johnfm

My C2 tip has 255 40 R17 on the rear and 205 50 R17 on the front.

1. Should I in fact put 40 profile tyres on the front. If so, for UK weather can anyone suggest besttyre combo.

2. How can I calibrate speedo to read correct speed with these tyres/wheels.

I think it is reading fast, as I know my tip cannot do 0 - 60 in less than 5 seconds!!

Adrian

Dear John,
Boy it has taken me a while to get back to these questions.
1/ Do not put 40 profile tyres on the front. It will not be good.
Tyres are purely dependent upon what you waant out of your 964. Plus how much money you want to spend. Please advise what you are looking for. I do not ever recommend all weather tyres anyway. Summer and winter tyres.
2/. No you cannot recalibrate anything. Manual or TIP. Everything is pre-programmed. With the TIP you have a few extra bits installed for speed calculations.
With the current tyre installation I would be interested to know what manufacture and type they are. The sizes are within the normal parameters. If your speedo is over reading I suggest you may have another problem. However to be sure I need to know what actual tyres you have installed.
Ciao,
Adrian
911C4

PS: I always recommend N rated or Porsche approved tyres but I would also recommend Toyo T1S Proxes. Many of our club members in the UK are using them and they are very happy with them.

Herbie

To make your ABS working properly, you need no difference between rear and front axle revolving speed.

Over a certain difference percentage ABS goes off.

Adrian

Dear John,
You can do a quick check to see how accurate your speedo is. Record the indicated speed that the spoiler raises at. Do two or three runs and record as accurately as you can. The spoiler will open at 80kph (50mph).
Please let us know what your results are.
Ciao,
Adrian
911C4

johnfm

Last night I did a few runs using the km/hr from my trip computer (blasted thing isn't calibrated in MPH for some reason..I digress).

Showed 165 km/hr in top at 4000rpm, 146 km/hr at 3600 rpm, 134 at 3200 rpm.

Comparing to the engine rpm vs road speed charts in my handbook, these speeds are faster than the gearing should indicate.

So, I measured my front tyre (probably my first mistake, as I don't know which tyre diameter the speed calculation will come from. (I'll do the rear tonight when the kids are asleep!).

Anyway, using the final drive of 3.67 and 4th gear ratio of 0.8 (gee I hope my handbook is correct for my 92 manuf. C2) I came up with this:

For 165 km/hr at 4000 rpm
165 km/hr = 2750 m/min
so, for each ENGINE rev, my car goes 2750/4000= 0.6875 m.

My tyre dia was 0.63 m, so circumference (distance/rev) = pi x dia. = 3.14159 x .63 =1.9792.

With my suspected gearing, I can calculate how far I should go per rev:

distance travelled per engine rev = 1 wheel rev / (3.67 x .8) x wheel circ.
=1/2.936 x 1.9792
=0.6741 m (674 cm roughly).

So, at 4000 rpm, the car travels:
4000 x .6741 = 2696.45 metres/minute
OR (multiply x 60) = 161.8 km/hr.

So, my speedo is reading 3.2 km/hr (2 %) fast.

THE END!

Of course, this all depends on tyre size measuremnt being accurate, the torque converter being locked and my handbook being correct!!

Adrian

You are allowed 6% so you are well within limits. This 2% is your tyres variation from pre-programmed original. Your speed in your Tip comes all from the wheel speed sensors via the ABS control unit. You also have a frequency converter unit (for this speed input) in the TIP. There is no speed sensor elsewhere as in 1989 and 1990 model 964s.
Conclusion, everything is quite normal for a 964 manual or TIP, fitted with 17 inch wheels with 50s up front and 40s on the rear, which by the way will be rotating at the same speed. If you install 40s of the same width on the front, the front wheel speed will be too high when compared to the rear and you will get ABS problems and I suspect maybe TIP problems.
Ciao,
Adrian
911C4