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Old 08-11-2012, 01:37 PM
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Wheels, tires, gears.

I'm buying new wheels and tires for my 78 firebird and want to get gears too. I'm going with a 15x8 wheel. Stock converter with a th350. Mild cam. Rpm range is supposed to be 1500rpm up.
What tire size would look/perform best?
I want it to have some pick-up but i also need it to be able to cruise at 75mph while getting the best mpg possible at that speed.
Thanks in advance

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Old 08-12-2012, 08:59 PM
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Depends on what gears you get. My '72 Camaro has 3.73s and I found that even with a 3000rpm converter and a pretty overlappy cam (3000-6000 suggested powerband) highway travel at 65 (about 3200rpm) was tedious, and you want 75. Rather than regear my car again or change out my th350 I went with 275/60-15s in the rear (I have the same size rims as you) and regeared the speedo to match, though I use a $100 gps speedo anyway. The downside for a car like a gen2 Camaro/Bird is that those big tires, which lower my effective rear ratio to about 3.50, can't take more than 18psi of air without bowing and wearing out the middle of the tread. I'm assuming your 'stock' tire size is, like mine, 235/60-15. Stock diameter is 26.1", and my rear tires are 28".

So considering your need for comfortable cruising at 75 I'd suggest stock tire size in the rear maybe with around 3.30 gears MAX, or even 255/60-15s in back (at least you can run around 25psi in them). You might even prefer 3.08s with stock tires. Big gears are great for stoplights but awful for the highway. In fact, if mpg enters your equation in a big way, you might even want 2.73s (or 3.08s with the 255/60-15s in back; that would be like 2.97 gears with stock size tires).

Big gears like 4.11s were popular hot-rodding options once because they were common in junkyards back when most cars had straight sixes. They gave excellent around-town pep for the ultratame motors but couldn't really get past 80 and weren't meant for travel at much more than 50. That's why 100mph was such a magic number up to the early '50s. Today you'd have to be nuts to hit the highways with 4.11s, IMO. So like the stick shift, big gears are really a throwback to the '30s.

I have a c4 Vette that I'm currently working on, and its normal package is something like 3.40, but the hi-po rear end (which I have) is 3.08 for higher top speed! So big gears aren't necessarily "high performance" unless you're at the drags (or have a motor that likes always running over 3000rpm, which only a 2L 4cyl does). My advice would be never to overgear a smooth-running street v8.

Last edited by Leoman; 08-12-2012 at 09:09 PM.
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Old 08-12-2012, 09:35 PM
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not that you care what I say,,,lol.I would use a 235/70/15 and 3.36 gears with a 3 spd auto. If you do a lot of driving at 75 mph an OD trans is worth the money.If you just do it on occasion the 3.36 is a good ratio or 3.08 if you have a lot of torque. I use 3.50 with .82 OD and that gives me 2050 rpm at 60 with 27.5 ish tires(225/70/15 on 15x8 wheels)
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Old 08-12-2012, 10:07 PM
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some gearing formulas

Perhaps this formula and its derivations will help you find some answers.

Based on a final gear ratio of 1:1,
rpm = (mph x rear-end gear ratio x 336) divided by rear tire diameter or

mph = (rpm x tire diameter) divided by (rear-end gear ratio x 336)

If you have an overdrive transmission, simply insert the final drive ratio of the overdrive gear (generally around 0.7) into the upper part of the formula as follows:

rpm = (mph x rear-end gear ratio x final drive ratio x 336) divided by tire diameter.

Of course, this is for a traditional rear wheel drive vehicle. If you are hot rodding a front wheel drive car (lol!!!) substitute front end gear ratio and front tire diameter appropriately.

Dennis W. Parks
Author of automotive how-to books
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Old 08-12-2012, 10:13 PM
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so a 29 inch tall tire gives 74 mph at 2900 rpm with a lock up converter in 1:1 drive. and under 2000 rpm with a 30% OD
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Old 08-12-2012, 10:34 PM
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You left the rear end gear ratio out of the equation.

Dennis
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Old 08-12-2012, 10:43 PM
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Perhaps a clarification is in order. The 1:1 ratio would be in the transmission. Most likely, an overdrive transmission would be somewhere around 0.7. However, you have to factor in the rear end gears at the differential.

So as an example, if you have 29 inch tall tires, are running 2900 rpms, and have 3.08:1 gears in the rear end, you would have

mph = (2900 x 29) divided by (3.08 x 336) = 81.27 mph

With everything else being the same, but with an overdrive tranny that has a final gear ratio of 0.7, you would be running that same 81.27 mph at just 2030.26 rpm.

Dennis
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Old 08-13-2012, 09:05 AM
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Get a th2004r OD trans and 3.90 gears. Get a 3000 stall converter with lockup.
Yank
Precision Industries.
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Old 08-13-2012, 09:41 AM
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Ok well it seems like the th350 isn't going to work well for me. I've alway wanted a 4 speed, manual of course, and I guess this is perfect time to convert since I'm going to have the car torn apart anyway. What's involved in a swap like that? Do they make 4 speeds with overdrive or at least a 1:1 4th gear? Or should I use a 5 speed?
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Old 08-13-2012, 10:02 AM
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If you find a donor car for the pedals and bell crank you can start from there.Its not going to be cheap as these items are drying up fast.The ford top loader is a very strong transmission if you use the bb item.The muncie is not very strong but the shifting is silky smooth. none are over drive but you can buy bolt on O/Ds that fit on the tailshaft location,sometimes called gear splitter.

