Large Tyres - more mpg?

[DeTomato]

So here's a conundrum.

The standard tyre size for the ZZE123R I have is 195/55-R16.

I have put larger 205/55-R16 tyres on. The Radio/CD MPG now shows much better mpg!

Is this because the slightly bigger 205 tyres rotate fractionally and slightly less on the road overall than the standard smaller 195 tyres? And if so, how is the mpg measured from the gauge on the Radio/CD so that these larger tyres give better mpg by at least 10%? 

[Gerg]

Tire Size Comparison

Specification  Sidewall  Radius  Diameter  Circumference  Revs/Mile  Difference
195/55-16         4.2in  12.2in    24.4in         76.8in        825        0.0%
205/55-16         4.4in  12.4in    24.9in         78.2in        811        1.8%

This table is off a US MX5 website. So, assuming it is correct:-

The circumference of the tyres is hardly any different, you have made the car slightly (1.8%) taller geared, so the car should show an under-reading on the speedo and odo, (i.e. the normal speedo exaggeration will be lessened) and the shown mpg would go down very slightly. That's also assuming that the rolling resistance of the sets of tyres were equal.  However, the slightly taller gearing may make the car fractionally more economical in real terms as the engine will rev slightly less for a given speed and gear.  Nothing to get you near 10% difference though.

There is a line of thought that the new tyres should have very slightly lower tyre pressures than the old ones, as the contact patch has been increased in area, but most manufacturers don't bother differentiating for this as it's so small.

Unless I'm missing something here.......

[DeTomato]

Interesting. Good stats above. Thanks for the info.

Here's some more info for what I have recorded.

The 195 tyres were 32 PSI as Standard. However, with fitting and testing for a week, the 205 tyres had edge slight wear at 32 PSI so I set to 35 PSI and this issue has now gone completely. The road grip is better on the 205 tyres and the contact patch should only be slightly higher.

The mph reading is under-reading on the CD/Radio as you mention - when the Odometer speedo shows 70 mph steady on the motorway, the CD/Radio is reading 66 mph steady so a 4mph measured difference with the 205 tyres on.

The mpg is averaging approx. 40 mpg with the 205 tyres on with a daily 25 mile run each way (reset each day to re-calibrate it) using motorway and small roads combined the same every day. This is against the 36 mpg with the 195 tyres that did the same daily run - same make and type of tyres too. This is where I made the 10% mpg saving (or 9% to be more precise).

And if the Revs/mile is down with scaled up 205 tyres at 811, rather than 195 tyres at 825 Revs/mile, it sort makes sense in my non-maths brain that maybe this means more mpg with the 205 tyres? Or is this just wishful thinking?

[JoaoM]

Rolling resistance of the slightly larger tyres should be smaller by 1-2%, that will make a difference specially at lower speed, where the tyre resistance is the dominant drag force. You have inflated the tyres a little higher meaning better mpg. New tyres should get better mpg, even if the brand/model is the same. A little extra from the larger diameter and you can add 6-9%. 

[DeTomato]

I've found the ride is better with the 205 tyres, and it gives slightly more height for those horrible speed bumps that take the bottom of your car out.

I was surprised these slightly bigger tyres give more mpg, but happy with the beneficial effect with petrol getting expensive again.

Also, because the odometer speed reads slightly higher by about 4 mph against the CD/Radio mph measure, then this has an added benefit for not speeding!

[oldcodger]

Might be a better indicator of mpg if you rely on petrol into the tank, though the actual miles covered will be a bit distorted; plus you will need to use that bit more power/gas to drive the larger wheels ..?

Worth checking your actual speed with a mobile app or similar and your odo against a ' measured mile' which you can find on public roads.

Also be aware that you have fitted non standard tyres , so that could negate your insurance if they were really tight during a claim.

As detailed in the week before last Auto Express magazine, it gave a surprising long list of what simple / basic mods you can make that could invalidate your insurance , unless you tell them , which may involve a small extra premium.  Also its not just your insurer who could spot the change, the other party could seize on things like that, which could make you personally liable, a thing many folk do not appreciate or seem to care about, judging by the modded cars you see about.

https://www.autoexpress.co.uk/car-news/107964/car-modification-impact-on-insurance-cover-revealed