Tyres: Speed Rating

[Guest]

Hi

I've replaced the rears with conti eco 5's due to the excellent reviews, still have the original Bridgestone's on the front. The speed rating is one category higher (T) than the standard Bridgestone's (S). I've noticed a 5-10 mpg (avg) drop since fitting.

The tyre performance is outstanding with plenty of grip and a significant increase in stability.

Am I seeing a drop in mpg due to increased tyre weight from a higher speed rated tyre and if so could I offset this by fitting conti's on the front too?

Seems like the loss in mpg is the trade off for an increase in performance.

Any comments or advice is greatly appreciated.

[Devon Aygo]

The speed rating difference is likely to be neglegable however the increased grip and road holding are likely to be the cause of the increased fuel consumption, increased grip = increased friction = increased drag , this along with the winter weather is the most likely cause.

[Guest]

Thanks, it was just strange how I'd gone from 57 avg mpg to a struggled 50 avg mpg immediately after fitting at the weekend. I guess, as you say, the wind conditions and overall poor weather has not helped much either. In any event, the performance improvement is more than worth it.

These tyres are highly recommended!

[s44gtw]

The tyres are not any heavier are they when weighed?

They are Rolling, so would make a difference going up hill if heavier, and compensate going down hill.

Unsprung Weight, but then racing wheels/tyres are light for acceleration,

but then weight gives a gyroscope effect.

(i am not being serious there, but it is a thought.)

The Speed & Load Rating rates them for being safer with heavier cars, more load and stresses when going above what national speed limit. *S to T, 6 mph !!!*

It does not mean a vehicle is faster because they are fitted, a driver needs to press the accelerator more to go faster.

(Or less resistance at the same power, equals more speed.)

'Improved Performance',

i take it that is Improved Grip/Traction. That is Friction on the road, more Friction equals more resistance.

Well if you can use the better Traction and cornering ability to drive faster, & you do,

you may well use more fuel.

ie, pushing the accelerator down further and cornering faster.

Are you sure that even though the tyres have the same size, (Numbers), that they are exactly the same Total Circumference/Total Diameter as the the tyres they replaced.

(when measured side by side.) As you have put them on the back, that is unlikely to have made any difference.

http://blackcircles.com/general/speedrating

??

Have you set your own tyre pressures since the fitter, fitted the tyres, or have you gone by his choice of pressure.?

george

[Guest]

Hi, thanks for the reply.

With the exception of the speed rating. Tyre dimensions are exactly the same (175x65xr15). I haven't weighed them (don't have the facility to) or placed them side by side.

I've set the tyre pressures to 34 psi all round.

The increase in grip and cornering is significant enough to understand a hit on mpg. As you say, increase grip means increase friction.

Will be interesting to see impact when same are fitted to the front...

[2ndGear]

Don't forget we've been having a mild winter and a cold snap can reduce the mpg quite a lot. Just a thought.

JC