Will 255/50/15 rear and 245/50/15 fronts mess up the speedo on a firebird that has 225/65/15 stock all around? OR will it mess up the computer shift points???


Also the car has 3.42:1 gears in the rear, but doesn't have posi... what posi unit will bolt into a 91 firebird?

thanks



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91 Firebird
3.1 w/
K&N filter, 160 stat, No TB heat, Direct Cold air, DynoMax CatBack, B&M MegaShifter, Performance Shift Kit,
Bosch platinum plugs,
7mm "stock" wires, Jensen CD330X Head unit, Jensen 120watt rear speakers
Stock front speakers,

0-30mph 4.25 Best 4.09
0-60mph 9.16 Best 9.09

 

i'm not a huge expert on tires or sizes, but my guess is that they'll fit. Also, Tom P is running larger tires on the rear, and i think things are o.k., but you might have to have your speedo re-calibrated, because of a tire size different than stock, also larger (taller) tires are going to give your rear axel a lower ratio, so 3.42, might be more around 3.23 or so. As far as the rear ends, any rear out of a 3rd gen camaro or bird will go in, just find a Z28, Iroc Z, Trans Am, or Formula, most of them should have posi units, you can also check any others by raising the rear end, and spinning one side, if the other turns in the same direction than you have posi, if it turns in the opposite direction you don't. K. hope that helps, also ask tom P about the posi end swap, he could helps lots, plus if you get one with rear disc brakes, than you'll have more work instore for you with the master cylinder and brake lines. but it's all fun stuff, at least your car will like it.

 

Thanks for the info on the tires, i figured i would have to recal the speedo, i am just worried that the new tires will mess up the computer shift points ect. I would think that the computer would "relearn" the shift points/RPM's tho. I know the 255's will fit in the wheel well, upfront tho is a different story, they rub from what i have heard.

On the posi i am looking at buying a unit new, not swapping rear ends, i was hoping to be able to get the parts new rather then have to pull a rearend outta something. I saw another post that 89 and newer used 7 5/8 28 spline in the 10 bolt rears, so i am assuming i am looking for a posi unit that will work in that. I am gonna make some calls today. I am thinking about going to 3.73:1 gears as well, the 342's that i have are great and don't want to spend money to lose pickup in the 0 to 60 department if i get different tires. Anyways thanks for the heads up.




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91 Firebird
3.1 w/
K&N filter, 160 stat, No TB heat, Direct Cold air, DynoMax CatBack, B&M MegaShifter, Performance Shift Kit,
Bosch platinum plugs,
7mm "stock" wires, Jensen CD330X Head unit, Jensen 120watt rear speakers
Stock front speakers,

0-30mph 4.25 Best 4.09
0-60mph 9.16 Best 9.09

 

Like 85f-bird said, I didn't have to recalibrate my speedo because I'm exactly where I should be.

I have 3.73 rear gears and P275/60r15 rear tires. This calculates to an effective gear ratio of 3.46. What this means is, my current setup is the same as if I had "true" 3.46 rear gears and the stock sized P215/65r15 tires on the back.

Since my effective ratio is close to my stock ratio of 3.42, my speedometer's close enough for me. However, when I was running the P275/60r15 tires with my 3.42 gears, my effective ratio was a 3.22- which, in turn, made my speedometer read about 12% slower than I was actually going- not good for speeding tickets!

I needed to "modify" my rear inner wheel wells with a sledge to fit these tires.


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-Tom P (Hot rodded 1986 Firebird 2.8l) from http://www.f-body.net/mailbag/3rd/3rd_mailbag.html message boards
---Think your car could be pic of the week? Visit http://www.f-body.net for details!

 

Ok i redid my message as i found better tires from tire rack. They are the Potenza RE71's. Anyways after playing with a tire gauge program off of ask.com i found out my stock 215/65/15's are 26.0 inches around and rotate 776 times per mile. To switch to a rear tire of 255/50/16 would do this, drop the sidewall .5 inches, widen the footprint of the tire and it is 26.0 inches around and goes around 775 times a mile for a tire to tire difference of .1%. That prolly wouldn't show on the speedo, the computer wouldn't know it plus i can grip better, and handle better. Also the effective gear ratio stays 3.42:1 which is great if i wanna goto 3.73:1 for a better 0 to 60 time. Whew.... Tom i do my home work right????


[This message has been edited by Red91Bird (edited January 22, 2001).]

 

Yup, looks good. You get an "A" on the HW! That error's really minimal. I usually use http://www.tolan-hoechst.com/cars/tirecalc.htm for a calculator, I'll have to check out that link you gave. On the one I just gave, change the "wheel rpms @ ?? mph" to the right mph, put in your gear ratio at "engine rpms with ?.?? rear gear", and change the transmission gear to "1.00" instead of 0.70. Don't worry about wheel width or wheel offset, I usually leave those alone.

