How Fast Was I Going?

Talewinds · Talewinds's Garage

Is there a standard formula for this?

I was following my wife yesterday afternoon and although the speed limit was 65, my speedo indicated 80mph the entire time. I know she was going somewhere north of 65, but I'm fairly sure she wasn't going 80.
So, if my indicated speed was 80, and my indicated RPM was 3200, and my transmission is a standard ratio/OEM G50, and my tires are Dunlop FM901 225/50/16, can I calculate how fast I was really driving???

Also, on that 170mi trip I burned a gauge indicated 1/4 tank. How accurate are those gauges anyway? That comes out to 32mpg-ish. Seems high.

ossiblue · 10-05-2009, 11:32 AM

Here's something you can have fun with:
Wheel / tire size calculator / comparer - WHEELSMASTER

Assuming your speedo is accurate with factory spec wheels and tires, plug that info into one of the charts, On the other chart, plug in the current tire/wheel info and you will get your speedo error calulated.

Further, someone will post, I'm sure, the table for your trans which shows the mph at various rpms, to help nail down accuracy.

Finally, I would not use the gauge for any precise calculations of mpg--just use it to estimate when you need to refuel. Use the standard: Fill the tank, drive, refill to the approximate same level, and note the gallons used. This is the only reasonable way to establish mpg. Doing this several times will give you a close average. Doing it with the gauge at various points (say, mid-way) when you refill will give you a close proximity of how accurate your gauge is reading.

nut11 · 10-05-2009, 11:38 AM

The speedo gage is a little over 5 miles on the high side in my experience. 32 mpg could be attainable. I've hit as high as 30 mpg dividing number of gals required to fill tank to first click release of pump handle by total miles traveled from last fill up. Hope this helps.

87 blk coupe

Wavey · 10-05-2009, 12:01 PM

Brian, I know in my '88, in 5th gear, 1000 RPM = 25 MPH. Therefore 3.2 X 25 = 80 MPH. So yours sounds right, unless both our speedos are off the same %.

JeremyD · JeremyD's Garage

Well the fuel gauge is not accurate. It tends to burn the the last 1/2 tank much faster than the first 1/2 tank. IMHO the carrera speedos are not accurate either - wildly optimistic. I have the iphone app "speed" that is very accurate. My speedo before being shipped to North Holywood was around 7% off at 55 and around 12% off at 100 and a little more over 120

teenerted1 · 10-05-2009, 12:18 PM

remember your odo will be off the same amount your speedo is. YMMV

sbrown.nw · sbrown.nw's Garage

I've read in the past and also personally experienced with my past 911sc and my current 911 that Porsche 911 speedos read optimistically. Both of my cars were about 5mph fast at 35mph and doubled at 70mph (actually going 10mph less). I have verified this with radar mph readings (roadside non-ticketing ones - never got a ticket in either of my 911 cars - knock on wood). My 78 sc had factory 16" rims and my current 17" rim setup on my 72 tub with Carrera flares has approx .5" smaller diameter than factory wheel tire / setup.

Scott

Superman · 10-05-2009, 01:00 PM

Mile posts are generally fairly accurate. Hold your speed at an indicated 60 mph and you should pass them every sixty seconds. Adjust your speed so that they go by every sixty seconds and you will be travelling at 60 mph.

SandyI · 10-05-2009, 01:09 PM

From my experience with my 911E, it's all about your present tire diameter compared to the tires your car left the factory with. My speedo runs 10%-15% optimistic. Seems like you're there too. Some GPS units can display your speed on the highway with a very steady foot.

nineball · 10-05-2009, 02:18 PM

Quote:

Originally Posted by SandyI

My speedo runs 10%-15% optimistic. Seems like you're there too. Some GPS units can display your speed on the highway with a very steady foot.

This is exactly the same as mine. I found that 70 = 82 or so on my trip home. Until recently I was using the free speedo app on my iphone, which coincidentally fits perfectly in the opening when the ashtray is open if you have one of those thin rubber cases. I just bought a new gps for the dd so now my old one will go into the 83SC.

ben parrish · 10-05-2009, 02:59 PM

So, that also means that our cars are racking up the miles at an elevated rate of 15%????

Dantilla · 10-05-2009, 03:12 PM

I used my hand-held GPS to track speeds at different speedo indications, then converted the knots to MPH (GPS is an aviation unit).

I knew my truck would be off, as it has larger than stock tires. My 944S is really close- only reads one or two MPH fast at freeway speeds.

burgermeister · burgermeister's Garage

My Tach is also a bit optimistic (maybe 300 RPM high at 6000 RPM).
This is verified by
1) the non-chipped DME cuts off ignition at an indicated 6800RPM (as opposed to 6450)
2) a DL-1 tach hookup reads a 300RPM lower number than the tach displays at 6000 RPM

I know of other tachs with this 'feature'.

Really, a GPS is the best way to get accurate speed ...

gtc · gtc's Garage

My speedometer reads about 6-7mph high from 20mph to ~75mph (haven't been able to test higher than that). I guess my needle is just off by a few degrees??