[bigwheelz]

For my Alto, I upgraded from stock MRF to Michelin 155/70 R12 two months ago. Before the upgrade, my full tank to tank used to get me 500Kms per tank on average.
After the upgrade, I have done a full tank to tank five times and observed that the average kms per tank is 560 Kms.
Other conditions like driving conditions, roads, driver remains constant.

Does it do so much difference without a full upgrade? I am still wondering.
Looking for answers.

[vigsom]

My experience has been no more than half a km/liter after replacing with the same make/size.

I guess Michelins are doing the trick - but a 1.7km/l difference is hard to believe(before 500/35=14.3kmpl ; after 560/35=16kmpl ; delta=1.7kmpl)

[devarshi84]

You have changed the tyre profile. That has led to your range increasing to 560. You still cover 500kms only.

Tyre pressure also plays a good role in affecting FE. I usually go 2 psi higher for city and 3 psi higher for highway driving. It gets me better figures.

[Chewbacca]

Originally Posted by bigwheelz

Does it do so much difference without a full upgrade? I am still wondering.
Looking for answers.

Not sure what you meant by "without a full upgrade"
Alto's tank capacity is 35 litres but you can squeeze in 39 litres. How much does it take when you do a tank full? were the old ones tubeless? and how long did they last (in kms)?

My family has 2 Alto's, the older 2006 model came with tubed tyres and I recently put tubeless. On stock tubed tyres, in 2009 it consistantly gave 18 -20 km/l. But that was my dad driving mostly outside the city limts and he never exceeds 80 kmph and never uses the a/c.

After 38000 kms / 4 years, I put tubeless and checked the milage thrice, got 15-16 km/l. Tubeless should have increased the milage marginally but the car is now 5 years old and I am using it more often, not as sedately as my dad

[dhanushs]

Originally Posted by bigwheelz

Does it do so much difference without a full upgrade? I am still wondering..

In fact if the stock tyres are 145/80 , then a 155 upgrade should technically reduce the fuel efficiency.

In your case, whats happening might be, the reduced tyre dia causes a speedo error. Somewhere around ~2-3%. Add to that, the reduced weight of the Michlien's and the reduced weight of the tubeless tyres when compared to the stock MRF's tubed, along with a probable increase in air pressure does the trick for you.

Please note that, 560kms in odo means, you are covering only ~540kms (speedo error). The 40 km range increase can be attributed to the weightlessness and maybe, increased tyre pressure.

[MAS]

What is the OE tyre size of an Alto? Increase in FE by 14% is not very realistic. Guess your speedo reading took a beating due to change in overall diameter of the wheels.

[Durango Dude]

A real tyre 'upgrade' should see the FE go southward and handling go northward, due to wider contact patch and also the steering must get a wee bit heavier. I can't comment on the FE you are getting, but that's my opinion.

[deep_bang]

As many have suggested it is due to speedo error. A 145/80 would have a side profile of 116mm while a 155/70 would have a side profile of 108.5mm.

The circumference of the tyre is related to the radius as 2 x pi x radius. The radius in your case has reduced by about 7.5mm. This is about a 7 % reduction. So the circumference also would have reduced by 7% making it seem like it has travelled more than what it actually has travelled.

However I would expect it to be 7% of 500 that is about 535. Can't explain how it can become 560. May be the older tyres had some problem that is fixed by the new ones.

[xingamazon]

Originally Posted by deep_bang

As many have suggested it is due to speedo error. A 145/80 would have a side profile of 116mm while a 155/70 would have a side profile of 108.5mm.

The circumference of the tyre is related to the radius as 2 x pi x radius. The radius in your case has reduced by about 7.5mm. This is about a 7 % reduction. So the circumference also would have reduced by 7% making it seem like it has travelled more than what it actually has travelled.

However I would expect it to be 7% of 500 that is about 535. Can't explain how it can become 560. May be the older tyres had some problem that is fixed by the new ones.

Perfect stats there deep_bang.

I would guess the remaining improvement can be attributed to the actual tyre change, the old tyre a bald one would have very less grip on the road.
With change of tyres it would surely impact the FE for sure

[kirantp]

Originally Posted by Durango Dude

A real tyre 'upgrade' should see the FE go southward and handling go northward, due to wider contact patch and also the steering must get a wee bit heavier. I can't comment on the FE you are getting, but that's my opinion.

