LeXusBoy

A while back I herd that the speedometer was off about 3MPH I was wondering what way that was. Like if it says we are going 50 are we really going 53 or 47MPH???

mooretorque

LB, the odometer is the thing that shows total mileage. The speedometer tells you how fast you go. And a rule of thumb is that if a speedometer is inaccurate (on stock wheel/tire), it'll be "optimistic", i.e., show 53 when really going 50. But there is always some variation from one vehicle to the next.

LeXusBoy

Thanks every one must think im really stupid. Well Ill delete this post and start over. Thanks for your help and so is the speedodometer off or not?

LexRX

The speedo is off - by a 2-3mph. It reads slightly faster than you are actually going. So, if you want, you can add slightly larger wheel/tire combo and have a more accurate speedo

-Nick

LeXusBoy

So if on that site you gave me with the tires sizes it says that my speedo will read +3 then im ok??? So if it says 50 im going 50.

LexRX

Yeah, it's not going to be exact, but close enough. Seeing as how the speedo reads a little fast - using a slightly larger wheel/tire combo will offset that slightly.

-Nick

mooretorque

Remember, though, that regardless of how accurate you may be when the wheel/tire combo is new, as the tread wears down, the circumference becomes (slightly but significantly) smaller and smaller, so that you are once again off by 1-2% once your tread is worn significantly. So you may start perfect, but you won't stay perfect, unless your tire budget is significantly greater than mine.

BTW, don't worry about speedo/odo confusion; we all have brain fades now and then. In some cases, such as mine, more often than others.

mooretorque

Remember, though, that regardless of how accurate you may be when the wheel/tire combo is new, as the tread wears down, the circumference becomes (slightly but significantly) smaller and smaller, so that you are once again off by 1-2% once your tread is worn significantly. So you may start perfect, but you won't stay perfect, unless your tire budget is significantly greater than mine.

willard west

Tire air pressure in the morning when the tire is cold varies enough once the tire is warm (hot) to contribute a 3-5% variation.

mooretorque

Willard, not that I'm challenging your numbers, but 3-5% sounds a little excessive to me, given psi change of, say, 5#. No figures to back that up, though, as I've never checked. Diameter would have to increase by about 1.25" to produce 5% change. Considering your penchant for data acquisition, have you pulled a wheel/tire off and measured change in circumference with change in pressure? (Seems like that would be easiest.)

wwest

Easier to measure the height of the vehicle just above the wheel in both cases. More realistic measurement with the weight of the vehicle.

You should see the look on a trooper's face when he is asked this question in court. They oftentimes check their speedo accuracy early in the morning against another trooper with radar. They typically have no idea how accurate the speedo is after the tires have warmed up.

mooretorque

Had not thought of the vehicle height method. Exactly right, though. I assumed that with or without the weight of the vehicle, iow, with sidewall deformation or without, the change in circumference would change the same amount with the pressure change.

Unfortunately, on the rare occasions when I've been nailed (Thank you, Mike Valentine, all the way back to 78 with my first Escort!), 3-5% wouldn't help much.

BTW, your 996 is beautiful!!

wwest

The point is that the trooper didn't know of this potential for his speedo to be wrong, there was no reason to bring the issue of by how much.

I'va gotten maybe three speeding tickets in the last twenty years and only lost one because I didn't want to directly confront the young (wet behind the ears) trooper for perjury after his older partner was forced to "out" him.

He had signed the ticket indicating knowledge of the radar that only his partner had.

The older partner came up to me after court to tell me how much he appreciated what I had(n't) done and told me he owed me one. That was back about 15 years ago and I'm still waiting to collect.

I think the situation is that so few people actually challenge these tickets that the officers become complacent about really knowing the circumstances and proper use of the equipment.

It's also pretty clear that they tend to write tickets more often for out-of-staters and of course that further reduces their court appearances.