Formula for calculating tire size for Hypertech PP III
Thread Starter
Racer
Joined: Jul 2001
Posts: 281
Likes: 2
From: Henderson, NV
Formula for calculating tire size for Hypertech PP III
Okay, I know I saw a formula on some forum somehwere that showed how to calculate a new tire size to punch into a Hypertech PPIII if you happen to have 3.42 gears which the Hypertech does not support. But for the life of my I cannot find the post anywhere now and I cannot find the piece of paper that I wrote the formula on. :cry
Can some kind soul, repost the formula for me? I need to verify my calculations since my speedometer is still incorrect as compared to several other cars running side-by-side on the highway.
Can some kind soul, repost the formula for me? I need to verify my calculations since my speedometer is still incorrect as compared to several other cars running side-by-side on the highway.
Pro
Joined: Mar 2001
Posts: 711
Likes: 0
From: Orlando, Florida
Re: Formula for calculating tire size for Hypertech PP III (BluFRC)
Here's some info that someone else previously posted...
(real rear tire diameter) * (new gear ratio) / (Ratio inputted into HPP3) = (Tire Size to use)
For stock rear wheels, use the 3.15 gear setting and a 29" tire height. Works great!
For 295/35x18, I would use a 28.25" tire height.
Diameters:
295/35x18 tires = 26.1"
stock (275/40ZR18) = 26.6"
(real rear tire diameter) * (new gear ratio) / (Ratio inputted into HPP3) = (Tire Size to use)
For stock rear wheels, use the 3.15 gear setting and a 29" tire height. Works great!
For 295/35x18, I would use a 28.25" tire height.
Diameters:
295/35x18 tires = 26.1"
stock (275/40ZR18) = 26.6"
Thread Starter
Racer
Joined: Jul 2001
Posts: 281
Likes: 2
From: Henderson, NV
Re: Formula for calculating tire size for Hypertech PP III (goldman)
Thanx, that's the info I found before... I double checked my math and I did input the correct ratio so I wonder why my speedo is still that far off.
It's reading 70mph when I am really doing 75mph (as measured by several other vettes traveling side-by-side and communicating with radios).
Very strange...
It's reading 70mph when I am really doing 75mph (as measured by several other vettes traveling side-by-side and communicating with radios).
Very strange...
Pro
Joined: Mar 2001
Posts: 711
Likes: 0
From: Orlando, Florida
Re: Formula for calculating tire size for Hypertech PP III (BluFRC)
Hypertech (& superchips also) did not properly correct a 3.42 axle change for me!!!
My Hypertech even had an option for 3.42, when that didn't work I tried the other "mathmatical" selections, but nothing worked properly. My speedo didn't seem too far off, but I was getting error codes...
Mine was showing error codes & (occasionally) getting the "check engine" light. The errors would sometimes only show up after a couple days of driving (more often during highway driving)... I don't remember what error codes they were, but they were related to Transmission Slip (P1861? maybe?) & Traction. Watch for these error codes!!! Hypertechs do not seem to work for all people, on all cars. Others forum users have also reported problems, although some people do not have problems.
I had to go to a Powerloader to get the 3.42 change properly corrected, But I'm glad I did, I'm MUCH happier with the P/L... it's better in all aspects than the Hypertech..
Read my related post here.... http://forums.corvetteforum.com/zerothread?id=133505
[Modified by goldman, 6:46 AM 9/1/2001]
My Hypertech even had an option for 3.42, when that didn't work I tried the other "mathmatical" selections, but nothing worked properly. My speedo didn't seem too far off, but I was getting error codes...
Mine was showing error codes & (occasionally) getting the "check engine" light. The errors would sometimes only show up after a couple days of driving (more often during highway driving)... I don't remember what error codes they were, but they were related to Transmission Slip (P1861? maybe?) & Traction. Watch for these error codes!!! Hypertechs do not seem to work for all people, on all cars. Others forum users have also reported problems, although some people do not have problems.
I had to go to a Powerloader to get the 3.42 change properly corrected, But I'm glad I did, I'm MUCH happier with the P/L... it's better in all aspects than the Hypertech..
Read my related post here.... http://forums.corvetteforum.com/zerothread?id=133505
[Modified by goldman, 6:46 AM 9/1/2001]
Melting Slicks
Joined: Jul 2001
Posts: 3,284
Likes: 3
From: Southwest Florida
Re: Formula for calculating tire size for Hypertech PP III (BluFRC)
It's reading 70mph when I am really doing 75mph (as measured by several other vettes traveling side-by-side and communicating with radios).
Very strange...
Very strange...
Mike
Thread Starter
Racer
Joined: Jul 2001
Posts: 281
Likes: 2
From: Henderson, NV
Re: Formula for calculating tire size for Hypertech PP III (goldman)
The weird thing is that I didn't change gears, my FRC has 3.42 gears from the factory but the HP III does not support that ratio when specifying a new gear ration. But when you change the tire sizes (which I did) you are forced to set the gear ration -- but alas we are back to square one since they do not list 3.42 as an option for the Corvette LS1.
It sounds like I'll end up with a PowerLoader eventually, but since I am still tweaking things and yet to do the serious mods (cam, head, intake) I hesitate to order something that would require me to spend more money everytime I need to make an adjustment... Plus the inabiity to go back to stock makes certain trips to the dealer a pain.
