glencase

This is the actual thread

https://rennlist.com/forums/924-931-944-951-968-forum/265773-tire-size-math-wizard-speedo-calibration.html

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This is the content of the request.

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.

Tom Pultz

The amount of adjustment (8.47% increase in rolling circumference) you are asking for is significant. The diameter of your stock front tire is about 24.88 inches (205*0.55*2/25.4 + 16). Since circumference is proportional to diameter you need a tire that's 26.99 inches in diameter or larger (24.88*1.0847). That's a big tire. Working backwards from the needed 26.99 and using a 16" rim means you'd need a tire at least as large as a 233, which means something like a 235/60-16 (if there is such a tire). Or you could go to a narrower tire with taller aspect ratio. In either case it would look stupid on your car... but I suspect this whole exercise is to show the court your speedo was off because you had the wrong tire sizes. Good luck.

moronputz

Are you sure you have the request the right way around?

You want to be travelling 5mph FASTER than the speedo reads?

flash968

sounds a lot more expensive than the ticket would be

glencase

Thanks for the info someone posted this site on the other thread
http://www.dakota-truck.net/TIRECALC/tirecalc.html if you enter correct size in the A slot 205/55-16 and put in 215/65-16 and choose compare tires it gives the entire breakdown.

I came up with a 215/65-16 which I hope will fit in the well

flash968

is this intended to defraud the court? if so, think twice - if you get caught, it's potentially a felony

glencase

Actually a friend of mine put me on the right track of thinking, so I'm getting a lawyer. I haven't even checked the speedo yet with a gps so we'll see how that goes. Thanks for the advice although I'm now thinking that way anyhow.

glencase

Actually I think you're right I'll probably just pay it, I don't really have the time or money to deal with a lawyer and a day off of work to go to court.

Thanks