Downsizing from 20" - Can Someone Check My Maths?

M

mkmatt

New Member
Hi folks. I bought a T32 Kombi which is lowered 40mm and sat on 20" Velare alloys with 275/40/R20 106Y tyres.
The ride is far too firm and really not a lot of fun on the pot-holed country lanes around where I live. The dogs are in crates in the back and it can't be much fun for them either.
Absent being able to change the suspension I was considering downsizing the wheels to get a bigger aspect ratio tyre on there to try and help the ride.
The MoT is due next month, and the front tyres need replacing anyway so I need to do something.
The speedo appears to read correctly based on GPS so I assume whoever did the wheels and suspension adjusted the speedo because based on my calculation the current combination is not a 1:1 replacement for the standard setup.
So, without having to adjust the speedo what wheel/tyre options are open to me?
It looks to be 19" - 225/55/R19 on a 19 x 8.5-10.5" wheel?
Is that about all I can do ?
Secondly - anyone have a set of 4 wheels and tyres for sale that will work for me? :)
 
Try willtheyfit.com for tyre compatibility check. I went from 20 inch as yours to standard 17 inch alloys - recoded the speedo with OBDeleven.

Have a set of 20 inch wheels with new (<1000 mile) CrossClimate 2 tyres 275/40/20, but that isn't what you are asking :)
 
With any factory setup, the speedo will over read for safety reasons. I increased my tyre size by 3% and the speedo reading was bang on with GPS. This was with a 255-45-19 tyre.
 
Your approach is misguided assuming that you are lowered 40mm on standard shocks. This is why the ride quality is awful. It will still be awful on the 19's. You need to change your shocks to an uprated set to match the resistance of the springs so they can damped their action and operate in a comfortable range.
Also if you do go 19's, then 225/55/19 are totally wrong, while they are the same rolling radius as 275/40/20 they too are the wrong size.

255/45/19's are a better size for the front and 275/40/19 for the rear, assuming you can get 104 load rating for the T32. Staggered rims need staggered tyres. 19's are generally hard to get in correct sizes though that's why a lot go for 20's. I'd keep the wheels and change the front tyres to 265/40/20 and change the shocks.
 
BenT5 said:
Your approach is misguided assuming that you are lowered 40mm on standard shocks. This is why the ride quality is awful. It will still be awful on the 19's. You need to change your shocks to an uprated set to match the resistance of the springs so they can damped their action and operate in a comfortable range.
Also if you do go 19's, then 225/55/19 are totally wrong, while they are the same rolling radius as 275/40/20 they too are the wrong size.

255/45/19's are a better size for the front and 275/40/19 for the rear, assuming you can get 104 load rating for the T32. Staggered rims need staggered tyres. 19's are generally hard to get in correct sizes though that's why a lot go for 20's. I'd keep the wheels and change the front tyres to 265/40/20 and change the shocks.
Click to expand...
Couldn't agree more.
 
It's usually the wheel size that gets the blame for a poor ride, but with the right suspension setup and appropriately sized tyres you can get 20s to ride comfortably.
 
BenT5 said:
Your approach is misguided assuming that you are lowered 40mm on standard shocks. This is why the ride quality is awful. It will still be awful on the 19's. You need to change your shocks to an uprated set to match the resistance of the springs so they can damped their action and operate in a comfortable range.
Also if you do go 19's, then 225/55/19 are totally wrong, while they are the same rolling radius as 275/40/20 they too are the wrong size.

255/45/19's are a better size for the front and 275/40/19 for the rear, assuming you can get 104 load rating for the T32. Staggered rims need staggered tyres. 19's are generally hard to get in correct sizes though that's why a lot go for 20's. I'd keep the wheels and change the front tyres to 265/40/20 and change the shocks.
Click to expand...

Thanks Ben. I realise the shocks probably need looking at, I was just trying to do a quick intermediate step so it can go for an MoT as the front tyres won't pass. Think I'll just stick the same size tyres back on the fronts and then get the suspension looked at next month.
 
mkmatt said:
Thanks Ben. I realise the shocks probably need looking at, I was just trying to do a quick intermediate step so it can go for an MoT as the front tyres won't pass. Think I'll just stick the same size tyres back on the fronts and then get the suspension looked at next month.
Click to expand...
If the fronts are 8.5" or 9" wide you'd be better off with 265/40/20 like I mentioned above
 
mkmatt said:
In what way would 265/40/20 make me better off than the existing 275/40/20?
Click to expand...
In 2 ways, 275 is too wide for a 8.5/9 rim especially on the front, so will feel a little wobbly and 265/40 is closer to the original rolling radius so keeps gearing more normal and will be better acceleration and up hills. 265/40/20 are available in 104 load rating for the T32, other wise I'd suggest 255/40 101 if you had a T28/T30
 
@BenT5 ,whilst not wanting to come across pedantic, the minimum load rating for a T32 is 103, not 104, it could make a difference to the OP. :thumbsup:
 
BenT5 said:
In 2 ways, 275 is too wide for a 8.5/9 rim especially on the front, so will feel a little wobbly and 265/40 is closer to the original rolling radius so keeps gearing more normal and will be better acceleration and up hills. 265/40/20 are available in 104 load rating for the T32, other wise I'd suggest 255/40 101 if you had a T28/T30
Click to expand...

Less chance of rubbing on full lock too.
 
Ali-G said:
@BenT5 ,whilst not wanting to come across pedantic, the minimum load rating for a T32 is 103, not 104, it could make a difference to the OP. :thumbsup:
Click to expand...
You can be as pedantic as you like Ali-G as I was merely referring to the more commonly available load rating of tyres rather than the load rating of the T32 itself as I'm not aware of too many 103 load rating tyres in a 19" or indeed a 20" tyre in a T32 size that we discussed ;)
 
BenT5 said:
Your approach is misguided assuming that you are lowered 40mm on standard shocks. This is why the ride quality is awful. It will still be awful on the 19's. You need to change your shocks to an uprated set to match the resistance of the springs so they can damped their action and operate in a comfortable range.
Also if you do go 19's, then 225/55/19 are totally wrong, while they are the same rolling radius as 275/40/20 they too are the wrong size.

255/45/19's are a better size for the front and 275/40/19 for the rear, assuming you can get 104 load rating for the T32. Staggered rims need staggered tyres. 19's are generally hard to get in correct sizes though that's why a lot go for 20's. I'd keep the wheels and change the front tyres to 265/40/20 and change the shocks.
Click to expand...
Looks like I have a new salesman in the making. Good to see better advice being given out , nice one
 
BenT5 said:
In 2 ways, 275 is too wide for a 8.5/9 rim especially on the front, so will feel a little wobbly and 265/40 is closer to the original rolling radius so keeps gearing more normal and will be better acceleration and up hills. 265/40/20 are available in 104 load rating for the T32, other wise I'd suggest 255/40 101 if you had a T28/T30
Click to expand...
Which size is best for a 20” T30 to keep the gearing and rolling radius as close to original?
Currently I have 245/45/20s
 
JonriceT6 said:
Which size is best for a 20” T30 to keep the gearing and rolling radius as close to original?
Currently I have 245/45/20s
Click to expand...
245 45 20 is my go to size to get the right look and comfort from the wheel /tyre , It is off course bigger in diameter than standard , but no touching . The speedo on aT6 is programmable and can be recalibrated to read accurately with these wheel / tyre combos. The 40 mm or so bigger diameter does affect a few things like acceleration and economy. The increase in fuel usage on the dash isn’t always what it seems as the wheel is turning less than a standard wheel so the figures the MFD throw out are under stated. iE. It looks like the van is using a
more fuel , in reality it’s the extra power needed to turn the bigger wheel that uses a little more fuel
 
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