Got my first......

Speeding ticket, first EVER!! State Trooper said I was doing 76 but my speedometer said I was doing 60, the posted speed, $250.00 ticket, can it be that far off, I mean I figured maybe 5 MPH or so but not 15 MPH, He could see I had new rims and tires cuz he said so, he was real nice about it, we talked for a bit about cars (he said he was a car guy) and told me I could get my speedometer calibrated/checked and if it's a good bit off he would take back the ticket after I e-mail him a copy of the slip.
Ohhh rear tires are 275/60/15

 

Taller tires will make a speedo slower. 275 /60/15 would be a lot taller than the E/78/14's it probably came with new....

 

what size rear tires did you use ? very nice clean mav

 

thoes tires can be 3-5 inches taller than stock. that could easily make your speedo off by 15mph. you just need to change the gear on the speedo cable that goes into the trans.

 

You are off by about 20%. Not a uncommon problem. You need to pull your speedometer cable and count the teeth on the gear. Then you need to find a smaller gear for it to get it to where it is closer to what you are actually running.

 

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


I get almost %10 off with that tire change. E78 converts to a 195-75-14 for the calculator.

Assuming this is correct.http://www.vintagecarconnection.com/vintage_tire_size_conversion_chart.htm

 

Use your cell phone as gps, and get your EXACT speed. There are several apps for droid and iphone.

I am using the smallest gear I can find on my speedo, and still off by 5% or so. So I keep a TomTom One in there full time for use as a speedometer.

Hmmm. That doesn't sound too nice to me. "Nice" would be "hey son, go a half mile out and come back at me again and I will radar you and tell you how fast you are going, and together we will figure out how far off your speedometer is so you can drive safely from now on."

 

The tires alone could have caused the problem, but if I remember correctly you changed the motor/trans and rearend in you car too..... So the new rear could have been a different gear which would change the speedo.....

Also C4 transmissions have the drive gear for the speedo cut into the out put shaft, the thing is there are 3 different ones/ 7, 8 and 9 tooth..... If the trans that is in you car does not have the original trans it could have a different the original tooth count on the output shaft which could cause a difference in the speedo.....

That all being said you can calculate what driven gear(goes on the end of the speedo cable) to correct it..... You will need to know what your tire size is(that's easy), you will need to find out what gear ratio your rear end has in it(not to hard to do), and also need to know what tooth count the output shaft on the trans has(pain in the you know what, you will probably have to remove the tailshaft housing on the trans to be able to count the teeth).....

 

OK with the calculator in electronic land it is 10% off. But in the real world, on the road, if he is running 60 mph on the speedo and is 10% off he would be running 66 mph. At 20% he would be running 72 mph. Just saying.

 

didn't you think something was off when you were passing everyone on that road...

I asked a Sheriff friend of mine to write me a warning ticket for 152 MPH...he said if he did, we both would go to jail...

...

 

Yep your correct. I was just showing him the tire difference. I'm sure hed figure out he has more issues if its out as much as he said.

Not to offend but I remember getting my tickets and I bet the speed I thought I was going was actually lower than reality.

Man thats a big seat of the pants difference from driving 60 to driving 76??

 

even changing the gear on the cable will get it close, still won't get it exact, but I do need to know about what it is off by.[/QUOTE]

get a gear that reads...faster...not slower... `

 

as you pass the reflector poles on the interstate count one thousand one one thousand two one thousand three if you pass the pole at one thousand three you are going 60 miles per hour lol:evilsmile

 

Well I thought I would let you guys know about my ticket and speed, had calibration on my speedometer done today and the results are in.......MPH of course top number is what I read, bottom number is actual speed.
speedometer reading.. 20...30...45...50...57...67...77
actual speed.............. 25...35...50...60...70...80...90
so it is indeed around 13 MPH off, I was reading my speedometer at 60 MPH when stopped.