I want the best of both worlds, the flexibility of the HPIII for the basics, but the tuner skills from the PL guys...
It sounds like I'll end up with a PowerLoader eventually, but since I am still tweaking things and yet to do the serious mods (cam, head, intake) I hesitate to order something that would require me to spend more money everytime I need to make an adjustment... Plus the inabiity to go back to stock makes certain trips to the dealer a pain.
I want the best of both worlds, the flexibility of the HPIII for the basics, but the tuner skills from the PL guys...
Pro
Joined: Mar 2001
Posts: 711
Likes: 0
From: Orlando, Florida
Re: Formula for calculating tire size for Hypertech PP III (BluFRC)
The weird thing is that I didn't change gears
If you go from a 275/40 to a 285/40 the difference is only 0.31"diameter... at 70 mph it should only be off -0.82 mph..
Here's a handy calculator that will compare tire sizes & speedo error..
http://www.c5-corvette.com/tirecalc.htm
[Modified by goldman, 3:33 PM 9/1/2001]
Thread Starter
Racer
Joined: Jul 2001
Posts: 281
Likes: 2
From: Henderson, NV
Re: Formula for calculating tire size for Hypertech PP III (goldman)
Did you try going back to stock & then install the HPP without changing the tire/axle options at all... Unless you made a drastic change in the tire diameter, it may not be far off just leaving it alone...
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Safety Car
Joined: Apr 2000
Posts: 3,802
Likes: 61
From: Wichita KS
Re: Formula for calculating tire size for Hypertech PP III (BluFRC)
BluFRC, you have to input the new tire size (height from the ground to the top, or make a mark on the sidewall and the floor, roll the tire untill the mark is again lined up on the floor, make another mark on the floor and measure the distance between the two marks and divide the result by 3.14 and that will give you the tire size to input), then when you come to the axle ratio select no (it will leave the stock ratio in there) then select no for change the tire size (you already changed it once). I did that yesterday and it worked like a champ. Went racing on 17" DRs with no problems.
If I confused the heck out of you e-mail me and I will try to be clearer.
cc
If I confused the heck out of you e-mail me and I will try to be clearer.
cc
Thread Starter
Racer
Joined: Jul 2001
Posts: 281
Likes: 2
From: Henderson, NV
Re: Formula for calculating tire size for Hypertech PP III (CCrane00)
When I input a new tire size, the PP III forces me to choose a new gear ratio size. The problem is that 3.42 is not one of the choices (even though they are stock on my FRC). Even the documentation explains this behavior. Does your PP III behave differently?
Safety Car
Joined: Apr 2000
Posts: 3,802
Likes: 61
From: Wichita KS
Re: Formula for calculating tire size for Hypertech PP III (BluFRC)
No, mine behaves the same way. But 3.42 is the stock ratio so I don't change it, I back out of the mods leaving only the tire size changed. Even though it looks like you are backing out of the tire size change, you have already change it in the HPP III so that is what it puts in. To verify, go all through the options a second time and you will see the new tire size is still there.
cc
cc
Drifting
Joined: Oct 2000
Posts: 1,395
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From: Stockbridge GA
Re: Formula for calculating tire size for Hypertech PP III
Why doesn't the Hypertech support 3.42 gears? Isn't that what comes with all manuals?
Sounds like Hypertech did a poor job if the didn't have a good reason for not supporting 3.42's.
Sounds like Hypertech did a poor job if the didn't have a good reason for not supporting 3.42's.
Safety Car
Joined: Apr 2000
Posts: 3,802
Likes: 61
From: Wichita KS
Re: Formula for calculating tire size for Hypertech PP III (rcwalters)
rcwalters, it does support 3.42 gears. That is the stock gears for a 6 speed so there is no need to change anything. The HPP III assumes 3.42 by default. The only time you have to change the gear ratio is if you don't have 3.42 gears.
Burning Brakes
Joined: Jun 2000
Posts: 1,122
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From: Annapolis, MD USA
Re: Formula for calculating tire size for Hypertech PP III (rcwalters)
My Hypertech allows selection of 3.42 gears, but not 3.73s (which I have installed). However, this doesn't matter for the speedo calibration, at least on the six speed. Since the speed sensor picks up the signal from the carrier gear, it essentially reads axle speeds, so no recalibration was necessary when I switched rear end ratios. The speedometer automatically compensated for the different ratio. I would assume this would be true also for automatics.
The Hypertech gives me no problem in changing tire diameters. However, I would caution against trying to calculate tire diameter theoretically - just measure it by rolling the tire one, two, or more rotations (the more the better for accuracy) and divide by pi x number of rotations for an exact rolling diameter. My 295/35-18 Bridgestones are 26.1 inches in diameter. 26 inches in the Hypertech is close enough.
The Hypertech gives me no problem in changing tire diameters. However, I would caution against trying to calculate tire diameter theoretically - just measure it by rolling the tire one, two, or more rotations (the more the better for accuracy) and divide by pi x number of rotations for an exact rolling diameter. My 295/35-18 Bridgestones are 26.1 inches in diameter. 26 inches in the Hypertech is close enough.
