance will be driven per revolution.

And this means the tripmeter will read less over the same distance as before. Also influences the average economy readings from the BC.

And than there is a difference in frontal area if the wheels are wider and the offset is different (this influences the aerodynamics more than the Cw).

Best way to compare is to make a brim to brim drive and calculate yourself as the readings from the BC can differ.

There is a calculator on some tyre fitting sites, I think Black Circles has one, it tells you how far out you are between sizes and calculates the speed differences, might be worth a look, it'll deffo through the OBC out though, brim to brim is the only real way.

Just done it, there isn't much in it, 0.38% resulting in 70mph being 70.27mph now, so remember you are 1/4 of a mph out at 70mph and a bad tempered policeman will tell you so, lol.

Edited 22 December, 2009 by Supurbia

[RMDrive]

There is a calculator on some tyre fitting sites, I think Black Circles has one, it tells you how far out you are between sizes and calculates the speed differences, might be worth a look, it'll deffo through the OBC out though, brim to brim is the only real way.

Just done it, there isn't much in it, 0.38% resulting in 70mph being 70.27mph now, so remember you are 1/4 of a mph out at 70mph and a bad tempered policeman will tell you so, lol.

Yeah AFAIK OEM wheels will maintain a similar circumference when you go up a size. It looks like it might be down to the rubber I'm using. If it is, then it's a massive difference

I'll track the mpg using a brim to brim method in the next few weeks and get an exact figure for the usage.

[magic62]

Just read a test of winter tyres. Difference between the worst winter tyre and a summer tyre was 3% more consumption........

[RMDrive]

Thought I'd give people an update.

Bad weather ... potholes ... damaged alloy :( ... back on steelies ... mpg back up

I've driven just under 1000 miles since the wheels were changed and the trip computer is reading 60.2 mpg versus the low 50's reading when on the solids. I guess it will be interesting to see what happens when I eventually replace the Avons.