Re-calibrating the speedometer?

[Prowelder]

Hey guys,I have noticed since my tire change that my speedo is off about 3-4 mph. I thought I read somewhere on how to re-calibrate it but to normal so it and my odometer works right?:shrug:Thanks,Winslow

[Ops]

Same sit here. I'm running 285s and I'm off by about 10% or so.Ops

[Mopar1973Man]

Smarty or Dodge Dealer... The flash has to recalibrate the ABS and Speedometer. Oh... If your speedometer is off say 10% then so is the odometer off by the same amount...

[Prowelder]

Smarty or Dodge Dealer... The flash has to recalibrate the ABS and Speedometer. Oh... If your speedometer is off say 10% then so is the odometer off by the same amount...

Awesome man thanks!! So wonder what 4 mph would be, percentage wise? I bet the dealer would be expensive, I might have to talk to a guy and see if I can work out a deal!

[Mopar1973Man]

According to the tire calculator on the internet at...

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

If your stock tire size was 265/75 R16 (637 Rev/Mile) then to 315/75 R16 (583 Revs/Mile) makes you...

9.3% off and if your travelling 60 MPH your actually travelling 65.6 MPH

[Prowelder]

According to the tire calculator on the internet at...

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

If your stock tire size was 265/75 R16 (637 Rev/Mile) then to 315/75 R16 (583 Revs/Mile) makes you...

9.3% off and if your travelling 60 MPH your actually travelling 65.6 MPH

Well with that information I did a little math: I run 310 miles according to my odometer. But since I'm 9.3% off that makes it that I ran (310x.093=28.83+310)=338.83 miles really. Divide 338.83 miles by 19.902 gallons=17.024 MPG!!!!! Now I like the sounds of that a lot better!!!!!!:hyper:

However with the 285s the speedo seemed to be spot on so going with that, I am now 5.4% off. So I ran 326.74 miles so my milage would be 16.417 mpg. That's not as good as I would hope, but is probably more realistic. I'm going from the speed check radar deal that I've used to check my speedo with. Today it showed 62 and I was going 58-69 mph. Either way, I'm getting better that 15.5 and that makes me feel much much better!

Also that milage is running like 70-75 real speed at times averaging a little over 60 mph. Once I start driving normal speeds again that should help the mpg a little too.

[Mopar1973Man]

Ummm... Just for tossing out for information value... 2 Cords loaded on my truck each trip for total of 3 trips at 38 miles one way... 38 x 2 = 76 Miles round trip x 3 trips = 228 Miles total. Then a cord of dry wood is roughly 2,000 pounds so between the truck and trailer I had 4,000 pound of wood on. I know my truck gross weight is 7,900 empty. With 235/85 R16 hauling this wood for 1 day I still pulled 16.76 MPG (Hand Calc'ed) hauling heavy...

Just to rub it just a little I'm looking at my SGII and its displaying 20.7 MPG right now...

Oh yeah my Record lowest MPG ever is 14.10 MPG...

[Prowelder]

That might be true Mike but my truck looks meaner!!!!

[Mopar1973Man]

That might be true Mike but my truck looks meaner!!!!

I got to admit its deserves a "Thank You!" good looking truck...

[Prowelder]

Thanks Mike, I agree that the larger tires aren't the most practical but I'm young so looks are important!!! And really I don't "need" half the stuff in my sig but it sure makes it a lot moe' better!!:whistle:Also check out my Control Arm thread, it is going to be my write on install them!

[Blueox01]

I know someone is going to dispute this,but it's true when i swapped my NV5600 for the FSO8406A my speedo was 5 MPH of go to www.grimmjeeper/gears.html after awhile the speedo and odometer has corrected itself.:banghead:

[elshadow001]

That might be true Mike but my truck looks meaner!!!!

It looks crippled! has a walker holding it up!

I don't think I would race with a walker under it.

[Prowelder]

Just a cast and crutch!!!

[Northwoods]

For those of us with 12 valves you still have to switch out a gear, but first you have to choose the right one. I made the mistake of trusting the stealership to get the right part for me based on their charts, and $80 later the speedometer went from too slow to too fast. :banghead:I tried to do the math to figure out exactly what I needed but I got caught in the paradox of 4.11/3.55=115.8% versus 3.55/4.11=86.4%. Did I need to change the gearing 15.8% or 13.6%? :banghead: At the same time I went from a 31 inch tire to a 32, so that muddied it up even more. In retrospect, I'm pretty sure that this is what caused the dealer to give me the wrong part and it makes me question the accuracy of all the online conversion tables.After a few beers and remembering some algebra from 20 years ago, I figured out how to make the truck tell me the answer. :thumbup2:Here's what you do:First of all, you need to know what gear you're currently running, so pull the gear and read the stamp which tells you the number of teeth. Second, reset your trip meter and gps (or pull up even with a mile marker out on the highway)Third, drive the same number of miles (measured by gps or mile marker) as the number of teeth on your gearWhen you get to that point, look at the trip odometer, and it will tell you exactly which gear you need (round up or down to the nearest number).Here's how it works if you're actually interested in the math:In my case, I was running a 29 tooth gear, and drove 29 miles. Taking those first two known values I had 29 teeth/29 miles for a 1:1 ratio. On the other side of the equation I had 30.3 miles on the trip odometer. Since the first side of the equation was already set to 1, I knew I needed a 30 tooth gear to balance the second side. After ordering the right gear and putting it in, I double checked the odometer against the gps and I'm less than a mile off after a hundred miles of driving.If you need the gear, its $80 an up at the dealer with some of them going for over $100. I ordered mine through wermopar.com for $70. Later on I found out that there are aftermarket ones on ebay for $40. The dodge/jeep part #is 520676xx with the last two numbers being the number of teeth you need. I've got a spare 29 and a 36 if anybody needs them.