I'd go with whatever the tire manufacturer says. You can always verify with gps.
just installed 315/35/20 tires. Is the speedo off or not?
- Thread starter black2007shelby
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No offense taken. I'm simply passing on a common error most people make by using inflated diameters to make comparisons and calculations when changing tire sizes.
Another way to determine the loaded diameter is to make a mark on upper portion of the sidewall and measure the distance between the points the marks make contact with the pavement. Divide 63,360 (inches in a mile) by that result and you have the number of revolutions per mile that mounted tire turns. It's pretty accurate when you can't find manufacturer's specs.
I'm running Toyo R888 315/30 20. Mfgr says rpm is 754. Online calculator says 735. I set my rpm at 735 and checked with Garmin gps. At 80mph showing on Garmin, 80mph is showing on my speedometer. I haven't tried it at 754.
<added> FWIW a cop friend of mine was running laser as I was going home. He's a Mustang guy and I stopped to talk him for a few minutes. He said laser showed 39mph. My speedometer showed 40mph. </added>
<added> FWIW a cop friend of mine was running laser as I was going home. He's a Mustang guy and I stopped to talk him for a few minutes. He said laser showed 39mph. My speedometer showed 40mph. </added>
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Speedometers are never all that accurate, so even using the correct data, it might not put you spot on with a GPS. With the 3% difference you have between the two factors, your speedo could be off by 2 mph at 80 (half that at 40) using the right data. As small as the scale is on the Shelby, it's hard for me to tell the difference. It's likely to show up more on odometer readings. Having the right rev per mile data is a little more critical with an auto trans. I'm not sure an error rate that small would have any impact on ABS functions.
SO right now mine gps and speedo read the same, if i changed the rpm on the sct , its probalby gonna read different. This is confusing
stock 3.31 gears, bought the sct new with custom jonlund tune in there , its should already be program for stock gear and tire. One other thing i should mention is that after install these steeda wheel and tire there are no "tire sign" on . The wheel i ordered should not come with any tpms sensor in there. This is weird.
So, the way I see it you did not tranfer your stock TPMS to the Steeda rims or purchase new/used TPMS for the Steeda rims. Is that correct?
Without TPMS on your Steeda rims you will always have a tire fault light.
Without TPMS on your Steeda rims you will always have a tire fault light.
How does the car feel with that height of tire and stock gears??
Looking at doing a similar setup.
Does it feel a little sluggish at low RPM's?
Looking at doing a similar setup.
Does it feel a little sluggish at low RPM's?
If it reads the same as your GPS then it's perfect. Don't mess with it. It simply means your speedo was off before the tire swap. Very common.
I run Nitto Invo's at 315 and my speedo is dead on. GPS, and Speedo dead even with each other. If you need to adjust, just do it in your xcal. Best way to know is to go to the manufacture web site, and look up your tire, and what the settings should be if needed.
Like others have said, if the GPS and speedo match, then your good.
Like others have said, if the GPS and speedo match, then your good.
been driving the car for a while with new wheel and no light on yet. Purchased wheels tires without tpms in there, even if it accidently in there i think u still have to program it . Very wierd
the front steeda/tire combo weight 6lbs each side more than factory wheel/tire , the rear 10lbs each corner . It feel a little sluggish off the line on low rpm , slow driving. Probably cuz the height and weight of the rear tires. When u floor it hard on 1 st gear the rpm tach kinda over rev fast other than that they hook up pretty good , 2nd gear did not spin at all
The weight is a factor too, but the biggest factor is that you are gearing the car down with a taller tire.
If I went with that tall of tire I would do 3:73's at least.
Thanks for the info!
Sick car BTW. Reminds me of someone else's i know...
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