SSpdDmon

iTrader: (10)

One of the first things I changed when I received my copy of EFI Live was the speedo. It was no wideband O2 SD tune, rather just a place to start. Anyway, I go to the right table and there was a sweet tire height & gear calculator that adjusted the right fields and all. So, I put in my new size (275/40/17) and it came back with 25.66". However, after a quick measurement the other day (after a discussion I had with a friend about real tire heights), I found that my 17's were only 25" high. Not a huge difference, but big enough when it's the difference between a 79 in a 70 vs. an 81 in a 70. No, there were no massive burnouts done on these tires and no I didn't get a ticket. My point in saying this is....don't always trust the calculators you come across. EFI Live is a wonderful product and I'm completely satisfied with it. But, there are just some things in the REAL world that calculators won't have the right answers for all of the time. So, keep an open mind when your tuning...and if there's something simple you can do (like measuring your tire height) to double check what your changing, check it and don't just assume the answer you have is golden. That's just my $.02 .

WS6FirebirdTA00

iTrader: (33)

good point. ill try this out on my car. i know from the factory the settings are lower than the calculator shows. how did you measure it? using the circumf.??

WS6FirebirdTA00

iTrader: (33)

im wondering if gm set it up to be the half way wear point in the tread?

soundengineer

iTrader: (16)

the only accurate way is by using the circumference...
not all tire manufactures follow *strict* standards...
I have had 3 different tires brands all say 275/40/17...but you can see a difference in the footprint it leaves...and all 3 measured a slightly different circumference

12secSS

iTrader: (30)

The circumference is not correct either, as it does not take into consideration the weight placed on the tire. You need to measure the tire height as it is used, hot and mounted. Take a trip around the block to heat up the tires (no, not burnouts ... just regular driving) then park the car and measure from the top of the tire to the ground. This is the most accurate way of determining the actual tire height. If you want to be ****, acceleration also plays a role in tire height, which is reduced. Be sure the tire is at the optimum pressure prior to making any measurements.

Note: I do not go this far to get the tire height, I an not that **** about the exact speed of the vehicle. If I was I would replace all the gauges with very accurate (expensive) gauges, which the OE are no where near, just good enough.

WS6FirebirdTA00

iTrader: (33)

most it will change is 1-2 mph anyway, less than the error the speedo output is anyway im sure

Blacker

iTrader: (3)

I used the 25.66" Tire height and this was for a 275/40ZR17. With HPT, I multiplied the tire diatmeter by .974 and used that diameter. The local police had a radar trailer set up and verified several speeds and all were spot on.

WS6FirebirdTA00

iTrader: (33)

whats the reason for that mult factor, just a number you came up with or is there more reason behind it. like is that the actual/calc.?

SSpdDmon

iTrader: (10)

No, Flame is right. You want to use tire height (diameter). At least that's how it's done in EFI Live. I just find it odd that it's off by over 1/2 inch. A tenth or two, I wouldn't care. But like I said, my friend and I were discussing tire heights and their effects on your final, final drive ratio (he uses 26" MT's, which is the equivalent of turning his 3.23's into 3.11's). Then I got side-tracked to whether or not my tires were the right height in my tune.

RedHardSupra

iTrader: (1)

i use 24.91 for my 315/35-17 BFG KD's. according to the calculator in HPT i should have used 25.7. quite a bit of difference there.

WS6FirebirdTA00

iTrader: (33)

tahts what i was getting at, the diameter from that calc. just didnt know how it was done. i was gonna say load and speed will change the size a decent bit

EFILive

iTrader: (2)

To clear up any mystery about how EFILive calculates tire diameter...it uses the following formula:

t = (r*25.4+(2*p/100*w))/25.4
where:
t = tire diameter in inches
r = rim size in inches
p = profile in %
w = width in millimeters

It does not attempt to take into account variations due to many of the reasons posted in the previous threads.

Regards
Paul