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Newbie, new tires, a lot less acceleration


APM
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Hey guys,

My dad just bought an old 2002 Focus 1.8 TDCi 101hp, it had noname old tires with barely any threads remaining, so he asked me to order new tires, I got him Dunlop Sport Blueresponse 91H 195/65/15 (B/A fuel/wet) tires and he says the car got simply too slow, doesn't accelerate.Before he had 195/60/15 front, 195/65/15 rear tires, now the dunlops are all 195/65/15 (my dad asked for this size). Is it possible that the frontal 5mm height difference slows down the car that much ? I asked my brother to test if the car really slowed down and he says the same thing ( I don't drive, don't even have drivers license but I trust my brothers word ). In the manual of the car it says use 195/60/15 or 185/65/14. I can't return the tires because they are not "new" anymore (drove maybe 50km or less with it), I just want to know, is that 5mm height difference doing this or the fact that it's new tires with full thread ?

I'm clueless about cars but it seems so is my dad. Should I sell them all and get 195/60, will it really change that much ? Or get a smaller 14 wheel with 185/65 ? No idea what to do.

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I can't see how it would make that much difference. I have done some calculations and I reckon using 195/65/15 instead of 195/60/15 ups the gearing by 3.1% . This means that if at a certain speed you are at say 2000 rpm before, at that same speed you would now be at approx 1940 rpm.  In thoery the acceleration would be slightly poorer but I would have thought you would not feel it much. But you might get better fuel consumption if you were doing a long motorway run in top gear.

I reckon the difference in height between 195/60 and 195/65 is 9.75mm.   5% difference in aspect ratio - 5% of 195 is 9.75.

Actually what you say about full tread compared to bald tyres can make a difference. add say 7mm tread and that would increase the 3.1% difference in gearing to 5.3% difference. perhaps that would be more noticable. I still not sure how dramatic it would feel. 

 "I asked my brother to test if the car really slowed down and he says...", but the idea had been put in his head and you can't now switch the tyres round. If you did a blind test on the old tyres and new tyres would he still have said that.

 

http://www.tyresizecalculator.com/tyre-wheel-calculators/speedo-error-calculator

185/65/14 is about 3% difference from 195/60/15 on above link. My own calcs says 3.4% .  so 185/65/14 will give you better acceleration.

I don't know why your dad asked for 195/65 if handbook says 195/60 

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12 minutes ago, isetta said:

I don't know why your dad asked for 195/65 if handbook says 195/60 

Because he's dumb and I'm dumb too for trusting him instead of searching the cars manual.. so new tires with 195/65 would feel slower than old tires with 195/60 because with the added thread it's still taller. He asked me for a tire with good grip in wet, got him A rated wet grip, can that "slow" down the car too ? I'm beginning to feel like he never in his life used new tires with full tread, just fully worn out, second hand, retreaded, no name crap that is barely legal to drive and now that he has grip, it's too slow for him,  or can the engine be too weak for these tires ?

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As, by your own admission, you're "clueless about cars", you should have put the correct sized 195/60/15 tyres on the car.

195/60/15                                       195/65/15

Rolling Radius: 193.21 cm            Rolling Radius: 199.33 cm

Total Diameter: 61.50 cm            Total Diameter: 63.45 cm

Sidewall Height: 11.70 cm           Sidewall Height: 12.68 cm

Note: Changing your wheels/tyres will affect your speedometer reading!

With this combination your speed will be 3.2% greater than indicated

• When your speedo reads 30mph you will actually be doing 31.0mph

• When your speedo reads 60mph you will actually be doing 61.9mph

It is not a 5mm difference in height, the 60 and 65 refer to the % of tyre wall height, compared to the tyre width (195 mm)

So, the car height difference, from the wheel centre is TD: 63.45 cm - TD: 61.50 cm = 1.95 cm / 2 = 0.975 cm

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I see, by the way, I did not put the tires on the car, I did not drive it, all I did was research which tires have good rating and test scores and order them. His (my dads) problem is not the speadometer reading, it's the car actually being slower from 0 to 60. Anyway, I'll tell him to try to sell these or exchange them for 195/60 but if his problem is the grip itself, then it doesn't matter what he puts on, he won't like it, unless it's close to slick. There more grip you have the slower the car becomes, is this correct ? (with this 100hp engine, not a monster that can handle any tire size). Also the better wet grip rating, the slower the car ? (again with this engine)

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i don't agree that tyres that grip better make the car slower.

And no tyre just grips better than another in all road conditions. Some tyres will grip better than others in certain conditions. eg. rain - because the tread pattern channels the water away better,  or snow because of the shape of the tread and softer rubber which means it stays flexible at lower temperatures, or in the dry - look at how little tread there is on a racing tyre - if any at all. the more rubber in contact with a dry road means more grip.

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5 minutes ago, isetta said:

i don't agree that tyres that grip better make the car slower.

And no tyre just grips better than another in all road conditions. Some tyres will grip better than others in certain conditions. eg. rain - because the tread pattern channels the water away better,  or snow because of the shape of the tread and softer rubber which means it stays flexible at lower temperatures, or in the dry - look at how little tread there is on a racing tyre - if any at all. the more rubber in contact with a dry road means more grip.

That's exactly what I thought, F1 cars have slick tires only (for dry racing), so the less thread the better acceleration, but worse wet grip ? (this seems logical)

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4 minutes ago, MrRedman said:

Someone is on the wind up here

never heard such rubbish 😂😂😂

I don't get this, what do you mean ?

