Tom M'Guinn

I'm curious about speedometer accuracy in general, and if the 180 mph speedometers are inherently more accurate than the 160 mph speedometers. Please check the box that applies. Please factor out non-stock tire sizes and respond based on whether the speedometer itself is accurate or not -- i.e., answer based on whether it would it be accurate with stock tire/wheel sizes. Thanks!

p.s. not looking for atomic clock accuracy here -- just whether it is as accurate as a typical car or noticeably inaccurate.

Crazy Eddie

As you can attest to

Paulyy

i have 245 45 r16 on the rear so its very nearly the same circle as the stock 225 50 r16

Cyberpunky

Although I voted not very accurate, as it is always wrong, it is actually very accurate in that it is out by exactly 10 KPH, regardless of what speed I do. So it is wrong by the same amount at fast or slow speeds and so easy to adjust to. 70 KPH is exactly 60 KPH, 140 KPH is exactly 130 KPH etc. This has been checked with 2 GPS units
peace
Cyberpunky

Paulyy

Yes this is exactly like mine! i do 110 on the hwy and wonder why im doing the same speed as everyone else and why i have had no tickets.

i've checked with a few qps units, including my phone and others and it's very close to 10k's off.

Thom

The 300kph speedo in my turbo is less inaccurate than the 270kph speedo in my S2.
At 130kph on the satnav the turbo speedo reads ~135kph while the S2 speedo reads ~140kph.
Both cars are on their respective stock tire sizes.

gruhsy

300kph and out by 10km/r to fast

993GT

my '87 with 180mph reads about 2mph UNDER with 265/35-18 and about 2mph OVER on 225/50-16...

fortysixandtwo

I should elaborate that I'm using a 968 cluster in an 86 951. The speedometer is waaaay more accurate at low speeds and, based on radar signs, very close at typical surface road and highway speeds. I have not checked its accuracy at higher speeds.

245/45-16 Hankook RS-2 rears

racerxrick

The 285/30/18 and 245/35/18 match almost exactly with the stock tire sizing for a Turbo S.

Using GPS at speed the stock 180 speedo (using much of it) was still accurate within 2 mph

nofalls

I tweeked mine to GPS accuracy by changing a tailor resistor but now the odometer reads slow. Plus side is my MPG appears to be better than it actally is.

blown 944

Not sure how to answer the poll. However, I got a pretty good idea of what my measured mph was going to be while watching the speedo at the end of the track.

For example, if it hit over 130, I would get a time slip at over 125

If it just hit 125 on the speedo, I would get a slip for 120ish.

This was with 275 40 17's. I forget the rollout, but they were on the smaller side (25" tall)

The 285's were a little more correct.

This is with a 160 mph speedo

Scott H

My 180mph w/ stock tire diameter (245/45-16) was surprisingly accurate whenever I drove past one of those signs that shows your speed. It was accurate up to ~80mph, never really tried to test accuracy past that speed.

Dougs951S

my 160 speedo seems accurate enough for an old sports car. It reads ~4 mph fast at all speeds that I've bothered to check it at ( read: less than 100 mph ) Its a world better than my old 83 which was ~15-20 mph off at highway speeds, and the error would change value, ie get smaller at lower speeds and higher as speed got higher. Both cars were on their respective stock tires sizes.

racerxrick

LOL...that would be difficult