TKO is a modified tremec with either .68 or .82 od ratios. they come in heavy and extra heavy duty.They do not shift as smooth as a muncie,not even close. I use a TKO 600 in my car,dont drag race,I chose .82 OD ratio.at 60mph my tach is at 2k.
TKO will need drive shaft shortened,cross member moved back. fit is good,no mods to floor or firewall. I paid $2100.oo for tko 600,plus adapter and all related parts,speedo adapter. shifter location has to be measured and checked,though not likely a problem for your car.
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Old 08-13-2012, 10:30 AM
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Quote:
Originally Posted by bigdog7373 View Post
Ok well it seems like the th350 isn't going to work well for me. I've alway wanted a 4 speed, manual of course, and I guess this is perfect time to convert since I'm going to have the car torn apart anyway. What's involved in a swap like that? Do they make 4 speeds with overdrive or at least a 1:1 4th gear? Or should I use a 5 speed?
Most if not all non-OD transmissions, manual or auto, are 1:1 in the top gear AFAIK. If you really want to increase your drive ratio for more acceleration you probably will require an OD, considering your mpg and cruising requirements. I don't know what to recommend as well as others do since the only OD I have is the Doug Nash in the old Vette, which is a real oddball.

But going to, e.g., a .68 overdrive changes everything. You could run 4.11s and the OD would make them look like 2.79s on the highway.
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Old 08-13-2012, 10:44 AM
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I'd want to but a new tranny, not messing with used stuff. I all I'm really worried about what tranny would be best. Id prefer a 4 speed since it was an option and goes with the car better, but if a 5 speed or anything else will work better I'm all for it. I need it to cruise at 75 comfortably because the car will be my daily driver when done. I'm also planning on building a 6-71 blown small block for it. Either a 355 or 383, haven't decided yet. So the tranny will have to handle at least 600 hp. Don't worry about the rest of the car I have it all planned already. I just need a manual trans and gear combo that will keep my car quick and albe able to cruise at 75 without revving sky high
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Old 08-13-2012, 11:26 AM
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Quote:
Originally Posted by bigdog7373 View Post
I'd want to but a new tranny, not messing with used stuff. I all I'm really worried about what tranny would be best. Id prefer a 4 speed since it was an option and goes with the car better, but if a 5 speed or anything else will work better I'm all for it. I need it to cruise at 75 comfortably because the car will be my daily driver when done. I'm also planning on building a 6-71 blown small block for it. Either a 355 or 383, haven't decided yet. So the tranny will have to handle at least 600 hp. Don't worry about the rest of the car I have it all planned already. I just need a manual trans and gear combo that will keep my car quick and albe able to cruise at 75 without revving sky high
Boy, this car is changing its spots fast Interestingly, if you build a street-friendly blower motor for it, you probably won't see the high side of 5000rpm very often anyway, as its powerband probably won't be much different from your current 'tame' motor (just a lot taller on the graph). The other thing that changes is that you really won't need big gears with a blower (you could easily use 3.08s or 2.73s and still leap off the line) so you could use a rebuilt Muncie (just as good as a new tranny if rebuilt right). But you could also use something like 3.42s with a 5-speed OD and still end up with a super highway-friendly 2.56 equivalent ratio in 5th gear (if the OD is around .75). You motor surely won't be knocking from the tall gears at part throttle!

BTW nice Wasp Major I know a guy who was a USAF maintenance mechanic on B-36s in the '50s, and he told me that when one came in for work they prayed it was for one of the jets, not one of the Wasps. Unfortunately it usually was for a Wasp. They had a million moving parts as you can imagine, unlike the turbojets.
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Old 08-13-2012, 11:39 AM
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I thought I was going to have to spend a lot more on the body when I posted this thread but it turns out its gonna be cheaper than I expected so now I have room in the budget to build the motor Ive been wanting to build. And plus, I don't want to even give those honda guys even the slightest chance of outrunning me! Lol.

I'll summarize so its not so complicated.
Blown small block aprox 600 horse
Manual tranny - open to suggestions
Gears that will allow me to cruise at 75mph at low rpm

And yea those waps are awesome!
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Old 08-14-2012, 09:24 PM
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Since you say you're on a budget, I'll just add one more thing as a back-in-the-day owner of a 1968 SS396 Camaro with the (so-called) 350 hp motor. I bought it in 1970 and I never did hole shots. By 1973 the relatively new unit-bodied car was hopelessly body-twisted. My current '72 with the naturally-aspirated 350 has custom-fitted (by me) traction bars and subframe connectors, and even then I'm starting to see signs of body twist (about 1/2" so far), despite having a nice soft 3000-staller and never pulling horror shows. Unless you plan to do a LOT of chassis work, the kind of power/torque you're talking about will probably have you looking for another vehicle to put the motor in fairly quickly. A body shop can straighten a frame, but not a unit body. Once it twists too much it's toast, so be careful.

As a friend explained to me once, subframe connectors aren't there to stiffen the body directly against twist really, they're to keep it from being crushed when the rear housing tries to twist its lower end forward. Do that enough and the metal fatigues, which makes it easy for the other twist component (the axle wanting to spin like a propellor) to twist the body. I trusted his explanation.

Pardon me if you know this stuff (or know better) already, but you're starting to scare me
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