Speedo compensation?

exit64 · Post by **exit64** » Mon Mar 19, 2007 12:09 am

I just got some new shoes for my 1970 1600. I went from sun beaten, cracked side wall, balding 165 R 14 to new Formoza FD1 195 60 14 from Les Schwab. Wow, what a difference the lower profile makes in the handling. What I want to know is how for off is my speedo reading now? Seems like it is reading fast. Is there a formula to adust for the different tire size or should I just wait until I get a ticket and see what the radar reading says?

Sound off.
Mike
aka exit64

Daryl Smith · Post by **Daryl Smith** » Mon Mar 19, 2007 2:32 am

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

The above site will give you some info to work with. You've just got to figure out what size tire is closest to the original size tire on the car!
My guess: 175/75/14 or 175/70/14..........

SLOroadster · Post by **SLOroadster** » Mon Mar 19, 2007 8:08 am

Thats a cool link. I wish I could factor in the rear end ratio to find the right speed.

Will

Daryl Smith · Post by **Daryl Smith** » Mon Mar 19, 2007 12:33 pm

There used to be a site with an excel spreadsheet which you could plug in your rear end ratio, each gear ratio, and your tire size to calculate your speed at any rpm in each gear.
It was great to go through when contemplating changes to see what effect the changes would have .......

It seems to have gone away....

robbocx · Post by **robbocx** » Mon Mar 19, 2007 6:23 pm

Or you could get a loan of a GPS and check it. That way you can be 100% sure of your speeds.

Steve

Post by **itsa68** » Mon Mar 19, 2007 10:07 pm

I believe the stock 5.60-14 tire size stated in the roadsters owners manual was 24.6" diameter.

For that calculator listed above, if you plug in your 165R14 (82 series) as a 165/80 it should be close to stock size, your speedometer reading of 60 mph is actually 57.1 mph .

When my new set of tires get put on, I will also have to find out how far the speedo will be out.

I think that I'll just ask the fella driving next to me how fast he is going.

Ray B.

Daryl Smith · Post by **Daryl Smith** » Tue Mar 20, 2007 3:04 am

"I believe the stock 5.60-14 tire size stated in the roadsters owners manual was 24.6" diameter. "

If that is the case then the 185/65/15 tires I'm running are pretty much right on the mark @ 24.5" diameter.

When I get hold of that calculator, I'll see if 185/55/15's and a 3.7 rear end will keep the revs/speed in the same range......

Post by **reblues** » Tue Mar 20, 2007 9:25 am

What I do to estimate the accuracy is:

Pick a highway with mile markers. Travel 10 miles (the further you drive the more accurate the estimate) making note of your odometer reading at the 1st mile marker. At the 10th mile marker note your odometer reading. If the difference between the two numbers isn't 10 miles then your speedometer is off. Figure the percent. If your odometer read 9 miles then you are off 10% and your speedometer will be slow 10%. So at 50mph you are actually doing 55mph. You can use that percent to decide if a different speedo drive gear will fix the problem. I think each tooth on the gear makes about 6% difference at the speedometer.

Now if a train leaves Cleveland @ 10pm traveling 31.1 mph.............

PPCLINDL · Post by **PPCLINDL** » Tue Mar 20, 2007 3:08 pm

I have a shortcut on my desktop to the afformentioned Excel spreadsheet but can't figure out how to get back to the original address so I can turn you guys onto it.

ppc

exit64 · Post by **exit64** » Tue Mar 20, 2007 3:46 pm

Thanks for the input, guys. I really like the GPS idea as I no longer have the original tire to work backwards from. I suspected that I was reading a tad fast becouse there's no way I do 0-60 in 6 seconds.

Howard R · Post by **Howard R** » Tue Mar 20, 2007 4:14 pm

Mike, I've got Falken RT615's, 195 60 14's. What tire pressure are you running? I've driven past those highway devices that tell how fast you are going, and the best I can figure is about 2mph faster than my speedo says at 60 mph. Not much of a difference. But I'm not sure if my speedo was recalibrated. I suggest that you drive side by side with a friend that has a digital speedo, and check the difference.

Post by **ppeters914** » Tue Mar 20, 2007 5:01 pm

PPCLINDL wrote:I have a shortcut on my desktop to the afformentioned Excel spreadsheet but can't figure out how to get back to the original address so I can turn you guys onto it.

* Right-click on the shortcut.
* Select Properties.
* You should now be able to see the URL.

Post by **dbrick** » Tue Mar 20, 2007 5:58 pm

90% of factory speedometers read high, most as high as 10% This prevents speeding ticket lawsuits, and also as a bonus to the manufacturer, effectivly shortens your warranty. From personal experience, if you ever have a warranty claim denied and are slightly over on your miles, get your speedo checked, the difference may get the work done. One of my customers did it to Volvo.

Post by **ppeters914** » Wed Mar 21, 2007 1:25 pm

dbrick wrote:90% of factory speedometers read high, most as high as 10%

Yeah, I recently discovered this on our 2002 Miata. Speedo reads approx 5mph higher than actual speed.

PPCLINDL · Post by **PPCLINDL** » Thu Mar 22, 2007 2:02 am

Pete,

I did what you said to do and just got the fact that it is in my documents, so I guess I don't have the URL, just the spreadsheet. As you may have guessed by now, I am not too computer savvy. Maybe that's why I love my old Roadster and 3 each 510's....

ppc

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