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DPs into Fuel useless at?

When spending Design Points in Fuel Economy become useless in Elite tier?

No in no refuelling leagues it is an important attribute 

I meant if there's a value over which doesn't give any advantage like I red about Tyre Economy that over 63 point is nonesense.

Not in no refuelling races,  but you need to consider it holistically taking into account how you mix and match it with Acceleration, Handling, Braking and Downforce.  Also, you need to consider where everyone else is researching and use this to your advantage to gain extra dp pts.  FE is not the most important attribute, but ensuring you are at least equal to your strongest competitor and hopefully further ahead will make you much faster in no refuelling races.  It’s all about finding the little edges against your opponents which all add up to a faster car.  The game used to based around the ‘big four’, but now with the tyre changes and no refuelling option both TE and FE are important attributes.  When I race in no refuelling I look to develop FE early and in my current league no refuelling league with 1 race to go it is my third most researched attribute behind Accel and Handling, whereas in my refuelling league it is the 6th most researched.

Car development is very important in Elite especially if you are in a strong competitive league and one of the skills is using your experience to maximise how many dp pts you can achieve each race.

FE isn't useless and there isn't a magic figure that it doesn't improve past. However, it doesn't improve linearly it plateaus out. So going from 10 to 11 will have a much bigger improvement to your fuel efficiency than 186 to 187. Yes they are both investments of the same amount, one design point, but one had a significantly larger impact.