Was talking with another board member about a way to make the first gear of my Isuzu 5 speed a little taller (and I guess it would make the other gears taller too). But, has anyone done any real calculations or have real life experience. It bugs me how fast 1st gear is over.
Thanks
Oh, it's an 88GT, 4.9, daily driver , currently running 315/35-17 on the back
[This message has been edited by michfiero (edited 08-23-2010).]
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09:26 PM
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TONY_C Member
Posts: 2747 From: North Bellmore, NY 11710 Registered: May 2001
You have to run 50 series tires on the rear. I have an 84 muncie with 4.10 final drive and 0.81 fourth gear. I went from 215/60/14 to 235/50/17 and reduced the gearing by around 8%. When I'm going 70, my speedo reads about 63mph. My 4.10:1 is around 3.8:1 now. go here http://www.miata.net/garage/tirecalcold.html
[This message has been edited by TONY_C (edited 08-24-2010).]
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09:40 PM
Aug 24th, 2010
Fieroseverywhere Member
Posts: 4242 From: Gresham, Oregon USA Registered: Mar 2006
I run a 225/55-16 on the rear which come out to a 25.7". This is a full 1.5" larger then the original tire size my car came with. I will be going larger just on the rear when I need to replace my tires again. Depending on width, strut/spring clearance, ect, up to 27" diameter may work on the rear. I plan on using a 245/50-17 (26.6") for the next setup. The front is a bit different depending on ride height and especially if you have an 88. On my 85 i'm already at my max diameter for the given ride height and wheel offset. Currently running a 25.6" diameter. You have to take into account the ride height increase adding larger tires does also. For a 1" diameter increase you will get a .5" ride height increase. It takes some math to get everything right. Its a system and you are changing a main part. It effects everything else so you have to take it into account.
As for the actual use. I have put many miles on these diameters with few issues. The only issues I"ve had were while I was dialing in the ride height. Have run the isuzu, getrag, and V6 4speed transmissions. Everything else was rebuilt when installed. Suspension bushings, shock/struts, stiffer springs, bearings, stronger brakes, everything. Brakes are very important for the increase in diameter. I would not recommend it if you have stock brakes.
MPG only got a slight increase (as did top speed). 1st gear is still rediculously short with the isuzu, better with both the other trans. It took a little bit to fully dial in the ride height. I like the setup and would deffinately do it again. IMO the fieros need the larger tires in the looks department also. Fills in not only the top wheel well gap but the sides aswell. This is another reason why the front is so limited.
Thats my experiance. Oh, also running a 4.9. Without a PCM you can program the tire size into (such as stock engines) any change will throw off your speedo. If your going to do it, weigh the pro's and con's and do your math. It worth it if done right.
[This message has been edited by Fieroseverywhere (edited 08-24-2010).]
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10:48 AM
TONY_C Member
Posts: 2747 From: North Bellmore, NY 11710 Registered: May 2001
I would not recommend it if you have stock brakes.
This is very true. I just went from the stock 14" to 17" rims and I noticed a difference in braking. The 12" Vette swap is next on my to do list. Besides, stock brakes look ridiculous behind big rims.
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11:05 AM
82-T/A [At Work] Member
Posts: 25625 From: Florida USA Registered: Aug 2002
I run a 225/55-16 on the rear which come out to a 25.7". This is a full 1.5" larger then the original tire size my car came with. I will be going larger just on the rear when I need to replace my tires again. Depending on width, strut/spring clearance, ect, up to 27" diameter may work on the rear. I plan on using a 245/50-17 (26.6") for the next setup. The front is a bit different depending on ride height and especially if you have an 88. On my 85 i'm already at my max diameter for the given ride height and wheel offset. Currently running a 25.6" diameter. You have to take into account the ride height increase adding larger tires does also. For a 1" diameter increase you will get a .5" ride height increase. It takes some math to get everything right. Its a system and you are changing a main part. It effects everything else so you have to take it into account.
As for the actual use. I have put many miles on these diameters with few issues. The only issues I"ve had were while I was dialing in the ride height. Have run the isuzu, getrag, and V6 4speed transmissions. Everything else was rebuilt when installed. Suspension bushings, shock/struts, stiffer springs, bearings, stronger brakes, everything. Brakes are very important for the increase in diameter. I would not recommend it if you have stock brakes.
MPG only got a slight increase (as did top speed). 1st gear is still rediculously short with the isuzu, better with both the other trans. It took a little bit to fully dial in the ride height. I like the setup and would deffinately do it again. IMO the fieros need the larger tires in the looks department also. Fills in not only the top wheel well gap but the sides aswell. This is another reason why the front is so limited.
Thats my experiance. Oh, also running a 4.9. Without a PCM you can program the tire size into (such as stock engines) any change will throw off your speedo. If your going to do it, weigh the pro's and con's and do your math. It worth it if done right.
