Hey guys so I switched my tires from BFG Sport Comp 305/35/20 to Goodyear Eagle F1 Super Car 3 305/30/20 and I couldn't find the tire circumference online anywhere so I took the measurement using a cloth tape measure and then inputted it into the speed odometer / tire size tab but my speed odometer is off by 4 mph. What do you guys reccomend that I do? Thanks
Joseph
There is a fudge factor in the original OEM tire size.. I don't know why.. it is what it is.
So if you put a "real" number for tire size the speedo will be off.
At this point do this.
GPS Speed on flat level straight road / indicated speed * tire size in hptuners.
The faster you are when you take the measurement the better it will be.. so do it at 70mph if you can.
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You can always just tweak it by the percentage it is off. If that 4mph at 60mph, add or subtract 15% to the circumference.
https://tiresize.com/comparison/
Do the math with the revs per mile and multiply or divide the rev per mile in the stock calibration depending on if it is a bigger or smaller diameter tire. Smaller diameter = more revs bigger = less revs.
Last edited by 2xLS1; 01-27-2023 at 09:02 AM.
That's really weird because with the tire size calculator that 2xLS1 posted, that is exactly the size I put in and it still reads 4 mph more than what I'm really doing. But how accurate are the gps in our phones, I thought that I had read their pretty much dead on but not 100% positive. Maybe I should just play around with it until I get it right. What do you guys think? I wouldn't have thought that going from a 35 series to a 30 series wouldn't be that big of a difference but it is because the finder gap now is huge and now I scrap the driveway every time I come or go.
Like was said you don’t put in the actual size a calculator says. You adjust by the percent difference between the two sizes.
The phones are very accurate. Just do a small size change of about a half an inch one way or the other. If it shows you are going faster than you actually are GPS lower than speedo just increase tire size a little. If GPS shows you are going faster than the speedo, reduce tire size. I like to do about 75 for accuracy sake.Originally Posted by morepowerjoe
On gen 4s if you use the wizard many times you will end up with a abs inop light or brake light on so I always adjust every revs per mile field then manually scale the shift and lock up tables by the inverse if an auto transmission. On gen 3 cals like a P01 or P59 just subtract .8 from what the T/S calculator says and use the wizard.
oh ok I get it now. Thanks. And thanks Alvin and Gtstorey
Thank you to sir
what do I do for the distance - trans? under sensor calibration, tire revs per, dist - trans 5006
Last edited by morepowerjoe; 01-28-2023 at 11:06 AM.
Did not know that. Do you know why? Never ran into this although I usually get it right on the first or second shot. Maybe some form of counter on a chip that says f**kit after so many adjustments?
Well I got it within 2 mph under the actual speed. So you say to increase the tire circumference by a half inch or so higher but do I need to adjust the tire revs per too or not? And one more question when I write the tune to the ecm do I need to write the transmission for it to work or just the engine or both? Thanks
If you are within 2 mph at 70 you are good. I as a habit program both. When you use the tire/gear wizard it does all the math for you so you don't have to.
Ok thank you for your help.
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