Audi TT Forum banner
1 - 12 of 12 Posts

·
Registered
2008 TT Roadster
Joined
·
42 Posts
Discussion Starter · #1 ·
I recently swapped from 18" wheels on 245/45r18 tyres to 19" with 255/35r19. According to one of those tyre/rolling radius calculators, this results in a slightly larger diameter (about 3mm).

Indicated speed on the speedo vs GPS is unchanged (sampled at indicated 30mph and indicated 60mph).

However, there's a big difference in fuel economy - over 10%. I typically get ~42mpg on a steady run up the M1 from Derby to Leeds and it's now down to 36ish with no other variables (well, the tyres themselves I guess should be considered). Granted, I'm only gauging the trip computer for MPG which will never be as accurate as actually measuring fuel used.

I'm not that bothered about a few MPG, but I would have only expected a very marginal increase in consumption if at all. Does anyone else's experience compare? It was suggested to me that I need to "tell" the car about the revised wheel and tyre configuration, but wasn't convinced by that (or even how to do so).
 

·
Registered
Joined
·
3 Posts
There are variables to think of, as the mpg calculation is worked out on the number of revolutions the wheels make. so even if it is only 3mm In diameter, which of course it isn’t it’s 25.4mm. that is only new tyre to new tyre, wear reduces the diameter as well, so I am assuming you are on new tyres now, so there is likely to be a bigger error. A 245/45/r18 will be 746 rotations per mile the bigger rim size will be 784 rotations per mile so that’s 38 revolutions more to achieve a mile, multiplied by every mile you do. That is why you will have an error. Your Speedo should also be out, unless you adjust the size.
 

·
Premium Member
Joined
·
6,139 Posts
There are variables to think of, as the mpg calculation is worked out on the number of revolutions the wheels make. so even if it is only 3mm In diameter, which of course it isn’t it’s 25.4mm. that is only new tyre to new tyre, wear reduces the diameter as well, so I am assuming you are on new tyres now, so there is likely to be a bigger error. A 245/45/r18 will be 746 rotations per mile the bigger rim size will be 784 rotations per mile so that’s 38 revolutions more to achieve a mile, multiplied by every mile you do. That is why you will have an error. Your Speedo should also be out, unless you adjust the size.
Basically using more fuel to achieve the same distance.
 

·
Registered
2008 TT Roadster
Joined
·
42 Posts
Discussion Starter · #4 ·
Yep, completely. Fair comments.

I didn’t bother doing the maths, but figured the variance would be less, particularly as my speedo hasn’t changed compared to GPS (indicated 70mph is 67 on GPS, as it was previously)

For what it’s worth, these tyres came with the wheels as a used set.
 

·
Administrator
2001 Amulet Red 225 coupe.Owned from new as well as a Nurburg VXR & an XR3, all from new.
Joined
·
96,573 Posts
Hi, Everything being equal at 60mph speedo will show 62 because wheels will be rotating 19 more times per mile.
Hoggy. :)
 

·
Registered
Joined
·
3 Posts
Speedo accuracy is a bit of a hole as well, legally they cannot sho less than the actual speed, but can show up to 110% + 6.25mph, I know it’s doesn’t account for the big difference, but as Tesco says, every little helps (or doesn’t in this case). I’m not certain if you can change the wheel size in the computer to fix the issue (not got that far) but a couple of my bikes have speedo’s that need to be adjusted for size to ensure the various readouts work correctly.
 

·
Registered
Joined
·
2 Posts
I recently swapped from 18" wheels on 245/45r18 tyres to 19" with 255/35r19. According to one of those tyre/rolling radius calculators, this results in a slightly larger diameter (about 3mm).

Indicated speed on the speedo vs GPS is unchanged (sampled at indicated 30mph and indicated 60mph).

However, there's a big difference in fuel economy - over 10%. I typically get ~42mpg on a steady run up the M1 from Derby to Leeds and it's now down to 36ish with no other variables (well, the tyres themselves I guess should be considered). Granted, I'm only gauging the trip computer for MPG which will never be as accurate as actually measuring fuel used.

I'm not that bothered about a few MPG, but I would have only expected a very marginal increase in consumption if at all. Does anyone else's experience compare? It was suggested to me that I need to "tell" the car about the revised wheel and tyre configuration, but wasn't convinced by that (or even how to do so).
blimey 42 mpg , whats wrong with my 3.2 dsg ???
 

·
Registered
2006 TT Mk2 3.2 VR6
Joined
·
609 Posts
I don't know what the % difference is, but different tyre compounds give different fuel economy, and they're rated A B C D when you buy them. Actulaly, I don't think I've ever seen an A-rated one advertised.
 

·
Registered
Joined
·
266 Posts
Slightly off-topic, I just noticed the wheels on this 2022 R8 RWD. I got almost the identical design last year 17" from UK brand Romac. It's called "Venom". I went with grey with a polished face and they still look like new a year on.

Carwow video

Wheel Tire Car Vehicle Automotive tire
 

·
Registered
2008 TT Roadster
Joined
·
42 Posts
Discussion Starter · #11 ·
Slightly off-topic, I just noticed the wheels on this 2022 R8 RWD. I got almost the identical design last year 17" from UK brand Romac. It's called "Venom". I went with grey with a polished face and they still look like new a year on.

Carwow video

View attachment 486368
Nice looking wheels, in both cases. I really enjoy Mat Watson's content too.
 

·
Registered
Joined
·
968 Posts
The rolling radius depends on the tyre size, the inflation pressure, the weight of the car and the stiffness of the tyre side wall. Your new tyres are close enough to the original not to make any significant difference: in theory at least. Don't forget that a new tyre is about 12mm bigger in diameter than an 'on the limit' one.

Tyres do vary a bit in rolling resistance i.e. the force required to continually 'bend' the tyre into the shape of the contact patch. Lower profile tyres are likely to be worse because they tend to have stiffer sidewalls. 10% is more than I would expect. Was the wind direction different to normal? Did you have the AC on on a hot day? Higher inflation pressure should reduce the rolling resistance but go to far and you get uneven wear (down the middle) as well as reduced grip.
 
1 - 12 of 12 Posts
Top