speedo accuracy

[newBDR]

You guys with the 17" wheels, do you think your speedo is accurate? Have you verified it? I have the M3 upgrade, 17" wheels, autometer reverse 180 speedo. Earlier car, #31. I think it registers on the high side. At an indicated 60mph I seem to be just flying past others on the highway. I've yet to drive a measured mile to verify. What's the simplest way to do measure it accurately? Does backdraft change the gearing for the speedo depending on which wheels, rear end you purchase?

[Ron61]

The easiest way to check the speeedometer reaing is if you have mile markers along your highway and use a stop watch. If you don't have mile markers, then take a stop watch and drive at what your speedometer reads as 60 MPH. You should go 1 mile in 1 minute at that speed. If you go less than a mile then your speedometer is fast. The other easy way is if you have a friendly patrolman around with a radar unit that will let you drive you car at various speeds and he can record on the radar what you were actually doing. The rest some of the Backdraft people will have to answer.

Ron

[SkipB]

newBDR,

Here is another method. Run your car up to an indicated 100mph. Then when you attract the attention of law enforcement, ask them what speed they clocked you at. Simple and easy to accomplish. Since you will have time on your hands in the cell they provide for you, do the math and figure out the speedo variance!

[Dave Bermani]

AAA conducts periodic speddmeter tests at their offices. It's like a dyno setup. It's free to members and they give you a printout of the actual vs. indicated readings.

Dave

[CJ428CJ]

Around here in California there are getting to be more and more electronic speedlimit signs that show you your speed as you drive by. Find one and see what it says compared to your speedometer.

[scootter]

I have the 17" wheels with 315/45 tires the rear of my SPF. Mine is off by 5 mph. When I am doing 60, it is 65. I just used my little hand held GPS to verify. I tried it in my pickup and wifes car first to make sure it was accurate.

[BMK]

newBDR

Your local instrument shop will have a hand held that they will run over a mile with you. Most have a mile marked out somewhere for that purpose. Some have the Sat Nat. GPS unit which is dead accurate.

The Cop with the radar is also a good one. Just remember the Kustom HR4 has a 2mph variation/tolerance.

If all else fails contact Tony at BDR he may have some specs. on tranny final, diff ratio, wheels/tires etc. to assist with the calculations

Bernie

[CoupedUp]

Or you can just have Bernie pace you. This will allow calibration up to 168 mph. If you chip in for a bottle of NOS, he can check you up to 195 or so...

[TampaFla]

GPS - speedo reads 70, GPS reads 68.2, speedo reads 60, GPS reads 58.3, speedo reads 35, GPS reads 33.2. So speedo is about 1.75 miles over.

17 inch wheel, 275/40s. Have a set of 275/45s but haven't tried those yet.

[BMK]

CoupedUp

Supply the NOS and I'm there... It's like someone has kicked your butt.

I did have a recent day at the track and found I was showing about .1 extra over the track circuit. The gauges on board are dead accurate.



I found that the wheel spin had contributed to the .1 variation.

And I thought it was the two 360's that I had managed to do each lap. So that wheel spin must have accounted for the extra 100 yeards that I was travelling each lap..

Bernie

[tfarhood]

The Autometer speedo is a mechanical speedo. (as opposed to electronic). There is no calibration for it other than choosing the most appropriate gear that connects to the transmission tailshaft gear. Based on rear end gearing and tire size you can calculate which color gear (they are color coded) will work best for you. As for checking the accuracy, a small hand held GPS is the easiest and most accurate way to verify it. If you don't have one, I guarantee someone you know has one. I checked my BDR with GPS and it was about 3-4 mph slow at 25 mph, 2-3 mph slow at 60 mph, and dead on at 110 mph! The interesting thing is to check all of your autos with GPS, you'll be surprised at how far off some factory cars are.

[Ron61]

Ok, a question for you guys with the hand held GPS units. Which one do you think is the best and could you point me to some links that I can look at them on?

Thanks,

Ron

[tfarhood]

Ron,
For speedo verification, any GPS that shows speed will work fine. That includes the ones people use for hiking. If you want a full road map, point to point, talking one, they are not cheap yet. Any Sports Authority or Dick's Sporting Goods has GPS starting about $100.
As I said before, I'm sure someone you know has one, you just need to ask around.
Garmin and Magellan are the two big names in GPS, but there are other good ones out there. Check E-bay, I'm sure you can get one cheap there.
If your speedo is more than 5-6 mph off, you need a different gear on the cable.

[TampaFla]

If you're tired of using road maps, get a GPS. If you're tired of being caught in traffic on the Interstate, get a GPS (accident). If you're tired of filling up in one state when 30 minutes later you could buy the gas for 10 cents less, get a GPS. It's always nice to know where you are but also nice to know what's ahead and on either side.

But, if you're only verifying a speedometer error, borrow a GPS or get a used one (GARMIN II, etc.).

I got one because I constantly get lost and can't ask for directions.

[Ron61]

Tfarhood,

Thanks for the information. I want one for a different reason. My speedometer is computer controled and I have checked it with the CHP radar at 60, 70, 80, 90, and 100 and it was right on the button. Also the tach was right to the exact RPM for each speed. I do some hinking up in the mountains around here looking through old gold mines and such and hoping to find that long lost treasure, and would like a way to pinpoint it so I can go back with the correct stuff to carry it all out. I Wish !!!

Ron

[tfarhood]

Ron,
When you are in that old Gold mine 6 miles back in the woods somewhere, make sure you have extra batteries in your pocket. Those handheld units are only as good as the batteries in them. We don't want to see any headlines that read: "Ron61, found dead in the woods with $2MM in gold nuggets. We followed his tracks in circles. He should have used Copper Tops in his GPS!"

[Ron61]

Tfarhood,

I can guarantee you that when I am poking around in those old mines, I carry extra batteries, lights, and ammunition. As for the gold, if I found any I probably wouldn't know what it is. We have found some old 1800s type coins and china way back in the canyons where the miners used to have camps. Sometimes I marvel at how they got that stuff miles into those 2 mile deep canyons and the only way in is to walk and I can barely do that without moving equipment and such.

Ron

[BonitaCobra]

NewBDR,

Check out this link to give you the speedo driven gear teeth calculator http://www.5speedtransmissions.com/c...ors.html#other Also, click on gear ratio's to find many comparison to tranny/tire size and gear ratio's. problems.

Larry