Has changing tyres for for a bigger load index changed torque figures?

[Emmit]

Hi All,

Our Hobby has been uprated from 3500Kgs to 3700Kgs. It was a paper exercise insomuch as neither of the axle loads were altered. (2000Kgs on the back, 1750Kgs on the front.)

The standard tyres 215 x 70 x 15 had a max load of 1030Kgs for each which meant that when fully laden the tyres at the back were operating at very close to their maximum.
No problem in UK but Oh! those french sleeping Policeman. I was in fear and trepidation of a tyre going BANG! when dropping down off every one.

So, in an effort to get some distance between the two weights, (Max allowed and Max on the tyres) I've fitted 225 x 75 x 15 all around. These tyres have a higher load rating.

This has had the effect of making the speedo nearer to the truth. Using a SatNav as a reference it's gone from reading 60mph when doing 56-57mph to reading 60mph when doing 59mph.

I have noticed however that, because the gearing has in effect been altered because for every revolution of the wheel on the road, the engine does not have to go as far, (the circumference of the road wheel is that bit more) the 'van doesn't seem so willing, (fractionally) to climb an incline/hill in top gear.

Is this simply a question of the slight alteration in gearing or has there been at the same time a slight alteration in the actual peak torque produced by the engine. According to Fiat, Max Torque is 300Nm @1800rpm. Has anything there altered as a result of fitting bigger circumference tyres.

Sorry if I'm being thick.

As usual, all answers gratefully received.

 

[2657]

I would think that the max torque is determined by the engine rpm, the wheel size will not affect that but will affect the road speed at which the max torque occurs.

 

[Silver-Fox]

I’m no physics guru but I would have thought the only thing to change torque figures would be internal alterations to the engine or a remap

Happy to be told differently though

 

[Xabia]

Have a look at this website which gives exact details of the effect of the change: https://www.tacomaworld.com/tirecalc?tires=215-70r15-225-70r15

I did the same as you, haven’t noticed any great difference in hill climbing performance but my 3.0 litre Fiat has been remapped by Wow Power to 190 bop, I do tend to change down on long inclines as whilst the motorhome would cope with the climb quite happily in a higher gear I don’t want to risk clutch damage.

 

[pappajohn]

the circumference of the road wheel is that bit more) the 'van doesn't seem so willing, (fractionally) to climb an incline/hill in top gear.

Got it in one.

Put little tiny wheels on and it'll rev its guts out but pull a caravan up the same hill in 5th gear.
Nowt to do with engine torque, it's the torque at the wheels which changed.

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[autorouter]

A 215x70R15 will have a diameter of 15 inches plus 2 x 70% of 215mm = 381 + 301 = 682mm

A 225x75R15 will have a diameter of 15 inches plus 2 x 75% of 225mm = 381 + 337.5 = 718.5mm

Max Torque is 300N-m.

The new tyre has a bigger diameter, the difference is 36.5mm, so if the torque is the same, the driving force at the tyre-road contact surface will be slightly smaller.

Force at will be reduced from 300/0.682 = 439.9N down to 300/0.7185 = 417.5N.
That's a percentage loss of (439.9 - 417.5)/439.9 x 100 = 5%.

I don't know what road speed corresponds to the 1800rpm that gives maximum torque, but I would expect it to be 5% higher with the new tyres. You may think it's the same speed, but in reality what's happened is, your speedometer has changed by 5% as well, so it will seem the same.

As you say, your speedo has changed by about 5%, so that ties in with this calculation.

 

[Emmit]

A 215x70R15 will have a diameter of 15 inches plus 2 x 70% of 215mm = 381 + 301 = 682mm

A 225x75R15 will have a diameter of 15 inches plus 2 x 75% of 225mm = 381 + 337.5 = 718.5mm

Max Torque is 300N-m.

The new tyre has a bigger diameter, the difference is 36.5mm, so if the torque is the same, the driving force at the tyre-road contact surface will be slightly smaller.

Force at will be reduced from 300/0.682 = 439.9N down to 300/0.7185 = 417.5N.
That's a percentage loss of (439.9 - 417.5)/439.9 x 100 = 5%.

I don't know what road speed corresponds to the 1800rpm that gives maximum torque, but I would expect it to be 5% higher with the new tyres. You may think it's the same speed, but in reality what's happened is, your speedometer has changed by 5% as well, so it will seem the same.

