Tire question for ALB and the other gurus

The VW/Porsche cable driven speedometer (goes through left spindle) is calibrated for a 24 3/4- 25 1/4" tire (5.60-15- remember those?, 155 or 165-15, 185/70-15) so anything smaller throws it and the odometer out. A 24" tire is only 1/25 (or 4%) smaller so it doesn't change things that much- at 50mph there's only a 2mph error. That said, going slightly bigger will only make the percentage of error smaller.

The gear ratio link below shows the effects of 195/60's and 205/60's with the same transaxle on speeds; feel free to change numbers and experiment. Changing to the taller tire in the rear will change things, but maybe not as much as you think? Put a 205/65 in #2 and see what happens...

http://www.teammfactory.com/gearcalculator.php?kmh=0&tirewidth=195&tireprofile=60&wheelsize=15&tirediameter1=24.21&maxrpm1=5500&finaldrive1=3.88&gearratio1=3.78&gearratio2=2.06&gearratio3=1.31&gearratio4=.89&gearratio5=0&gearratio6=0&trannytype=&tirewidth2=205&tireprofile2=60&wheelsize2=15&tirediameter2=24.69&maxrpm2=5500&finaldrive2=3.88&gearratioa=3.78&gearratiob=2.06&gearratioc=1.31&gearratiod=.89&gearratioe=&gearratiof=0&trannytype=&tranny=Calculate+Transmission+1+%26+2

Marty.

I thought I read that your car came with the new GPS speedo, but I guess not. That might be an option vs buying new tires.

Marty, what size tire gives you a diameter of 24"?

Did I read you wrong, or are you trying to do two things here: correct your speedo and decrease rpms.

Do you know how much off your speedo is?  I compared my speedo with a GPS to find out how much it was off at various speeds.

By knowing what size tire you have and the speedo error you can calculate what size tire you need to bring your speedo as close as possbile to reading the correct speed.

Troy-I have a Speedo cable and I think my Porsche Trans has an electronic tach.  Ron,

The 24 inch is this bizarre 175/65 sport tire made for Minis and Fiat sport models.  I am off about 5mph at 75 (shows 75 and I am going 70 on GPS I-Phone app) Its used to be very accuracy with the 195 60s.  I like the idea of the 185/65 front easy steering and more meat with the 205's in the rear.  I checked out all the cars at Carlisle and the look I liked the best is the Pirelli P4's which have all these sizes.  

Marty, I'm running the same size tires - 175/65-15 - on my VS and have the same 5 mph speedo error above 60 mph.

I chose this tire size for a little more clearance at full lock up front, but the clearance is pretty tight in the back, too.

Marty, here's two comparisons-surprisingly, not a big difference

175/65-15 vs 185/65-15

175/65-15 vs 195/65-15

Thanks Ron. ALB, can you repost that chart with these specs:

Tires 185/65 front and 205/60 rears.

911 Carrera 3.2 Turbo-look (USA/Japan) 1984-86

5-speed synchronized transaxle

Manufactured by Porsche (ZF)

Plate-type limited-slip differential available

Rod shift

Single-plate clutch

Gear ratios:

1st gear: 3.18 (11/35)

2nd gear: 1.83 (18/33)

3rd gear: 1.26 (23/29)

4th gear: 1.00 (26/26)

5th gear: 0.79 (38/30)

Ring and pinion: 3.88 (8:31)

As far as tires go, if you go with a 185/205 combo I'd recommend this tire:

For those of you out there that run 185/65s, front and back, this is an interesting tire.  It's one of the VERY few tires offered in 'summer' tread.

Marty- you should be able to change any of the gears or the tire sizes- just left click on any of the variables. For example, click on the number box for the tire size and then enter your info. Go through the whole gearset like that and then click on the calculate transmission button and it will refigure the numbers and graph with your specs.

PS- Here it is, with your transaxle specs-

http://www.teammfactory.com/gearcalculator.php?kmh=0&tirewidth=205&tireprofile=60&wheelsize=15&tirediameter1=24.69&maxrpm1=5500&finaldrive1=3.88&gearratio1=3.18&gearratio2=1.83&gearratio3=1.26&gearratio4=1.00&gearratio5=0.79&gearratio6=0&trannytype=&tirewidth2=205&tireprofile2=60&wheelsize2=15&tirediameter2=24.69&maxrpm2=5500&finaldrive2=3.88&gearratioa=3.78&gearratiob=2.06&gearratioc=1.31&gearratiod=.89&gearratioe=&gearratiof=0&trannytype=&tranny=Calculate+Transmission+1+%26+2

PPSS- Good call, Ron, I usually look at tirerack and they don't carry Falkens.

Here's a couple of screen shots comparing 205/60 and 205/65 with your gear ratio.

First, 205/60

Next, 205/65

Ok, I am catching on.  Ron, can you post that same two charts comparing the 185/65 to the 175/65?  Thanks 

Marty, those two charts are for your drive wheels.  It won't work with your front wheels.

Switching from your 175/65 to 185/65 will correct your speedometer by 1.28 mph @ 60mph and switching to 195/65 will correct your speedo by 2.56 mph.

My speedo was off by around 5 mph at 60 mph, so I had North Hollywood recalibrate my speedo when they were installing LED lighting.  The only information they needed from me was my tire size.

I can't tell you if they solved my speedo error problem, because I don't have my IM on the road yet.