A. Was on 255-60-18" General Grabbers getting 24.5 mpg (edited)

B. Now on 255-50-20" Nexen/Pirellis getting 26.9 mpg (edited)

I measured the diameter of each type: A. 77mm and B. 74.5mm. Both sets have @ the same amount of wear.

A. Circumference = 2420mm meaning in 1000 revolutions of the wheel I would travel 1.5125 miles

B. Circumference = 2340mm meaning in 1000 revolutions of the wheel I would travel 1.4625 miles

I should be getting a better mpg from the 18" General Grabbers but get a better mpg from the 20's?

Increased "friction" drops mpg by 2.5 miles per gallon ?

Explain....  .
Experience is the only genuine knowledge, but as time passes, I have forgotten more than I can remember
Volvo V70 P2 2006 2.4 Petrol 170bhp Estate SE
MG Midget Mk1 1962

Previous: L322 Range Rover TDV8 3.6 2008; L322 Range Rover TD6 3.0 2002; P38A Range Rover V8 1999

Last edited by Joe90 on 14th Jun 2012 8:18am. Edited 1 time in total

tyre compounds and tyre pressures do make a difference tim, just be happy your getting a better mpg now..  ... - .- -.




Y. O. L. O.
.

Less rolling resistance (some how) Like the loss of MPG when fitting Off road tyres.

Enjoy the extra Mpg

what are the tyre pressures for each??

I tend to run towing pressures as i like the way they feel and make the car more predictable for me but seem to have a similar kind of affect to the one you've noticed  FFRR MY06 facelift With TDV8 Alloys Zeros/ATR's
Mantec Sump Guard, Rigid Load liner, MY10 BT upgrade.

Tyre compound and tread profile / contact area makes a huge difference to the rolling friction of a tyre - if you get a chance you should watch teh Michelin "Mega Factory" on Discovery - very insightfull..

However, i think you're measurements are out -diameter of new 255x60 R20 should be 81.4cm =2557mm circumference = ~1.59 miles/1000 revs.

Sure you measured you're 20's correctly?

Just do the "recalibrate after tyre change" thing in the Nav menu. Graham

____
Gone: 10MY L320 RR Sport HSE, Ipanema Sand
Gone: 20MY Jaguar iPace HSE, Silicon Silver
Gone: 17MY RR Evoque SE Tech, Loire Blue
Gone: 08MY Discovery 3 XS, Stornoway Grey
Gone: 07MY FFRR TDV8 Vogue, Stornoway Grey

^^^^^^
is that available on older ffrr models?? FFRR MY06 facelift With TDV8 Alloys Zeros/ATR's
Mantec Sump Guard, Rigid Load liner, MY10 BT upgrade.

@ allycraven

I measured all four tyres and averaged and it came out at 745mm (a new one should be 763mm according to the etyres calculator)

But that is by the by. The fact is my FFRR is defying the laws of maths and physics (if you ignore friction and RollingRoadResistance) Just remarkable that less resistance and smaller wheel circumference can create such a diffference in mpg

@ A1GSS (nearly typed Antonia's Dad )

Not seen that one in the menus  .
Experience is the only genuine knowledge, but as time passes, I have forgotten more than I can remember
Volvo V70 P2 2006 2.4 Petrol 170bhp Estate SE
MG Midget Mk1 1962

Previous: L322 Range Rover TDV8 3.6 2008; L322 Range Rover TD6 3.0 2002; P38A Range Rover V8 1999

255-50-18" = 712mm diameter
255-60-20" = 814mm diameter

Giving a difference in diameter of 14.33% according to Etyres calculator.

So the 20" wheels are giving false readings for speedometer and odometer compared to those with the 18" wheels.

Are you sure you have quoted the correct tyre sizes here as the difference makes one of them at least illegal?

I think correct/standard size for FF is 255/50/20?
That's what is on mine and the dealer replaced two tyres prior to purchase, hopefully they are the correct size  
Had it, loved it!
2007 Vogue SE TDV8
Stornoway Grey / Ivory
Climatic Seats, RSE, AFS, Deployable steps, iPod kit.

Joe,

Think you need to check the figures stated gave in your OP... 255 x 60 R20 is definitely not 763mm diameter, as both oldcro and i have stated, that combo would be 814mm dia.

763mm dia would be 255 x 50 R20...

Also, I think ignoring resitance/rolling friction might just negate your comment about defying the laws of maths and physics.....

Are your MPG figures from the OBC or calculated? If from the OBC, then 1 mile = X revolutions (regardless of tyre size) so an OBC "mile" in your grabbers isn't the same distance as a OBC "mile" in your nexens/pirellis.
On the smaller diameter wheels you've actually moved a shorter distance per OBC "mile", hence used less fuel... Between that, better efficiency tread profile and tyre compound I'm actually surprised you haven't seen a greater increase!

What distance are you're averages taken over and have they been exactly the same roads, driving style, time of year, weather etc. as all of these also play a massive part in fuel consumption figures as well.

I definitley see a fair drop in consumption in the summer compared to winter as i'm more likley to be a bit heavier on the right foot when tyres are warmer and roads are dry....although i try to tell myself it's because the engine is more efficient in cold winter air...

the 3 standard sizes for the fullfat are ,

255/60/18
255/55/19
255/50/20


possibly tim [joe90] has had variants of these.. ... - .- -.




Y. O. L. O.
.

Sorry am a plonker - got my 50's and 60's round the wrong way in the original post. Have edited this now.

Anyway fitted some 13" mini wheels last night and am getting 60mpg

Will be trying go faster stripes next  .
Experience is the only genuine knowledge, but as time passes, I have forgotten more than I can remember
Volvo V70 P2 2006 2.4 Petrol 170bhp Estate SE
MG Midget Mk1 1962

Previous: L322 Range Rover TDV8 3.6 2008; L322 Range Rover TD6 3.0 2002; P38A Range Rover V8 1999

pics please re the mini wheels....  ... - .- -.




Y. O. L. O.
.

No problem: