[Zappo]

Recently I made an observation that set me thinking. I may need to change the tyres on my Indica NA soon. Now, what I observed is that when I put on my Garmin GPS device and check and compare the speed that the GPS device displays vis-a-vis my speedo (VDO make) I found that my speedo was consistently showing around 8% more. That means when my speedo shows 100 kmph the Garmin tells me the speed is around 92 kmph.

Now all speedos have some calibration error. But this big a difference? I checked it again in a colleagues Indica NA. Again the same thing! It shows around 7-8% more on the speedo than what the Garmin tells me. Then I had a hunch. I put the device in my close buddy's Indica Turbo (which we use for long journeys). It has 185/60 R14 spec tyres (Yoko ES100). I was amazed when my doubts proved correct. The speedo in this car shows almost exact speed as what the Garmin says the speed is!

Here are some important notes

* Both the Indica NAs are post 2005 vintage (BS III) and sport VDO speedos.
* The Turbo is a 2007 model car and has the same VDO speedo console as the other two Indicas.
* Turbo comes with 165/65 R14 spec tyres which has been upgraded to the 185/60 R14 spec in the Turbo.

Now the the Turbo's wider tyres (upgraded spec) hardly makes any noticeable difference in the rolling diameter from its original spec. While the sample may not be good enough to make a statement about this my hunch is that Tata's do not have two different calibration sets for the two sizes. They use(d) the same speedo consoles in both the Turbo and the NA version. As a result all NA versions show a much higher speed than the actual.

I do not know how accurate the GPS device's speed display is. It is no Racelogic VBox. However my hunch will be that it can not be very wrong as it does not rely on anything mechanical. As long as it has a lock on a satisfactory number of satellites it should be calculating the speeds fairly accurately.

Now what does this all mean? To start with I calculated the difference between the rolling radius and a resulting difference in speeds that a 165/65 R13 replaced with a 165/65 R14 will bring in. I found that the difference in displayed speeds is around 7% almost the same difference that the Turbo and the NA has for the two sizes. Thus the assumption that the same speedo is used by Tata in both the cars without any recalibration. It also makes sense because the Indica's wheel-wells were actually designed for the 14 inch tyres but later Tatas decided to put in the 13 inchers, till they came up with Turbo.

More importantly, for me and for all the Indica NA owners it means that we can simply switch over to the 14 inch tyres with the Turbo specs without any related headaches (like forced reduction in the sidewall height and the resulting pain due to the potholed Indian roads). The speedo will show actual speed for a change. Nice, very nice!

Finally, if some of you have the post 2005 Indica NA (because that is about when the VDO console came in and I guess Tatas also planned for a future version with 14 inch wheels) and have a GPS device can you check and confirm whether your car's speedo has a similar speedo error as mine? I have checked and found the same error in 2 similar cars. If a few more confirm the same then it becomes a reasonable assumption that all Indica NAs with 13 inch wheels can just switch over to the 14 inch spec of the Turbo without any modifications.

P.S. And yes... before someone raises the point the suspension setup in both the Turbo and the NA version is the same except for some difference in the way the 2005 version NA Indicas had a soft sprung suspension for a comfy ride. However that is mostly immaterial here.

[greenhorn]

I've noticed that the DLS dials are a bit different to my DLG turbo dials. not sure about the calibration.

PS: I think you might lose out on the initial torque and pickup on 14"ers

PPS: I've heard that the 14" wheels are not as prone to bends as the 13 inchers

[sgiitk]

It is not the 12" or 13" or 14" wheel rims, it is the overall wheel diameter, which is the rim diameter + 2*tyrewidth*aspect ratio. Remember to convert the units to all mm or inches. Upto 5% variation is considered acceptable.

[Zappo]

Originally Posted by greenhorn

I've noticed that the DLS dials are a bit different to my DLG turbo dials. not sure about the calibration.

PS: I think you might lose out on the initial torque and pickup on 14"ers

PPS: I've heard that the 14" wheels are not as prone to bends as the 13 inchers

Not really. I use both the Turbo and the DLS on a daily basis. There is absolutely no difference in the dials, or at least I did not notice any. Both are the same VDO dials. Of course I am talking about the post 2005 NA versions which started having the VDO units with the Tacho and all.

About the torque and initial speeds, let me see. I will put the NA on trial with the Turbo's wheels (the old rims and tyres are lying around since we shifted the Turbo to alloys and new Yokos). Let me see the results first. Will update.

