Fluidplay

Thought I saw a thread a while back regarding this, but my search came up empty. The speedometer in my 86 951 is off. Had a friend pace me and found that at 65mph I'm really going approx. 58mph and at 100mph actual speed 90mph. I've gone through those road side speed radar markers and I'm generally off about 5mph off at 45mph. Conclusion, margin of difference is relative to increasing speed. Is this a problem with the speed ref. sensor? Appreciate your help!!

ausgeflippt951

It was a thing done at the factory, particularly back in tha day w/ high-perf cars, in order to stave off going "too" fast. You can get it recalibrated, but honestly, why bother? I just remember to go 5-10 faster than my speedo says.

The margin of error is actually more like 10% of the actual speed (90 vs 100; 45 vs 50, 18 vs 20, etc).

You're actually more off than you think, because your friend's car is also calibrated (unless he recalibrated it) to also run as smidge fast, tho certainly not as much as our cars.


You want a real reality check? Ride the new Yamaha R6 (or any bike, really) and they can be off by over 15%, as is the case w/ the new R6.

billindenver

Increase your rear tire size to compensate.

sweanders

Use a GPS to verify speeds and memorize or take notes of the speeds.

Fluidplay

My bud's car wasn't a 951 or a P for that matter. Brand new "Saleen". Anyway, did a check against the engines rpm 4500/gear ratio thru gears 2-4 and according to that measurment my speedometer reads correctly, but it's not! Running up to 4500rpm the car according to P should be @ 48mph in 2nd / 74mph in 3rd / 100mph in 4th / 125mph in 5th. In this test I found that I was averaging 10-15% less in actual speed. I would like it to read correctly. Re-calibrating sounds like a job for a speedo shop? Has anyone tried it? Thanks for your input as always..

ryanm6100

More importantly, how did the p-car hold up against the Saleen???

billthe3

It doesn't matter what brand car your friend has, all cars are set with their speedometers fast from the factory. Different companies are just off in different amounts - BMW's are the worst.

The simplest way to fix it is to change tire sizes. If your tires are to small the speedo will read fast, if the tires are to big the speedo will read slow. Ask anyone who has put large offroad tires on a truck - their speedo reads crazy slow. Get some larger diameter tires and the error will go away. Of course getting smaller tires means you the car will go slower at max speed, but the speedo will be correct.


...I don't have the speed differences mixed up, do I?

Darwantae951

^^^^^ Sounds ok to me.

-Darwin

washington951

get one of these:
http://www.abbott-tach.com/era.htm

i had one on my 89 due to a wheel size change. had the 'new' speeds dialed in exactly.

Fluidplay

I completely understand the theory of larger tires first hand. I have a raised truck with larger tires. The wheels/tires are stock Phonedials. But what I'm hearing is that there's a conspiriscy out there to get us to drive slower, I won't have it!!

JarmoL

Hi there,

I removed the speedo needle and put it back so that it points little more below zero than it used to. (There are tiny splines both in the needle and the axle.) It took me a couple of tries to get it right but now it's pretty accurate at least up to 100 km/h.

Jarmo

ApexCars

Maybe my PO did something but I've checked my speedo against a GPS up to 75 and it reads pretty much correct.

toddk911

Is there a formula to know the differences of the speedo in adding bigger/smaller wheels/tires?

I.e. what size wheel/tire do you need to move from the factory 10% off to accurate??

toddk911

Yea, my wife's Pathfinder has factory wheels/tires and reads 8-10% faster than actual.

So in speed zones that are 35 she is at 35 on the speedo but radar display shows 30-31.

Another good check is to do a 3rd gear pull to redline and if you are reading 100 or over, you are reading 10% over, give or take. Assuming stock wheels/tires.

turboted

Here is a calculator that determines how tire sizes affect speedometer readings.

http://www.1010tires.com/TireSizeCalculator.asp