If he had 40's on he wasn't getting 28mpg in the first place. The speedo over reads on standard tyres about 8% on new tyres. Increases as they wear. The speedo was over reading nearer 16-17% on 275/40's. Probably around 20% when worn. Showing 40 mph when the car was doing nearer 32/33 mph. So the Odometer was also over reading.
275/ 40 = 110 mm x 2+ 508mm gives the diameter x 3.14 = 2285 mm rolling circumferance.
508mm being a 20" rim
255/50 r 20 is standard. Gives 2395mm rolling circumferance
255/55r 20 is 30mm higher sidewall. So 60mm overall height. They give 2475 mm rolling circumferance.
That's 190mm or nearly 4" difference. Add to that 6mm of extra tread and it's even more.
It will affect the computer readout but not real MPG.
Holding the gears will, of course, affect MPG. Maybe get the gearbox ECU remapped. That will sort it.
The speedo will now be just about spot on.
Re-map the gearbox and you will get accurate MPG and a nicer drive too  2007 Vogue TDV8 in Zermatt Silver with Charcoal

^^ very well explained^^

My thoughts exactly plus it will probably be holding lower gears for longer.

Verbosely explained but not quite correct.

275/40/20 = 110mm sidewall
255/55/20 = 140mm sidewall

An increase of 30mm, not 4" - If you're talking about sidewall height.

If you're talking about circumference then
the 275/40R20 are 90"
the 255/55R20 are 97.5"

so 7.5" not 4"

Yeah my mistake. 100mm is 4" . The Circumferance is increased by 190mm. 200mm being 8". 30mm extra side wall x 2 for diameter = 60mm.
60 x 3.14 = 188.4 mm Add 12mm for the tread depth on new vs old tyres. Then you have 72mm x 3.14= 226 mm or about 9".
I know 9 extra inches in the wife's box would certainly mess it up
Anyway, maths aside this my help.

https://www.4x4australia.com.au/gear/1710/...explained. 2007 Vogue TDV8 in Zermatt Silver with Charcoal

Fantastic info!
I will check the turbo hoses and investigate the gearbox re map, anyone had one done?

Ive had the top two hoses off no apparent damage, they were replaced just before I bought it about 10 months ago, any more I should check?
Bespoke remap starts at £350 locally, I’ve to discuss the finer details with them tomorrow 👍

bespoke remap ? are they going to rolling road it then pal ?

Yep rolling road custom remap, they give it a health check to see what’s what and what’s do-able, then check it all again after, they also offer a stage1 one remap offering an extra 30 Bhp and 100NM of torque for a lesser price but I prefer the idea of the bespoke remap to fit the car, lifetime guarantee on software or any problems related to the remap too

blimey pal , seems a bit OTT on a 4x4 to me but let us know what you think ? obvs going to be a big improvement over standard , are you fitting free flow filters or anything to make the most of the map ??

Guess I’ll see what they say about filters etc, I’m more interested in the gearbox working less as it’s hanging in the gears after the tyre change to be honest,

hmmm , well I guess the rolling radius has grown due to new tyre depth but im surprised its altered how the transmission responds pal ?

@andy, you'll need a rolling road that accommodates both the rear and front axles at the same time .. ... - .- -.




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Have you tried resetting the gearbox ECU. They learn driving habits so a reset on the new tyres maybe worth a try. 2007 Vogue TDV8 in Zermatt Silver with Charcoal

@stan they’re aware it’s a FF if they’ve not thought if that I’m off!!
@grofus how do I carry a reset out?

that would be hilarious if they've given you all the shpiel and they don't actually have a 4WD RR to map it on