Speedo running fast.. advice please

[RJ Gilliam]

Okay wizards, next issue. It appears that either this is the fastest car in the world or my speedo is off. I know the previous owners has changed the tires and possibly the rims, but is there a gear in the trans that can be changed?

I was told that on my "next" set of tires I should go to 245's...If I fix the speedo now will it just have to be redone later?

After I put the car back together, gonna have the wife pace me at the usual speeds to get an idea on how fast the speedo is...

Is this a in the garage fix or does it have to go in for professional recalibration?

Thanks in advance,

RJ
86 930

[RJ Gilliam]

Tire size is 225/50ZR-16 the car is a 1986 911 turbo ( 930 )

Thanks again

Richard

[emcon5]

I think all Porsche speedo's are a little optimistic.

If you have your owner's manual, there should be a chart with rpm=mph for each gear, you should be able to get an idea how far off it is. You may need to send it off to Palo Alto speedo or North Hollywood Speedo to get calibrated.

Not much help I am afraid,

Tom

[johnd]

isn't there a potentiometer in the back of the speedo? and can't, by tial & error, the adjustment be made to either lower or raise the shown speed?

[masraum]

I'm assuming stock tires should be 245 45 16 while the tires you have on now you say are 225 50 (besides the fact that going to skinnier tires is VERY weird). 225 50 tires are actually just a smidge taller than 245 45. the 225's should be 24.9-25" tall while the 245's should be 24.6-24.7" tall. By going to a taller tire than stock your speedo should actually be reading slow not fast. Taller tires will turn slower at the same speed than shorter tires.
As someone else stated, it's my understanding that the adjustment is in the speedo somewhere. Most domestic speedo's are/where mechanical/driven from a gear and cable in the trans. Your car is driven from a set of 8? magnets on a pickup in the side of the tranny and is all electric with no cables or gears.

[RJ Gilliam]

Thank you all very much for the information. I have no idea why the previous owner put on these size tires, maybe to save a buck or two?? LOL

I will have the wife pace me this weekend and see if it realy off or not.

If someone knows where this pot for electronic adjustments is please by all means drop me a line.

This is the best most responsive site on Porsche I have been in contact with, and it is my pleasure to be a part of this.

RIchard

[ohecht]

The potentiometer on my speedo ('82 SC) was inside the speedo assembly itself. If I remember correctly, it was a small flathead screwdriver adjustment towards the bottom of the assembly. I needed to get to it so often during calibration I ended up drilling a small access hole in the back of the case and then sealing it with an RTV plug after I got the adjustment right.

Olivier

[johnd]

when looking at the back of the speedo, does one turn the pot clockwise or counterclockwise in order to lower the shown speed.

If the former owner changed tire size check the owners manual for the size that came with the car.
Go to www.tirerack.com and look under "tire specs" for the tire it should have and what you have now or will use in the future.
Look in the column labeled "rev's per mile" this is the information that the speedo shop will need to calibrate your speedo.
you may have to pick a brand/model of tire ie Firestone 500 and then check tire specs. This also gives you recomended rim sizing and overall section width etc.

The spec book calls for
Front rim 7J x 16
Standard front tire 205/55 VR 16

Rear rim 8J x 16
Standard rear tire 225/50 VR 16

Since 1986 models
Rear rim 9J x 16
Standard rear tire 245/45 VR 16

Also the "revs per mile " will be different manufacture to manufacture