As you say, your speedo has changed by about 5%, so that ties in with this calculation.

Thank you very much for taking the time to explain this. I now understand.

 

[Emmit]

Have a look at this website which gives exact details of the effect of the change: https://www.tacomaworld.com/tirecalc?tires=215-70r15-225-70r15

I did the same as you, haven’t noticed any great difference in hill climbing performance but my 3.0 litre Fiat has been remapped by Wow Power to 190 bop, I do tend to change down on long inclines as whilst the motorhome would cope with the climb quite happily in a higher gear I don’t want to risk clutch damage.

With regard to hills I now do the same. Thank you for your response.

 

[Pausim]

My insurance broker makes a point about needing to inform the insurers of any modifications affecting performance. Obviously something like a remap would have to be reporrted but would a minor change to tyres be covered without notice? One would hope that the very small impact of a tyre size change would be considered unimportant.

 

[Hettie's Crew]

@Emmit Interested to know which tyres you chose in 225/75 15 size. I couldn’t find any when I was looking and used 225/70 15.

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[2657]

With regard to hills I now do the same. Thank you for your response.

Best way for climbing hills is to use the gears to keep the engine speed at, or just above, the engine speed that produces maximum torque.

 

[Paul1958]

@Emmit Interested to know which tyres you chose in 225/75 15 size. I couldn’t find any when I was looking and used 225/70 15.

I have just done the same. My Fiat certificate of conformity states 215 and 225/70 15 anyway, so thought no problems with insurance.

 

[Emmit]

I have just done the same. My Fiat certificate of conformity states 215 and 225/70 15 anyway, so thought no problems with insurance.

Sorry Paul, You are quite correct. Old age and decrepitude took over. the tyre size is of course 225x70x15

Regards

 

[Emmit]

My insurance broker makes a point about needing to inform the insurers of any modifications affecting performance. Obviously something like a remap would have to be reporrted but would a minor change to tyres be covered without notice? One would hope that the very small impact of a tyre size change would be considered unimportant.

In these particular circumstances the change in tyres is a positive addition to the vehicle. The tyres put more rubber on the road, increase the load capacity and all within the specification laid down by the manufacturers. They do not increase performance but DO increase safety factors.

 

[tacr2man]

Dont forget that due to uprating you will also most likely now have more load , as well as a drop in torque for same road speed at wheel contact point . The bigger tyres will also have a flywheel effect due to increased weight/diameter.

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[Guigsy]

Torque is the twisting force the engine pushes out. It's not changed. However, by increasing the diameter of the wheels, that turning force is now pushing a longer lever... so the force it pushes is reduced... but then you're going further on each revolution. Bigger and higher rating tyres will also be heavier and spinning mass holds a lot of energy, so you are going to lose a bit of acceleration too.

I too have gone from 215s to 225s to increase my load capacity and so I could run lower pressures to get a better ride. My speedo is also now much more accurate. My mpg has gone down from 35mpg to 32mpg, so about 10% worse. As I'm now going 5% further for every mile the van thinks I'm going, that'll account for half of it. The new tyres are obviously heavier and also 4 season (with a chunky tread), so that was bound to make it a bit worse. But I think most of the rest of the change is that my wheels are now balanced and don't vibrate at 65mph, and combined with the speedo change, I think I'm actually going quite a bit faster than I was before...

 

[Emmit]

Torque is the twisting force the engine pushes out. It's not changed. However, by increasing the diameter of the wheels, that turning force is now pushing a longer lever... so the force it pushes is reduced... but then you're going further on each revolution. Bigger and higher rating tyres will also be heavier and spinning mass holds a lot of energy, so you are going to lose a bit of acceleration too.

I too have gone from 215s to 225s to increase my load capacity and so I could run lower pressures to get a better ride. My speedo is also now much more accurate. My mpg has gone down from 35mpg to 32mpg, so about 10% worse. As I'm now going 5% further for every mile the van thinks I'm going, that'll account for half of it. The new tyres are obviously heavier and also 4 season (with a chunky tread), so that was bound to make it a bit worse. But I think most of the rest of the change is that my wheels are now balanced and don't vibrate at 65mph, and combined with the speedo change, I think I'm actually going quite a bit faster than I was before...

I learnt very quickly that although my perceived speed hadn't changed as looking at the speedo, I was in fact travelling faster and, as a result, I was using more fuel.

