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Old 5th January 2005, 20:00   #1
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Default lower top speed after tire change

guys,
my zen has done 27000kms in 3.5years.it had a stock 145/80r12 goodyear gps2 tires for about 25500kms.
the max speed i had touched with those tires was 160kph on the speedo and this was regularly done on any open stretch .

but after i changed to 155/65r13,the top speed on the speedo has reduced.i can barely touch 150kph now.

so i wanted to know the reasons for this reduced top speed.
just before i changed the tires i had got my annual servicing done.in which i had changed all the fluids,oils and had also got the tappet setting adjusted.
so is it because of improper tappet setting ?????????the maruti engineer has said that because of all my wheel spins in first gear ,my clutch plate has worn out slightly and thats the reason. that the power is not being transmitted properly adn so my lower top speed.

is this because of the worn clutch plate.
or is this beacsue of the changed gearing.
i know the speedo will change ,but on the contrary it should show more speed with lesser OD.
i know my top speed is less now because i checked this with a friends xing which previously was never able to keep up with me.

help.


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Old 5th January 2005, 20:24   #2
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maybe because a diff. in the OD of the tyre...
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Old 5th January 2005, 20:39   #3
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also theoretically speaking, 155 over 145 means ur tires will have more surface area contact with the road.... which means more friction... which means some power from the engine will go into overcoming that friction. U were not using alloys before so when u switched to alloys, the reduction in weight would have helped. But was it enough?
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Old 5th January 2005, 20:51   #4
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Bigger tyre = more surface area in contact with road = more traction = better grip = better handling = less mileage = marginally lesser top speed.
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Old 5th January 2005, 21:04   #5
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but my top speed with steel rims was 160kph.so with alloys it shud be more.
but wat abt the clutch plate theory
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Old 20th June 2005, 20:19   #6
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Default Lower top speed

Here is a comparison table for the tire size you had earlier (on the left) versus the new one (on the right)

Tire size: 145/80R12 155/65/R13
Section width: 5.70in 6.10in
Diameter: 21.13in 20.93in
Diameter difference: - 0.95%
Circumference: 66.38in 65.75in
Revs per mile: 984.3 993.7
Speedo read: 60mph 60.5mph
Speedo read: 100km/hr 101km/hr
Speedo diff: - 0.95% too fast

Your overall diameter has reduced by 5% -- which is above the recommended limit of 3% for tire sizing. Proportionately, your top speed will reduce assuming your engine can turn the same RPMs it used to earlier.

Your wheels will now need to turn 993 revs to cover the same distance you did earlier in 984 revs. Assuming there a 5% reduction in top speed, your car should now be doing 152km/hr versus the 160hm/hr on your older tires.

However, your speedo should show a marginally higher figure than it's actually doing. So for 152km/hr actual speed, your speedo will show 153.5km/hr -- which is still less than the top speed you had earlier. The reduction in speed is of a higher magniture than the speedo read error [Hope this makes sense].

I don't think it might be a clutch wear problem. If it were, the slip should be noticable while driving, especially when you take your foot off marginally from the throttle [it will appear to accelerate].

Here is a tire size calculator if you need one http://www.1010tires.com/TireSizeCalculator.asp

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Old 25th June 2005, 12:51   #7
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With the smaller wheels your speedo should actually read more than 160 kmph. Even if you consider the drag caused by the wider tyres, it should still be around 160kmph. 150kmph is way to less.

There can be many reasons for this. Try doing it on some other road, preferably on a road which will let you achieve 160kmph without much fuss (like on an expressway).

Shan2nu

Last edited by Shan2nu : 25th June 2005 at 12:52.
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Old 25th June 2005, 19:26   #8
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hey ,
i maybe incorrect on this, but isnt the speedo calibrated for a 12inch wheel.
so if the wheel size is increased then the reading changes....in this case lower top speed.

i mean the car could be travelling at the 160km/h but the meter shows 150km/h.

Shan2nu...i remember a post where you had mentioned that the wheel size and the tire profile would affect the speed reading.

pankaj T
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Old 25th June 2005, 19:47   #9
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Quote:
i maybe incorrect on this, but isnt the speedo calibrated for a 12inch wheel.
But, you also have to consider the reduction in aspect ratio, which supports the increase in rim size. So, the diff isn't as big as you think it is.

Quote:
i mean the car could be travelling at the 160km/h but the meter shows 150km/h.
Even though the rim size has increased from 12" to 13", the overall circumference has decreased (thanks to a lower profile which has been reduced from 80 to 65). In this case, the new tyre is actually smaller than the older one, which is bound to increase speedo error.

Since the newer tyre is just -0.95% smaller than the older ones, the diff should actually go un-noticed (in terms of revvs per km). Even if the tyres are 10mm wider, it doesn't justify the 10km drop in top speed.

Given the right condiditons, your Zen should be able to touch atleast 158-160kmph (on the speedo). 150kmph is unacceptable.

True top-speed of your car however, is a completely diff chapter (which will require a GPS device in order to be tested).

Quote:
Shan2nu...i remember a post where you had mentioned that the wheel size and the tire profile would affect the speed reading.
Like i've mentioned, the diff between the 2 tyres is just -0.95%, which should alter the speedo reads but by just 1.5-2 kmph and not 10kmph.

Shan2nu
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Old 27th June 2005, 03:01   #10
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Broader tyren also mean more friction and can reduce the top speed
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Old 28th June 2005, 03:24   #11
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its not a problem of the clutch

everyones right the tyres are larger so more surface area and so more friction so lower top speed but improved grip
but by putting alloys ur top speed will increase by little but if you get alloys and increase the pressure in the tyres by a little bit your top speed will rise but be careful if the pressure is too much the surface of the tyre on the road will decrease and grip will decrease so if pressure is to high you will not be able control the car at high speeds
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Old 11th July 2005, 21:01   #12
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Hey satya the low pros give u more traction to increase handling so its kinda holding on more tightly to the road hence reducing your top end.....Please correct me if I'm wrong guys
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Old 11th July 2005, 21:16   #13
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i have now changed to 165/60r13 .now the speedo shows 160+ : due to reduced OD.

and the lower top speed problem solved by itself.i could do 160 kph speedo afterwards

Last edited by speedsatya : 11th July 2005 at 21:18.
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Old 11th July 2005, 23:26   #14
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Thats great news...keep up the revs
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Old 26th December 2005, 18:08   #15
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Lightbulb The question of contact patches and grip

Quote:
Originally Posted by aazam
everyones right the tyres are larger so more surface area and so more friction so lower top speed but improved grip
A wider tire does not mean better contact path or better grip. See here (look for "Fat or thin:The question of contact patches and grip") http://www.carbibles.com/tyre_bible.html



The *better* handling is caused by the reduced sidewall and less flex in cornering.

sAP

Last edited by sAP : 26th December 2005 at 18:10.
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