The rim bending problem is there in both Turbo and the NA versions. Ask me. I have bent enough number of rims in both the cars. Actually the sidewall height is the same in both the cars irrespective of the dia. And with the car's weight if the tyre falls on a decent sized pothole at some speeds it immediately affects the rim in either cars.

Originally Posted by sgiitk

It is not the 12" or 13" or 14" wheel rims, it is the overall wheel diameter, which is the rim diameter + 2*tyrewidth*aspect ratio. Remember to convert the units to all mm or inches. Upto 5% variation is considered acceptable.

You are correct. And that is why I said I did all the calculations and realized that the difference is close to 6.5% between the tyre specs of the Turbo (14") and the DLS 13" wheels. Now the 2 DLS for which I tested the speeds they consistently show between 7-8% more speeds than the actual speeds. Whereas the Turbo has an accurate speedo. Hence my hunch that the Turbo spec tyres will immediately rectify the erroneous speedo and may actually not need any calculations, the kind you talked about.

[Nikhilb2008]

Zappo, somehow I;m not convinced. Please do the test you plan on and post your results here.

[Zappo]

I agree with you Nikhil. In all these years it never struck me that such a possibility could even exist. It's just a happenstance that one day while the GPS was on in a city drive I noticed that there is a difference in speed between what the Garmin says and what my speedo displays. And since the difference looked substantial to me I decided to do some further digging around. And I came up with this startling fact.

Anyways, I will put on the tyres and rims of the Turbo, after balancing, on my Indica NA and see what happens next. And I will keep you informed as this may also be a helpful data for you.

[Zappo]

Thought I will put in an update here. As mentioned earlier, one day after all these years I discovered suddenly that my Indica NA has a hugely optimistic speedo calibration. The actual speed is upto 8% less than what it displays. I planned to put the Turbo's setup (tyres, rims et al) as is into the NA to check whether my intution and rough calculations about that size rectifying the speedo is correct.

Well, I went one step over that. Instead of the Turbo's setup I recently put in the tyre-rim combo of the Indigo. That is a 175/65 R14 setup. The reason for opting for this was nothing but sheer laziness. While the Turbo's tyres and rims are all lying disassembled the Indigo went for a straight alloy/tyre replacement. So the old inflated tyres on the rims were just taken out as is.

Now here is what I have observed in the last 4-5 days of this swap.

• The tyres by virtue of being even bigger (higher rolling radius than the Turbo which is 165/65 which in turn is much bigger than the NA spec) have now nicely filled in the wheel wells of the Indica NA.
• The car feels infinitesimally more comfortable to drive around.
• The body roll has gone down perceptibly.
• No hardening of the steering or drop in acceleration was observed.
• The speedo is now almost error free. In city, my GPS showed that for a displayed speed of 60 kmph the actual speed is around 59 kmph.
• And here is the kicker. My car seems to have suddenly become more sprightly!

Now this last one I am still trying to get a hang of. Can a substantially bigger tyre cause the car to respond quicker? I can not think of a solid reason. And yet after having driven the NA for long I know it's acceleration capabilities (or rather more of an incapability, depending on how you see it) pretty well. I know it all the more as I also drive a Turbo frequently plus having the experience with the Indigo. I know the NA is the slowest of them all. And yet now I know that the NA is behaving perceptibly better, faster. Of course it is still a slouch but not as much as earlier.

What is this phenomenon guys?!! I am pretty sure though that it has happened for some unfathomable reason.

I am now trying to figure out that when I go for new tyres soon (remember the current Indigo tyres though not bald are actually 32000 kms old) what size should I go for. My initial plan was 185/60 R14 but now I am wondering whether 175/65 R14 is a better idea.

[greenhorn]

I was trying out my new android phone with gps in my indica turbo running stock 14" rims and stock 165/65 tyres, and it was showing an increased reading by about 10% or so. at an indicated 40kmph, the gps showed 34~36.

not sure what to make of it !

[mercedised]

@zappo: With increase in the RIM size the acceleration will drop but, you will end up in having a much higher top-speed than before. It's a nice feeling to have an upgrade at no cost merely. BTW, Could you please explain me about the turning radius; whether it is decreased or the same as was the case with stock tyre + RIMs.

[dkamath]

Maybe an O-T post. Though i haven't got an idea on the realtime speedo error if swapped to 14", but can safely say that the odo calibration is fine with the Indica 13" wheels (atleast on our 9+ year old car, hehe). With the Safari as a reference, the distance from hometown to B'lore is exactly the same on both cars. Now, either both are erroneous or vice versa .

PS: Good to hear from you after some lull, greenhorn, unless i missed reading any of your comments on Indica lately . And congrats on your new Android based cellphone acquisition!