If the gearing is right then the mpg should be the same. Mpg used to be worse on a lot of early megas that were not factory built. Tyre wear is definately worse though (unless the rubber compound is different)
Another reason some companies use smaller wheels is speed, take a unit for its tacho calbration with small wheels, 56mph. Take em off put normal sized wheels on 60mph. 
Unfortunately noy yhe outfit I drive for 
dowahdiddyman:
Another reason some companies use smaller wheels is speed, take a unit for its tacho calbration with small wheels, 56mph. Takeem off put normal sized wheels on 60mph.
Thats ok doing that but the calibration sticker for the speed limiter has Tyre size on it. I know of someone who did that, ordered a new truck with 295 tyres, stuck 315 tyres on, put speed up to 58/60mph still registering 56mph. Driver got stopped in France and the Gendarmes noticed this, whacking great fine and truck had to go in and get recalibrated. I got stopped by VOSA in Dover last year, one of the first things they looked at was the tyres and calibration sticker.
Also if you put bigger tyres on after calibrating on little wheels it may then put you overheight. A few cms may not sound a lot, but try telling that to the swiss 
Coffeeholic:
It says ERF not RAF:
philgor:
europe has a max running height of 4mts where ours is 4.8mts, and its heavly inforcedI thought the UK didn’t have a max height limit ■■?
It doesn’t, yet.
maybe soon
4m height limit on trailers muted
New vehicle and trailer heights could be limited to 4 metres under new proposals being developed in Brussels, according to the Society of Motor Manufacturers and Traders. The obvious threat to much of the UK transport network would be to limit high-cube trailers.
The SMMT’s commercial vehicle affairs manager, Robin Dickeson, says that if this makes it onto the statute book we should “expect anguish”. Details of the plan will be published shortly as part of European Whole Vehicle Type Approval proposals, he says.
We are aware that a number of proposals are being considered in Brussels and we are keen to establish the likelihood of their being adopted and the extent to which there would be derogations in the UK.
Denis F:
Coffeeholic:
It says ERF not RAF:
philgor:
europe has a max running height of 4mts where ours is 4.8mts, and its heavly inforcedI thought the UK didn’t have a max height limit ■■?
It doesn’t, yet.
maybe soon
4m height limit on trailers muted
New vehicle and trailer heights could be limited to 4 metres under new proposals being developed in Brussels, according to the Society of Motor Manufacturers and Traders. The obvious threat to much of the UK transport network would be to limit high-cube trailers.
The SMMT’s commercial vehicle affairs manager, Robin Dickeson, says that if this makes it onto the statute book we should “expect anguish”. Details of the plan will be published shortly as part of European Whole Vehicle Type Approval proposals, he says.
We are aware that a number of proposals are being considered in Brussels and we are keen to establish the likelihood of their being adopted and the extent to which there would be derogations in the UK.
That’s why I used the word yet. 