@Durango Dude: I agree with you. I heard the same from my ANHC dealer that tyre upgrades will definitely reduce FE and on the other hand it enhances the road grip. However, as you have mentioned, we will have to live with steering being on the heavy side.

[bigwheelz]

Originally Posted by vigsom

My experience has been no more than half a km/liter after replacing with the same make/size.

I guess Michelins are doing the trick - but a 1.7km/l difference is hard to believe(before 500/35=14.3kmpl ; after 560/35=16kmpl ; delta=1.7kmpl)

Vigsom, Same here. I dont believe the difference should be so high. But as given by fellow members, the odo is erroring due to chane in tyre profile.

Originally Posted by devarshi84

You have changed the tyre profile. That has led to your range increasing to 560. You still cover 500kms only.

Tyre pressure also plays a good role in affecting FE. I usually go 2 psi higher for city and 3 psi higher for highway driving. It gets me better figures.

Devarshi, The normal pressure is 30 psi, so for city driving can I do 32?

Originally Posted by Chewbacca

Not sure what you meant by "without a full upgrade"
Alto's tank capacity is 35 litres but you can squeeze in 39 litres. How much does it take when you do a tank full? were the old ones tubeless? and how long did they last (in kms)?

Chewbacca, I dont top the tank, when the auto filling gets cut, I ask the attendant not to sqeeze any more in it. This practice remains same before and after. Yes, the old ones were also tubeless. I changed them after 32k.

Originally Posted by dhanushs

In fact if the stock tyres are 145/80 , then a 155 upgrade should technically reduce the fuel efficiency.

Please note that, 560kms in odo means, you are covering only ~540kms (speedo error). The 40 km range increase can be attributed to the weightlessness and maybe, increased tyre pressure.

Dhanushs, yes I changed from 145/80 to 155/70. Do you think the odo is behaving very bad?

Originally Posted by MAS

What is the OE tyre size of an Alto? Increase in FE by 14% is not very realistic. Guess your speedo reading took a beating due to change in overall diameter of the wheels.

MAS, The diameter hasn't changed IMO. It's still R12. The side profile has changed.

Originally Posted by deep_bang

As many have suggested it is due to speedo error. A 145/80 would have a side profile of 116mm while a 155/70 would have a side profile of 108.5mm.

However I would expect it to be 7% of 500 that is about 535. Can't explain how it can become 560. May be the older tyres had some problem that is fixed by the new ones.

Bang on, deep_bang. So it mean's it's a mix of odo error and increase in fuel efficiency that is giving it 560 per tank.

[Chewbacca]

Originally Posted by bigwheelz

The diameter hasn't changed IMO. It's still R12. The side profile has changed.

Earlier it was 80% of 145 = 116 mm (sidewall)
Now it is 70% of 155 = 108 mm
Thus overall diameter has changed from 536.70 mm to 521.70 mm

This also means the car lost 7.5 mm of ground clearance but will perform better while cornering.

Nothing wrong with the odo, wheel now rotates 17 circles more per km. If speedometer shows 100 kmph then it's actually 97.2 kmph.

[MAS]

Originally Posted by Chewbacca

Earlier it was 80% of 145 = 116 mm (sidewall)
Now it is 70% of 155 = 108 mm
Thus overall diameter has changed from 536.70 mm to 521.70 mm

This also means the car lost 7.5 mm of ground clearance but will perform better while cornering.

Nothing wrong with the odo, wheel now rotates 17 circles more per km. If speedometer shows 100 kmph then it's actually 97.2 kmph.

This is exactly what i meant. Tyre size is generally xxx/yy R12. The yy is the percentage of xxx. This changes the dia of the tyre as clearly explained by Chewbacca.

[Sn1p3r]

I hope you see light from this discussion. I am sad that the tyre change was done without the right calculation.

As mentioned earlier an upgrade of rim size or tyre width leads to decrease of mileage as a general rule. However it's impact can be reduced by some innovative rubber compounds. E.g., Michelins XM1+ have mileage + fuel efficiency improvement over a similar size tyre.

[GTO]

With all else being the same, your fuel efficiency should actually have decreased. Primary reason : you've lowered the overall diameter. Think of it like shorter gearing. Of course, the speedometer will tell a different story.

Here's a comparo chart that will explain it better (1010tires calculator)

Originally Posted by dhanushs

In fact if the stock tyres are 145/80 , then a 155 upgrade should technically reduce the fuel efficiency

Bingo!