glencase

I need some assistance from a tire/math wizard.

I won't get into details but I need to make my speedometer read 5+ mph slower than it is currently.

This is for a 968 so everything that I have read and researched references the abs system for the pickup that calculates the speed. What I am unclear of is if it's one of the front sensors, one of the rear sensors, or info from the abs brain calculated from all 4 sensors.

So what I need to do is figure out which tires would effect the speedo and what size I would need to increase them to for a 5+ mph slower than actual speed adjustment.

I need the speedo to have an indicated speed of 59mph or less for an actual speed of 64mph.

Current tire size
205 55 16 front
225 50 16 rear


Thanks.

KuHL 951

I've used this one before. Maybe it will help.
http://www.dakota-truck.net/TIRECALC/tirecalc.html

Matt H

Let me be the first to ask WHY?
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josephsc

Let's see: current rear tires are 225/50/16, so the 2 sidewalls at 225mm*50%/(25.4mm/inch) + 16 inches = 24.86in diameter. If you goto 225/40/16, then it's 23.09in diameter, which is 92.9% of initial diameter. Therefore, 92.9%*64.0mph=59.4mph.

Am I right?

AllenM

I'm assuming someone was going a little too fast in a 35mph zone and is trying to keep their license. or going to fast in a 45mph zone and is trying to avoid getting points on their license.

Mike1982

If you REALLY want an accurate reading, see if anyone you know has a GPS. Then go out on the highway and find out your actual speed compared to visual on the guages. I plan to do this soon so I know exactly when passing a state trooper, "I mean..", so I know my right speed.

KuHL 951

I don't think so, going to a smaller diameter will make the speedo read higher not lower (i.e. more revolutions/mile)

glencase

Good site thanks for the info.

Now I need to know front or rear sensor(s) that generate the pulse for the speedo. Keep in mind that this is a 968 which supposedly uses the abs sensors instead of the transmission sensor.

Everything I've read and found on thread searches references the left front but I want to be sure.

Anyone.

Matt H

Glen, curiosity in me just has to know why. I believe the answer to your question is front sensors. What year is the car?
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josephsc

Ah, you're right -- need to go larger.

glencase

A car started passing me on the right almost hit a mini van and started coming into my lane, I had to straddle the shoulder and speed up to avoid an accident. When I sped up he got behind and threw on his lights (tinted undercover small impala), promptly giving me a ticket (pace). Very unfortunate for me he was going after someone else almost hits me gets embarrassed and gives me a ticket.

Unfortunately speeding for necessity is not a viable defense.

It's a 94.

Matt H

If you can find someone with an OBD1 scanner (which there are plenty of) they should be able to scan the car while it moves to see which sensor sends the pulse. Since that would take time I suggest you try the 968 forum (there is another one not the one here).

Still not sure I understand how this is going to help you (unless you are going to try to prove that you didnt know you were speeding, which is still not a viable defense).

Slap some 225/60R16s on there, should solve your problem but it is going to look like hell.
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glencase

By using the site that Kuhl 951 provided I was able to come up with a size for the front that I think will work 215/65-16. Does anyone know if that will fit in the wheel well before I pickup a pair of them used?

Yes I know it will look like hell, the sidewall will be about an inch taller all the way around.

87944turbo

This is another good one. Just below the image of the tire are the speedo calculations.

http://www.miata.net/garage/tirecalc.html