Speedometer correction using larger tires?

[dannyo' in houston]

I am a contributing member but have only been a lurker for several months. I have a 2002 C2 Cab, 3.6 liter, Tip, 18" Carrera II wheels (8x18-50 front and 10x18-65 rear). I bought it used a year ago with Sumitomo HTR III tires, sizes: 225-40-18 front and 295-30-18 rear. My speedometer reads 3 miles per hour faster than my GPS at speeds between 40 mph and 90 mph. Since I need new tires, I called one of the local Porsche dealers here in Houston and was told that the original equipment tires were 235-40-18 front and 295-35-18 rear. (This dealer quoted over $1450 for N-rated Michelin PS2s plus labor. Whew...expensive!) Though these sizes are larger than what I learned was OE here, I am leaning toward buying them (probably not from the Porsche dealer) with the assumption that the slightly larger diameter tires will make my speedometer read closer to accurate. What do you think?

Also, where does the speedometer get its input; that is, front wheels, rear wheels or pitot tube? ;)

[ericb]

I own a MY99 C2 6sp with 265/35R18s in the rear. I've also observed that my speedometer is high by about 8% according to my GPS, and every radar speed sign in the state. As far as knowing my true speed I just adjust by 5-10%. But another concern is that when I go to sell the car the odometer will be high by 8%. If I sold it at 54,000 miles it would actually be 50,000. But I'll assume I can explain away that little fact. My Porsche dealer rep says anything within 10% is within legal limits. That's fine, but this is an automobile that seems to exude uber-engineering, which makes an approximate speedometer an anomoly. Anyway, a definitive answer would be great. And I wonder if this is a typical experience for 996 owners.

[PAULSPEED]

The tires should read the same because they are both 18". If

you went with 19" then maybe it would change. The 235 number

is the width of tread. The 35 is the aspect ratio of the

side of the tire.

What I do is go to tirerack.com. I then print out the price and go to the

tire store. I tell them that I will buy the tires and have them put them

on. They then make some good deals.

I do not know where the speedometer gets its input.

Paul

[dgcook]

Interesting. My '99 C2 does exactly the same thing. It's approximately 3 mph over while reading 65 mph. I've puzzled over the same thing. It seems very odd since it is fitted with factory 18s.

Dave

I own a MY99 C2 6sp with 265/35R18s in the rear. I've also observed that my speedometer is high by about 8% according to my GPS, and every radar speed sign in the state. As far as knowing my true speed I just adjust by 5-10%. But another concern is that when I go to sell the car the odometer will be high by 8%. If I sold it at 54,000 miles it would actually be 50,000. But I'll assume I can explain away that little fact. My Porsche dealer rep says anything within 10% is within legal limits. That's fine, but this is an automobile that seems to exude uber-engineering, which makes an approximate speedometer an anomoly. Anyway, a definitive answer would be great. And I wonder if this is a typical experience for 996 owners.

[Modavi]

Even with original factory 18" wheels and tire sizes the speedometer reads 3-5mph high. Do a search, there are other threads about this, IIRC this is done deliberately to meet a German or European law that requires speedometers to never under-report the speed of the vehicle.

[dannyo' in houston]

I thought that this remedy for the "fast-reading" speedometer would have been investigated for a 2002 C2 by now, but I haven't found a thread that addresses this issue. So....I gather that from the lack of definite answers that no one here is sure whether the 235x40x18 and 295x35x18 tire sizes will cause a problem (such as rubbing the fender in a turn especially while the springs are compressed or cause an ABS problem of some sort); nor does anyone know for sure where the speedometer sensor gets its input. Maybe the reason that Porsche makes the speedo read fast with OE tires is so that for legal reasons it will not read slow with the largest diameter tires that can be accommodated??...just thinking out loud.

[L986]

From 996 Service Information manual:

"Vehicle Speed

The DME control unit receives information regarding vehicle speed at terminal 79. The DME control unit requires this information for various calculative functions (system adaptation, diagnosis functions, etc.), The speed signal is picked up from the rear, right-hand wheel."

"Speedometer

The speed signal is taken from the *** speed sensor (48 pulses) and the frequency division (split into 8 pulses) occurs at the instrument."

Hope this helps.

[Dharn55]

Different 18" tires will have different diameters depending on the size. On the Tire Rack site you can look a the specs for each tire and you will see the revolutions per mile. Some examples of this for a PS2:

265/35-18 815

265/40-18 790

295/30-18 832

295/35-18 797

So you can see that several percentage points in the speedometer error can be due to different size tires.

[gusmagoo]

I’m afraid they all over read speed but only by a fraction. They are meant to for reasons touched upon above (so I've been told)

Personally, I wouldn’t worry about it. Just look at your sat nav if you want a more accurate speed but why would you?

As for tyres, whatever the stock size is for the wheels that came with the car, assuming that the wheels are original to that car are the ones you should stick with.

That’s what years and years of research finally settled on and in a car capable of post 150 mile an hour speeds you’re only risking killing yourself if you start meddling with the setup. You might also invalidate your insurance. Just live with it.