One thing tho- you said you found P255/50/16 tires... originally you wrote P255/50/15 tires. I thought you were going for 15 inch rims? You know you'll have to buy 16 inch rims to fit the P255/50r16's, right?


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-Tom P (Hot rodded 1986 Firebird 2.8l) from http://www.f-body.net/mailbag/3rd/3rd_mailbag.html message boards
---Think your car could be pic of the week? Visit http://www.f-body.net for details!

 

Yeah, unfortunately i will have to get new rims, which bites... I am gonna look at getting some Cragar SS tho, not sure if they make em in 16's, is there a site that tells ya what tire fits what rim? I.E 255 fits a ?? inch rim... I went with the 16's cuz i want to keep the error to a minimum and i want the ability to goto a steeper gear (3.73:1) in the future. I got a BIG project underway and the 3.1L block just came back from the shop <----- hence i need traction. All i need now is the SLP drive shaft and the custom chip. I am figuring 210 to 235 rear wheel HP before the 90 shot of funny gas. Anyways thanks for the checking... Mind has been numb as of late.




------------------
91 Firebird
3.1 w/
K&N filter, 160 stat, No TB heat, Direct Cold air, DynoMax CatBack, B&M MegaShifter, Performance Shift Kit,
Bosch platinum plugs,
7mm "stock" wires, Jensen CD330X Head unit, Jensen 120watt rear speakers
Stock front speakers,

0-30mph 4.25 Best 4.09
0-60mph 9.16 Best 9.09

 

If your original tires are 225/65/15s (26.52" tall), then 255/60/15 would only be slightly taller (27") than your original 225/65/15s. 255/50/16 (26.0") or 275/50/15s (25.8) are slightly smaller than 225/65/15s. All these differences are within 2%. But those are your choices in common tire sizes.

I have my own converter that calculates it based on the mathematical formula. But, the overall diameter will vary from tire to tire based on the manufacturer's tread pattern and depth. I have found a 1% change just from the tread wear. But if your speedo is slightly off (they tend to be slightly under), you will probably find 255/60/15s corrects your speedo perfectly.

[This message has been edited by Glenn91L98GTA (edited January 22, 2001).]

 

just to add.... you should be running 215/65/15's or 235/60/15's w/ the 15" wheels from the factory.

4 days ago I finally went from 15's to 16's and i'm indeed happy. it handles better and the car is faster (in my head) i went from 275/60/15 in the rear to 235/55/16 (kid's tires were the wrong size) so soon I'll remedy that by replacing all the tires w/ 245/50/vr16's it feels faster cuz the speedo is finally on.

hey tom, isn't the fact that you are running 275/60's actually ruining the fact that you have 3.73's on the rear? wouldn't running the stock size actually make your car faster? in spring I'll be able to pick up an '85 firebird that's a 4 banger so it has 3.73's. (darn winter) first thing I'm gonna do is rip out that rear, and swap it into my maro.
and how does it feel now? Cuz the second I went down the road w/ the right size tires, and larger diameter wheels, I could spin em, easily. do the 3.73's fool people in thinking more is under the hood than they think?

had to delete some, I ramble too much.

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• 1988 Camaro Sport Coupe
• 2.8 V6
• TCI 3200 Stall TC
• B&M Transkit
• 2.5" 80 Series Flowmaster & Catback
• Hi-Flo Kitty
• '83 Z/28 Hood
• '87 IROC Wheels
• '92 Z/28 Bowtie Grille
• IROC Gauges
• IROC Taillights
• IROC Foglamps

[This message has been edited by SlowCamaro88SC (edited January 25, 2001).]

 

Actually I put the 3.73 axle in to "correct" for my large sized tires! But yup, you're right, the car might be slower because of it.

Heck with the 3.42 axle and the large tires my effective ratio was a 3.18! (I typo'd the 3.22 previously, I went by memory, now I went by the webpage I just posted.)

How-ever, in 1996, with NO mods, and even larger tires (P255/70r15) on the 3.42 axle, my effective ratio was a 3.08 - and I ran a 16.87 1/4 mile. Obviously, "just" the 3.73's and shorter P275/60r15 tires should've made me faster- ignoring all the mods since then. Something else is slowing me down.

I'm not out to fool anyone, I don't know what you mean by that. It wasn't my intent when I swapped the axle in. If anyone was fooled, they were fooled long before I swapped axles.


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-Tom P (Hot rodded 1986 Firebird 2.8l) from http://www.f-body.net/mailbag/3rd/3rd_mailbag.html message boards
---Think your car could be pic of the week? Visit http://www.f-body.net for details!

 

no I mean doesn't it make them think there's a v8 in there since it's launching (incredibly, I'm guessing) hard, and it's running through the R's pretty quickly? I'm curious to see how it will feel before I even do the swap in spring.