FRporscheman

Hahaha, nice website! I guess I spent the last hour making my own spreadsheet for nothing!

odurandina

different options for the different audi motors.... V8, intercooled turbo 5 cylinder, etc....





i could tell you where to buy everything, but then i'd have to put you on ice...





soon as i get the guy on the phone.........






25" tires = your speedo is lying to you. subtract 3 % from what your speedo says....


25.6" tires = your speedometer is telling you the truth.

FRporscheman

I think I need to physically measure the distance from the ground to the hub center. This way, there can be no mistake. It's easy to calculate a tire radius, but the tire does squish at the bottom, effectively reducing the radius...

My speedo is fairly accurate according to police courtesy speed sensors, and my gps unit. I don't remember how accurate, but I remember thinking the error was negligible.

I suppose tire pressures will affect things as well. I tested my speedo when I had the recommended 36psi. Lately I've been driving on 30-32psi. I'll raise it up again to see what happens.

odurandina

just run 275-35-18 25.6" tires back there and you'll be golden.

FRporscheman

No way! 18" ride quality SUCKS! Also, the cost of wheels and tires goes up dramatically. And don't forget that I'm looking for a tall 6th to save some money on fuel. Wider tires and heavier wheels only mean less fuel economy.

Maybe when I graduate and get my 6 figure job, and promote the car to weekend-only status. For now I'm sticking with 17".

kappa87

18" ride quality is not bad in my opinion. Didn't notice much of a change from the 16".

FRporscheman

Hey nice Horizon Blue car! My '93 is the same color! Are those 18" cup-2s??

Ride quality is a completely subjective thing, I should have added "IMO" to my comment. I do tend to drive my car frequently enough that I can't continue to tolerate the roughness of the ride, especially since the road quality in my neighborhood is sub-standard.

kappa87

Yes, they're 18" cups. The roads here in Texas aren't bad so, I guess you're right. It is subjective. Not sure I completely understand the gearing discussion here but, my understanding is that you can change the ratios to help out accelleration(0-60) with the compromise of top end speed?? Would be interested in knowing the ratios to use for this type of application.

mattipuh

I'd prefer getting better acceleration from 45mph to 100mph, 0-60 is not needed.

odurandina

earlier this evening i made a claim of taking my 968 up to 161 mph on the 944 vs 968 thread...


it was an estimate..... it all started a few days ago....









monday's dyno test:


i dynoed the car @ ~ 161 mph when i hit the limiter with the tach reading just a tad over 6,400 rpm on a long, long straight on monday (in a midwestern state)... the car ran up to about 150 mph quickly where it encountered a wall of wind and the increase of speed slowed...


the last 7 or 8 mph took about 5 more miles of tarmac.... an eternity.



the interesting part was that the car felt like a brick, even with my lower lip having blown off on the last top speed run... this was my first time after installing the resonator. i might add that the sound this was absolutely incredible at top speed running the no "od" exhaust w/ straight pipes.


i'm running slightly over stock Dunlop 9000 275-35-18s which measure 25.65" at birth but now have about 2 mm of wear..... on interstate 80 i ran 129 miles of mile markers. and the odometer read 128.3 miles.



here's the bad part.... something started to smell horrible. don't know if it was the bearings on the torque tube, or the input shaft, clutch or what.... didn't smell good. no oil refinery smell so i know the engine was doing okay.



maybe the sealer from the gaskets from the recent 120,000 mile service, which included everything.... ???






so, i am estimating about 250 hp due to the big fat tires which should be slowing the car down about 5 mph....


or did i miss something ?





to which i responded that i'd get back to Tim in a few minutes.... since i hadn't done the calculation since when i first selected tires for the new wheels about a year before... about 40 minutes later i came up with this;









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FRporscheman

Just thought I'd report my measurements, as I finally got to drive the 968 today.

65mph: 2625 rpm
70mph: 2800 rpm
74mph: 3000 rpm
80mph: 3200 rpm

Of course they are only as accurate as eyeballing a gauge with 5mph ticks and 200 rpm ticks, so +/- 3mph and 100 rpm I suppose.

The 6th ratio is better than I remembered it. It's almost good enough to just leave it alone. If for whatever reason I ever have to open up the transmission, I'll consider a slightly taller gear (as in, 3000 rpm at 78 mph), otherwise I'll leave it be.

Jfrahm

Doesn't the 968 use the ABS signal for the speedo, not a pickup off the rear axle like the late 944?

Offhand I think it is the LF wheel sensor that provides the speed signal.

I would not not expect that signal to be particularly good at high speeds, the error is generally cumulative and I would expect an indicated 160 to be anything +/- 10 mph.

-Joel.

odurandina

we don't use the speedo to measure speed. the instrument you want to test is your tach... anyway,

i turned my speedo back ~ 3 or 4 mph. and with the large tires it reads 60 @ 65 mph, about 97 mph @ 100 mph and about 160 mph @ 160 mph. that's just what you have to do to compensate for Porsche's well-known speedo defect.

FRporscheman

well-known speedo defect???


I will try to remember to do another speed test with my GPS unit.

odurandina

the speedo defect has been touched upon on off topic and on the 944 turbo, 993, and 928 forums quite a few times since i been around..... and the minions are spot on about this... running 25" tires or slightl over on nearly ALL the cars people drive, speedo readings are decent at the lower speeds (below 50 mph) with an exaggeration of about + 2 - 3 mph, but you'll end up with a defect by as much as + 8 - 10 mph nearer to the redline...

when you run the correct sized tires, the defect is closer to + 4 - 5 mph at the redline, but you'll be going like 2 mph over what the speedo is reading @ 50 mph.




http://www.boxsterforum.com/forums/a...p?t-13778.html


"During my drive today, I was using a GPS. The speedometer was consistently reading 8-10% higher than the GPS.... Is there a way to have the speedometer adjusted/calibrated to provide accurate readings ?.... "





this is why i turned my speedo back about 4 mph. this requires a very gentle touch with your fingers...


after doing the calculation from Porsche spec i came out with roughly a 25.5" tire as being the right mulberry to put on the car. stock 275-35-18s or even used 285-35-18s will work nicely to solve the speedo issue.




when i reached top speed the other day, my speedo was reading just over 160 mph @ 6,400 rpm.


however, when i go through the posted traps on the highways at lower speeds, i'm always going 4-6 mph faster than my speedo says. too bad there's nothing to do west of Fredricksburg, eh ?




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