ThirdMC Posted September 16, 2010 Posted September 16, 2010 Has anyone ever checked to see if their speedo was reading correctly? Checked mine against my 05 Silverado today. Truck read 72 while the Monte read 80. As far as I know tranny and rearend are stock. Also still has the stock rally wheels. Tires are B.F. Goodrich Radial T/A's, size 245/60-15. (Everything on the truck is factory too) Quote
7tonemonte Posted September 16, 2010 Posted September 16, 2010 Im running the same size tire on mine, going past police speed signs with radar, I read about 3 mph faster at 45. Someone put up a link before for tire size calculators, I think it was 2 or 3% off with that size tire as compared to the stock one. Just a different diameter Quote
kc8oye Posted September 16, 2010 Posted September 16, 2010 easiest way to check it for sure is with a stop watch. point the needle at 60.. time yourself between mile markers.. at 60mph it should be 60 seconds. at 70 mph should be about 52 seconds. or.. take 3600.. divide it by your actual time in seconds and that's your mph. say you timed yourself at 45 seconds.. 3600 / 45 = 80 mph 3600 comes from 1 hours * 60 minutes.. and 60 minutes *60 seconds so 3600 seconds = 1 hour.. that's how you can go from seconds to mph Quote
502ci Posted September 16, 2010 Posted September 16, 2010 That's quite a bit off, your rear gears might of been changed. Jack both rear tires off the ground, put the car in neutral and spin the tire (by hand) one full revolution while counting how many times the driveshaft rotates...I'm gonna bet it rotates 3.73 times. The best way to test your speed is a GPS, I have a navigation system on my phone that also will tell you how fast you are traveling. Quote
Phil Posted September 16, 2010 Posted September 16, 2010 That's quite a bit off, your rear gears might of been changed. Jack both rear tires off the ground, put the car in neutral and spin the tire (by hand) one full revolution while counting how many times the driveshaft rotates...I'm gonna bet it rotates 3.73 times. The best way to test your speed is a GPS, I have a navigation system on my phone that also will tell you how fast you are traveling. Tell me more about that, my wife got herself an IPhone, maybe it's on there. Quote
502ci Posted September 17, 2010 Posted September 17, 2010 Tell me more about that, my wife got herself an IPhone, maybe it's on there. I have Verizon and they have a navigation feature you can pay extra for...I use it all the time, it's by far not as good as a typical GPS system but it get me around. Anywho it has in the options screen where you can check your longitude/latitude, mph, and other stuff...took me about 2 years before I actually came across it...it was pretty cool to watch the mph (which was dead on for my truck) speed up and slow down as I stepped on the gas... Quote
ThirdMC Posted September 17, 2010 Author Posted September 17, 2010 I have a Garmin nuvi in my work car. Picking the Monte up from the transmission shop this evening so I'll take it along and see what the Garmin says. Quote
7tonemonte Posted September 17, 2010 Posted September 17, 2010 Originally Posted By: 502ci That's quite a bit off, your rear gears might of been changed. Jack both rear tires off the ground, put the car in neutral and spin the tire (by hand) one full revolution while counting how many times the driveshaft rotates...I'm gonna bet it rotates 3.73 times. The best way to test your speed is a GPS, I have a navigation system on my phone that also will tell you how fast you are traveling. Tell me more about that, my wife got herself an IPhone, maybe it's on there. Thats it, you guys are doomed.....now "The Man" can always keep an eye on you Quote
Canuck Posted September 17, 2010 Posted September 17, 2010 I run BFG 255/60/15s on my 70 Monte (all original drivetrain) and my speedo reads about 7% too high. I think it's because my tires are a slightly smaller diameter than stock. I tested it during my drive to the western Meet using the mile markers. I tested it several time during the drive and got the same 7% result each time. You are running 245/60/15s which would throw it off even further (11% in your case). Quote
502ci Posted September 17, 2010 Posted September 17, 2010 Thats it, you guys are doomed.....now "The Man" can always keep an eye on you I think of that all the time...unfortunately no way around it that I know except turn the phone off and use a road map...can't even use mapquest and print out the directions as I bet that's traceable... Quote
Phil Posted September 17, 2010 Posted September 17, 2010 Originally Posted By: 7tonemonte Thats it, you guys are doomed.....now "The Man" can always keep an eye on you I think of that all the time...unfortunately no way around it that I know except turn the phone off and use a road map...can't even use mapquest and print out the directions as I bet that's traceable... The man is at the window..... the man is at the window!!! Quote
Sam (Bones) Posted September 17, 2010 Posted September 17, 2010 mine is about 5 mph off at 55, I just use my TomTom GPS Quote
ThirdMC Posted September 17, 2010 Author Posted September 17, 2010 OK, According to the Garmin...at 55-60 the speedo is about 3-4 mph off...speedo is faster. And 114 on the Garmin is just past 120 on the speedo (Or so I hear---Looking innocent) Quote
kc8oye Posted September 17, 2010 Posted September 17, 2010 if you go up one driven gear size you can probably correct that to be spot on Quote
ThirdMC Posted September 17, 2010 Author Posted September 17, 2010 You mean rear gear or on the tranny? Quote
stangeba Posted September 18, 2010 Posted September 18, 2010 Here are a few places to check out that should help. speedo calc site one more speedo calc site Quote
kc8oye Posted September 18, 2010 Posted September 18, 2010 driven gear.. the one that goes into the side of the tranny (speedo cable screws onto it) Quote
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