Chances are the 17s wouldn't fit over the brake callipers on your ER, especially the front ones.
That's what I need to determine by putting our Braumach spare on the SE 51 and then putting the 17" wheel onto the ER 77. I was hoping someone might have done this already to save me the trouble
If I were you I'd check the overall diameter of the tyres on your ER vs the tyres on the Excite ... you may find that there's not much in it, so little or nothing to be gained by swapping.
I don't believe it's the diameter of the tyre as such that determines its efficiency: -
Question
are thinner tyres more energy efficient?
AI Overview
Yes, thinner tires are generally more energy efficient than thicker tires because they have less rolling resistance.
Explanation
- Rolling resistance: Thinner tires have a smaller contact area with the road, which means they require less force to move.
- Fuel consumption: Thinner tires have lower fuel consumption levels.
- Road noise and vibrations: Thinner tires have a reduced surface contact area, which means they produce less road noise and vibrations.
- Winter traction: Narrow tires offer better traction in winter conditions.
Other factors that can affect fuel efficiency include:
- Tire pressure: The pressure of your tires can affect your fuel consumption.
- Tire tread: Fuel efficient tires have tread compounds that reduce drag and friction.
- Tire shape: Fuel efficient tires are more streamlined and aerodynamic.
- Tire stiffness: Fuel efficient tires are stiffer and rigid to retain their shape longer.
So... I'm wondering if I can get a meaningful increase in range by using thinner tyres such as the 215/50 R17's on our Excite 51 instead of the 235/45 R18's currently installed on our Essence 77.
If I could get a reliable 450km vs 420km @ 115kmh on the open road that would be worthwhile for my long distance drives between South Australia and New South Wales.
If the 51's tyres will work on the 77 it will be easy to test.
If they don't fit, then I won't bother investigating any more
I think the 18” are more about looking a bit fancier. Keep in mind that the overall diameter is different between the 18” and 17” so potentially it could throw out your speedometer. Although we are also curious to see if the car auto corrects as it seems to be using gps in its calculations of speed.
I looked into tyre & rim scenarios quite a lot when trying to source a usable Space Saver tyre for our two MG4s, before giving up and just getting the Braumach!
There's a bunch of calculators that make it simple to determine what wheel combos "work".
Here's one: -
Tire size comparison the easy way. Use our tire size comparison calculator to compare diameter, width, sidewall, circumference and revolutions per mile.
tiresize.com
Using this you can see that the 215/50 R17 are only a little "smaller" than the 235/45 R18 and would give an overestimation of the speed by only 2mph @ 70mph. Not enough to really cause angst.
That being said 205/55 R17 would be even better still as it only overestimates by 1 mph @ 70mph
If I do find I can run the 17" rims and that there is a material range benefit then when the tyres next needed to be replaced I'd see if they could be fitted (I'm pretty sure they can) to the rims and go that way.
People tend to find the smaller rims more comfortable because the tyres have a taller sidewall and therefore flex a bit more, but I don’t know if that is a bit of placebo effect.
That is of minor interest to me but maybe? I'd be giving up cornering ability and some braking performance for the range benefit so a bit of added comfort wouldn't be a drawback...
I don’t know if it will really help with range, although some people are all about unsprung weight and what not, I had a search but can’t seem to find the thread anymore.
I'm hoping I can provide a real world test...
I'll probably give it a go when I take the wheels off to make it easier to fit my (Aliexpress) mudguards.
But then I wouldn't have had an excuse to buy more tools 