I went from 195/70/14s all around on HiTechs to 235/60/15s with in the rear with 86GT wheels (and 205/60/15s in the front I think).
How would that have affected my speedo? I made no change to my speedo at all. I would love to know. At one point in my really immature days like 11-12+ years ago... I pegged my speedo past the 120 mark (to what would be I guess 130).
I still had another 800rpms left on the tach before the red area, but it just didn't have enough power. FLOORED, the car just maintained speed at about 130. Was I actually going 130? Or was I going slower? Maybe faster? I'd love to know.
So I don't get persecuted for my immaturity... I haven't been over 80 (even on the highway) in like the past 6-7 years. I usually drive 5 miles under the speed limit... (I have a daughter now... really wakes you up).
Originally posted by TONY_C: You have to run 50 series tires on the rear. I have an 84 muncie with 4.10 final drive and 0.81 fourth gear. I went from 215/60/14 to 235/50/17 and reduced the gearing by around 8%. When I'm going 70, my speedo reads about 63mph. My 4.10:1 is around 3.8:1 now. go here http://www.miata.net/garage/tirecalcold.html
something I need to do as well. I got the 4speed w/4.10 gearing & still have the 215/60/14's, and freeway driving is quite the rev-fest turning 3500 to do 70 I did the 4.10 gearset with the intentions of putting larger wheels on one day - just hasnt happened yet
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11:41 AM
TONY_C Member
Posts: 2747 From: North Bellmore, NY 11710 Registered: May 2001
something I need to do as well. I got the 4speed w/4.10 gearing & still have the 215/60/14's, and freeway driving is quite the rev-fest turning 3500 to do 70 I did the 4.10 gearset with the intentions of putting larger wheels on one day - just hasnt happened yet
Yeah, I drive about 600 miles a week and it's almost all at 70 so the revs were killing me, especially on a Duke. It dropped down to where at 70mph, I'm right about a 3000 rpm. Hasn't had any effect on my gas mileage though, I was hoping to see some increase, maybe 2-3 mpg more but it's about the same.
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11:45 AM
82-T/A [At Work] Member
Posts: 25625 From: Florida USA Registered: Aug 2002
Any change larger or smaller will throw it off. Usually within 3% is hardly noticable until you get above 120mph. Its a percentage so low speeds may not be off much at all. The faster you go though the worse it gets. Without the speedo correction at 60mph I would actually be doing ~64. This is with ~6.6% difference@60mph. 215/60-14 (24.2") to 225/55-16 (25.7").
quote
Originally posted by TONY_C:
This is very true. I just went from the stock 14" to 17" rims and I noticed a difference in braking. The 12" Vette swap is next on my to do list. Besides, stock brakes look ridiculous behind big rims.
I'm using the 12" Vette rotors with 88 fiero calipers (on this 85GT). They work good for this. I'd like to go with wilwood calipers for a future upgrade. A little more clamping force and possibly the booster upgrade couldn't hurt.
Its actually worse when you use larger tires over just larger wheels. The added weight is carried at the very outside edge of the rotating wheel. This makes it even harder for the brakes to do their job because of the momentum. Even if you manage to get the total weight of the wheel/tire combo down from stock the placement of that weight has still changed. Weight placement is a big one. Its the little details that make this difficult to do right.
I love my setup but it was deffinately a learning process.
[This message has been edited by Fieroseverywhere (edited 08-24-2010).]
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02:35 PM
FastIndyFiero Member
Posts: 2546 From: Wichita, KS Registered: Aug 2002
------------------ My Web page | The Turbo Super Duty Build. You know that little voice that says it can't be done? I duct-taped its mouth shut and pushed it down a flight of stairs. (Leader of the Insurgency)
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03:51 PM
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aaronkoch Member
Posts: 1643 From: Spokane, WA Registered: Aug 2003
Hah... no, really. I've REALLY calmed down over the past several years. The last time I drove anywhere near that fast, it was the Richard Petty Driving experience on a track. I honestly haven't gone that fast on public roads in like probably 10 years. I think I might have hit 100 once in my Grand Am just because I was being an idiot... and that was like in 2001 or something.
I was still a little "hyper" when I got my 2006 Pontiac Solstice in December of 05... but I never speeded with that thing. I would just wind out the gears from a dead stop... but I never really drove over the speed limit. I had really calmed down some.
Since my daughter was born almost 2 years ago, I've REALLY, REALLY calmed. I drive an Explorer now... and I just putz along in the right lane now.
My Fiero hasn't run well in like 5-6 years. Heh...
See cave... Gear Ratio in axle & trans section, "How are final ratios and tires related?"
Taller tire Will effect all gears... But "Final" gear will fix it for trans. (assume already not lowest final...)
------------------ Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should. (Jurassic Park)
