Ritter v4.0

What wheel and tire size is a 92 C2 speedo calibrated for?

Bill Verburg

Standard wheels and tires were 8x16 w/ 225/50x16, options were 9x17 w/ 255/40x17, both are ~25" OD

That said I've never found Porsche speedos to be very accurate

Ritter v4.0

Thanks- I was doing about 162 mph or so with 275/35 18's and wondered what the true speed was.
I have correction software somewhere for rolling diameters but could remember what stock was.

dfinnegan

Aside from the tires, the speedo, itself, is not that accurate. Mine reads ~3 mph high above 60 mph and looks to be going toward 4mph high above that. According to my GPS navigation system, at any rate.

I've read other similar comments.

What was your tach reading? That should be more accurate.

Even so, that's very fast! How'd that feel?

Ritter v4.0

It was superstable. Its the silver street car with only JIC's and Ruf chin spoiler. It feels more stable at 160 than my C4S at 120 did- but that could be tires and other factors. I think its about 5700rpm and it feels like there is more there.

There is a new road in Nassau that is over 4.5 miles long (L-shaped actually) with no ingress/egress, superb visability and very smooth new blacktop. Just a couple of gentle rises and dips
Think of New Mexico- er, sort of.

There are also virtually no cars using the road right now so . . . I do a early morning drive on the w/e. Don't mean to drive quite so fast its just so "easy"- I look down and I'm 120 before I know it. The car just has legs and tracks steady. So then I got curious.

I'll try again sometime with my GPS.

Ritter v4.0

BTW- for a 275/35 18 rear wheel with dia of 25.6" the correction is -2.2%. That is, if the speedo is reading 60mph you are really going about 61.3 mph.

ilko

With stock diameter wheel/tire combo your speedo will read a bit higher than the actual speed of the vehicle. But that's the lawyers' fault, not the engineers'. Most newer cars have a built-in speedo "error" to avoid lawsuits.

dfinnegan

I should sue 'em for that!

porschapete91

You could not believe my speedo over 60 mph,so I work on 25 mph per 1000 rpm in top which makes 5700 /142 mph and 6500 rpm 162 mph my sat nav pretty well agrees with the theory as well

newsboy

Here is a calculator that lets you compare tire sizing and gives you speedometer error.
http://www.1010tires.com/TireSizeCalculator.asp

Jonnygo1

Ritter.....did you feel a bit like this???

http://www.youtube.com/watch?v=mk7VWcuVOf0

Ritter v4.0

Almost exactly like this but without the special effects ( and John Candy).

andrew911

[QUOTE=dfinnegan;6154473]
What was your tach reading? That should be more accurate.

QUOTE]

But the original question was in reference to the speedometer reading being thrown off by different OD of non-stock tire size....the tach wouldn't correct for this (if we saw off his fenders and put 30 incher's on there, the effective engine speed to road speed ratio is thrown off for instance). But with stock tire sizes, you're right- armed with the effective transmission ratios the speed calculation would be more accurate than the speedometer

Bull

FWIW, From my first 911 in 1969, to my current car, I usually have seen about 4-5% error on the high side with stock diameter wheels/tires.

Bill Verburg

GPS can be pretty good if you have time to read it.

Another good approach is to do a rollout of the tire to find as close to it's actual rolling radius as you will get out side of an instrumented run.

find a flat straight piece of road, mark a pt on the tire and roll through as many revs of the tire as you have room for, the more the better.

take the distance rolled in inches, divide by the # of revolutions, this gives you the tires real rolling circumference. From there it's simple to calculate speeds accurately from rpm and known gear ratios.