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8 hours ago, APM said:

he says the car got simply too slow, doesn't accelerate.Before he had 195/60/15 front, 195/65/15 rear tires, now the dunlops are all 195/65/15 (my dad asked for this size). Is it possible that the frontal 5mm height difference slows down the car that much ?

No!

I do not believe that the 3% or so change in gearing would be noticeable without a stopwatch & careful A-B testing. The changes in road noise, and in the feel of the car, will be much greater, and wreck any subjective judgement.

The Focus has quite a lot of road noise, so this would be significant.

The other option is that something has changed a bit in the engine, at the same time, nothing to do with the tire size. On an older engine like that, you could try a fill of premium fuel with cleaning additives, though I have never noticed any change from this. Or try a fuel additive claimed to clean injectors. They sometimes do work, if the injectors are getting a bit gummed or sooted up. A new air filter (a decent one) can also help.

Turbocharged, common rail diesels are complex beasts, and can be a bit moody if not in tip top condition. One time they may feel great, next time a bit sluggish.

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9 hours ago, Tdci-Peter said:

Turbocharged, common rail diesels are complex beasts, and can be a bit moody if not in tip top condition. One time they may feel great, next time a bit sluggish

...even affected by temperature and humidity!

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9 hours ago, Tdci-Peter said:

No!

I do not believe that the 3% or so change in gearing would be noticeable without a stopwatch & careful A-B testing. The changes in road noise, and in the feel of the car, will be much greater, and wreck any subjective judgement.

The Focus has quite a lot of road noise, so this would be significant.

The other option is that something has changed a bit in the engine, at the same time, nothing to do with the tire size. On an older engine like that, you could try a fill of premium fuel with cleaning additives, though I have never noticed any change from this. Or try a fuel additive claimed to clean injectors. They sometimes do work, if the injectors are getting a bit gummed or sooted up. A new air filter (a decent one) can also help.

Turbocharged, common rail diesels are complex beasts, and can be a bit moody if not in tip top condition. One time they may feel great, next time a bit sluggish.

This is what I told him at first, that it must be in his head but then my brother tried the new tires, the next day they changed back to the old tires and they both said it's faster... and I know my brother doesn't BS me. Could it be that the new tire is wet A rated and that affects acceleration somehow ? But yeah... first time he told me this, I told him to get stopwatch 0-100 test and do brake distance test with both tires to see if there's a difference or it's all in his head only.

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1 hour ago, APM said:

Could it be that the new tire is wet A rated and that affects acceleration somehow ?

When it comes to power (acceleration & speed), the important rating is the rolling resistance one. A G rated tyre would absorb more power than an A rated (rolling resistance / ecomony rating). Thus less power is available to accelerate.

See: http://www.blackcircles.com/general/tyre-labelling/tyre-label

But again, this would be a small difference.

During acceleration, rolling radius need have no impact at all, you just stay in lower gears a tiny bit longer to compensate. It is just a change in gearing effectively. No power is lost.

I still maintain it is road noise that is the cause. That has a very strong impact on the perception of speed.

I am not sure how they changed back to the old tyres! New set of wheels?, de-mount & re-mount themselves (hard unless one of them is a mechanic with the right kit)?, or pay someone to do it?.

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5 minutes ago, Tdci-Peter said:

When it comes to power (acceleration & speed), the important rating is the rolling resistance one. A G rated tyre would absorb more power than an A rated (rolling resistance / ecomony rating). Thus less power is available to accelerate.

See: http://www.blackcircles.com/general/tyre-labelling/tyre-label

But again, this would be a small difference.

During acceleration, rolling radius need have no impact at all, you just stay in lower gears a tiny bit longer to compensate. It is just a change in gearing effectively. No power is lost.

I still maintain it is road noise that is the cause. That has a very powerful impact on the perception of speed.

I am not sure how they changed back to the old tyres! New set of wheels?, re-mount & mount themselves (hard unless one of them is a mechanic with the right kit)?, or pay someone to do it?.

Guess they paid a mechanic to change back to the old tires (same wheel). The new tires are B (fuel), A (wet) rated, it's as good as it gets.. it's just insane that he thinks the new tires are shait, now he's trying to sell them and buy used 195/60..

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  • 4 months later...

At the end of the day it's a 15 year old diesel Focus! If speed and acceleration were important you/your dad should have bought a new RS/ST/Mustang etc etc

 

Personally I would be happy knowing that the 4 tyres were all new and good quality with a good wet rating :wink:

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On 23/07/2017 at 11:23 AM, Tdci-Peter said:

When it comes to power (acceleration & speed), the important rating is the rolling resistance one. A G rated tyre would absorb more power than an A rated (rolling resistance / ecomony rating). Thus less power is available to accelerate.

See: http://www.blackcircles.com/general/tyre-labelling/tyre-label

But again, this would be a small difference.

During acceleration, rolling radius need have no impact at all, you just stay in lower gears a tiny bit longer to compensate. It is just a change in gearing effectively. No power is lost.

I still maintain it is road noise that is the cause. That has a very strong impact on the perception of speed.

I am not sure how they changed back to the old tyres! New set of wheels?, de-mount & re-mount themselves (hard unless one of them is a mechanic with the right kit)?, or pay someone to do it?.

Rolling radius will impact acceleration. It won't affect power but it will acceleration, 0-60 times etc That's why people change tyres size/diff ratios for drag racing etc but...... That's timed by a ticking clock not guesswork :wink:

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