[balefire]

I thought that this remedy for the "fast-reading" speedometer would have been investigated for a 2002 C2 by now, but I haven't found a thread that addresses this issue. So....I gather that from the lack of definite answers that no one here is sure whether the 235x40x18 and 295x35x18 tire sizes will cause a problem (such as rubbing the fender in a turn especially while the springs are compressed or cause an ABS problem of some sort); nor does anyone know for sure where the speedometer sensor gets its input. Maybe the reason that Porsche makes the speedo read fast with OE tires is so that for legal reasons it will not read slow with the largest diameter tires that can be accommodated??...just thinking out loud.

speedometer error is common in several cars, both american and foreign.

for the manufacturer better to err on the side of too fast than the opposite.

in some cars, you can trick the ECU to show the correct speed.

i don't think anyone has desired or found a way to do this on a 996.

your dealer is incorrect.

those tire sizes are for a 997 which has a larger overall diameter.

nevertheless, i'm running those exact 997 sizes without problem on my 996 40th, so they will fit.

i typically like to run larger sizes on my DD cars as it both corrects the speedometer error, reduces by 1% or so the actual miles driven, and cosmetically reduces the "wheel fender gap" making the car appear lower.

http://www.miata.net/garage/tirecalc.html

Edited March 2, 2011 by balefire

[wyovino]

(From Rennlist.com)

Porsche AC Diagnostics

(undocumented, unsupported feature)

To access diagnostics:

Hold down the Recirculating & Air up buttons for 5 seconds. The + - buttons go up and down through the list of "c" codes. The center vent button switches the left display between the "c" code and its actual value. Press the Auto button to exit.

0c - ERL

1c - Oil Temp?

2c - Inside temp. Sensor mounted in the aspirator assembly at the side of the dash.

3c - Outside temp. Sensor located inside the air inlet of the A/C unit.

4c - Outside temp. Sensor located in front grill of the radiator. The data is fed to the Climatronic from the instrument cluster. When not moving, the instrument cluster OBC temp display retains it's last setting until moving. This is to prevent heat emanating from the radiator affecting the temperature sensor. The A/C unit uses the lower of the two outside air temp values in determining fresh-air temp.

5c - Outside temp. (matches OBC outside temp display)

6c - Coolant temp.

7c - Footwell discharge temp.

8c - Sun sensor (dash top)

9c - Sun sensor.

10c - Passenger compartment fan speed.

11c - Passenger compartment fan voltage.

12c - Temperature mix Flap command 1=COLD, 100=HOT

13c - Temperature mix Flap position

14c - Central Flap command

15c - Central Flap position

16c - Footwell/Defrost Flap command

17c - Footwell/Defrost Flap position

18c - Recirculation Valve command 1=OFF, 100=RECIRC

19c - Recirculation Valve position (feedback)

20c - Vehicle speed in kph, updating only once per second. (real speed, not speed+safety margin as in the speedometer)

21c - Engine RPM in hundreds. This too only updates once per second.

22c - ?

23c - ?

24c - Sun sensor, exterior lights switch & panel lights control (term. 58 & 58d voltage) - used for A/C panel display illum.

25c - ?

26c - ?

27c - ?

28c - Fan speed?

29c - ?

30c - Engine run time in seconds (255 max.)(=0xff)

31c - Timing counter

32c - Displays test

33c - Software version? Mine states 3.4

34c - ?

35c - Outside temp. from inlet sensor (filtered?)

36c - temp?

[Loren]

(From Rennlist.com)

Porsche AC Diagnostics

(undocumented, unsupported feature)

To access diagnostics:

Hold down the Recirculating & Air up buttons for 5 seconds. The + - buttons go up and down through the list of "c" codes. The center vent button switches the left display between the "c" code and its actual value. Press the Auto button to exit.

0c - ERL

1c - Oil Temp?

2c - Inside temp. Sensor mounted in the aspirator assembly at the side of the dash.

3c - Outside temp. Sensor located inside the air inlet of the A/C unit.

4c - Outside temp. Sensor located in front grill of the radiator. The data is fed to the Climatronic from the instrument cluster. When not moving, the instrument cluster OBC temp display retains it's last setting until moving. This is to prevent heat emanating from the radiator affecting the temperature sensor. The A/C unit uses the lower of the two outside air temp values in determining fresh-air temp.

5c - Outside temp. (matches OBC outside temp display)

6c - Coolant temp.

7c - Footwell discharge temp.

8c - Sun sensor (dash top)

9c - Sun sensor.

10c - Passenger compartment fan speed.

11c - Passenger compartment fan voltage.

12c - Temperature mix Flap command 1=COLD, 100=HOT

13c - Temperature mix Flap position

14c - Central Flap command

15c - Central Flap position

16c - Footwell/Defrost Flap command

17c - Footwell/Defrost Flap position

18c - Recirculation Valve command 1=OFF, 100=RECIRC

19c - Recirculation Valve position (feedback)

20c - Vehicle speed in kph, updating only once per second. (real speed, not speed+safety margin as in the speedometer)

21c - Engine RPM in hundreds. This too only updates once per second.

22c - ?

23c - ?

24c - Sun sensor, exterior lights switch & panel lights control (term. 58 & 58d voltage) - used for A/C panel display illum.

25c - ?

26c - ?

27c - ?

28c - Fan speed?

29c - ?

30c - Engine run time in seconds (255 max.)(=0xff)

31c - Timing counter

32c - Displays test

33c - Software version? Mine states 3.4

34c - ?

35c - Outside temp. from inlet sensor (filtered?)

36c - temp?

Sorry, this will not work on MY2001 and newer cars.

[Dharn55]

I don't think the AC diagnostic trick will work anyway. It gets the input from some sensors somewhere, probably the same ones that the speedometer uses. There may be less of an error, but there will be some. And as the size of the tires varies the input from the sensors will also vary.

[wyovino]

Oops - I guess I should have read the fine print. Sorry.

[dannyo' in houston]

Thanks to all for your input. I've ordered the larger tires and will find out out if there is any fender or wheel well rubbing and will see if the speedometer error is improved. I'll warn everyone if there are consequences. However, I'm no race car driver and doubt that I'll be able to discern slight changes at the limits of cornering. My usual comment is that my car is more of a parade float than a race car. It is a tip cab after all. :)