I haven't been able to do much driving lately but last Tuesday I did the M5 and A38 and I got into the groove as to how the 'van reacted to my right foot. It's a matter of reeducation.

 

[WESTY66]

I wouldn’t have thought going from 215/70/15 to 225/70/15 would change the speedo reading one jot, or give a better ride. The only thing you’ve changed is the WIDTH of the tyre (which could explain a tiny difference in mpg as it has a slightly bigger contact patch on the ground). The overall circumference of the wheel hasn’t changed (except maybe the new tyre has more tread on).

 

[funflair]

The tyre circumference will be slightly higher actually as the tyre wall height is related to the tyre width via the aspect ratio, that's the 70 bit, so the tyre diameter will will increase by 10x70x2/100 = 14mm.

Martin

 

[DBK]

I wouldn’t have thought going from 215/70/15 to 225/70/15 would change the speedo reading one jot, or give a better ride. The only thing you’ve changed is the WIDTH of the tyre (which could explain a tiny difference in mpg as it has a slightly bigger contact patch on the ground). The overall circumference of the wheel hasn’t changed (except maybe the new tyre has more tread on).

Nope, the diameter is influenced by the aspect ratio, so with the same 70% aspect the diameter increases with the increase in width. See the 15" table on this page.

http://www.angelfire.com/on/geebjen/tires.html

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[WESTY66]

The tyre circumference will be slightly higher actually as the tyre wall height is related to the tyre width via the aspect ratio, that's the 70 bit, so the tyre diameter will will increase by 10x70x2/100 = 14mm.

Martin

Nope, the diameter is influenced by the aspect ratio, so with the same 70% aspect the diameter increases with the increase in width. See the 15" table on this page.

http://www.angelfire.com/on/geebjen/tires.html

Thanks guys, every days a school
day, for the last 30 years I always thought the 70 bit was 70mm tyre wall height

 

[Hettie's Crew]

I wouldn’t have thought going from 215/70/15 to 225/70/15 would change the speedo reading one jot, or give a better ride. The only thing you’ve changed is the WIDTH of the tyre (which could explain a tiny difference in mpg as it has a slightly bigger contact patch on the ground). The overall circumference of the wheel hasn’t changed (except maybe the new tyre has more tread on).

I know this has been answered already but the circumference increases by 43mm, so for every revolution of the wheel you go 43mm further. The load rating is higher, thus allowing lower pressures for the same weight on each axle, significantly improving ride. I haven’t noticed any torque/power/MPG difference on our 2.3 150 but did notice the speedo became more accurate by 1 MPH.
Highly recommend doing this. We did it for axle weight uprating but knowing what we know now, would have done it anyway just for the ride.
https://blobs.continental-tires.com...998ef8b3/download-technical-databook-data.pdf
Page 82 shows the size differences.

 

[Guigsy]

I'm running lower pressures because the tyres are rated with a higher load and Continental produce a very helpful load to pressure lookup book for many of their products. Instead of 65psi, I can use 55psi. I doubt the size would have much direct effect.

 

[bigtwin]

Thanks guys, every days a school
day, for the last 30 years I always thought the 70 bit was 70mm tyre wall height

And your wife has a different view on how big 9” is!

Ian

 

[WESTY66]

And your wife has a different view on how big 9” is!

Ian

She’s never known what 9” is, unless it’s been snowing for a week

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[jezport]

Personally I would have changed the wheels and tyres for 16" and reduced the profile slightly to give the same rolling circumference as the original.

 

[2657]

Personally I would have changed the wheels and tyres for 16" and reduced the profile slightly to give the same rolling circumference as the original.

Apparently not possible without a lot of new mechanicals as they are different PCD for the bolts.

https://www.motorhomefun.co.uk/forum/threads/fiat-wheels-change-from-15inch-to-16-inch.104577/

 

[Emmit]

Thanks for the input on this guys and gals.
It has made a detrimental difference on the ability to climb inclines and overall mpg.

It does have the 128bhp engine and the comments made by Hetties crew re the lack of change in performance has got me thinking.

 

[tacr2man]

Thanks for the input on this guys and gals.
It has made a detrimental difference on the ability to climb inclines and overall mpg.

It does have the 128bhp engine and the comments made by Hetties crew re the lack of change in performance has got me thinking.

Re mpg dont forget , indicated distance covered will be less (under read ) so mpg will drop if used in calculations. Incline capability will decrease only if same gear used. (other than when in 1st when will be not possible to avert ) .