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Discussion Starter #1
Did some searching but couldn't find answers I was looking for.

Here is my debate, 64 Gal, 390, 3.75 posi rear end, C-6. I love how she runs with 14" and 15" wheels/tires I've been buying cheapies from guys on craigslist cause I just tear them up. BUT on the interstate I turn about 3,000 rpm's to run 65-70 and it guzzles the gas.

So, rather than sink $600-800 bucks in a 3.50 gear rear end and deal with all that stuff, what if I go to a 16" or 17" wheel?

I do need some more ground clearance anyways for my full length MadDog headers...

So, anyone have a chart or advise around how many RPM's I'd drop by going up in wheel and tire size. I would like the look of a nice fat tire that fills the wheel well so I could do a decent height sidewall.

Thanks fellas!
PB in Nashville
 

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Need tire diameter to tell you rpm.
 

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Discussion Starter #3
Ok, here are a few sizes in inches. THANK YOU!

31.055

31.606

30.055

26.630
 

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31.6 shows 2850 and 26.6 shows 3380 at 70 mph.
 

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Discussion Starter #7
What size tire do you run now?

$200 for a set of gears or $1500+ for 17" tires and wheels... Sounds like you want new wheels.
Ah, good question. I don't know how to build a Rear end so I'd be buying the chunk with 3.50 gears & posi for 450 to 500 bucks. Most I find are 31 spline so I'd have to get axels and bearing as I have a 28 spline setup today, 200 more bucks.

I'm finding wheels and tires on Craigslist around here easily, 400 to 500 for wheels and tires. I'm burning off a set of crown Vic cop wheels/tires now, got 5 new 16 with 225 60's for 100 bucks Plus wheels are a lot easier to bolt on than building a rear end hahaha!

Any idea if 17" wheels from a 2003 mustang with 6.5" back spacing will fit my 64?
 

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Discussion Starter #8
However, the 31.6" will show 13 mph lower on the speedo. boooo!
Right now my speedo shows 20 mph faster than I'm actually moving. I've been using my old android phone with a GPS speedometer app and a windshield mount :)
 

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Did some searching but couldn't find answers I was looking for.

Here is my debate, 64 Gal, 390, 3.75 posi rear end, C-6. I love how she runs with 14" and 15" wheels/tires I've been buying cheapies from guys on craigslist cause I just tear them up. BUT on the interstate I turn about 3,000 rpm's to run 65-70 and it guzzles the gas.

.............I do need some more ground clearance anyways for my full length MadDog headers...
My '64 4spd Gal runs 3.50s, and I can tell ya it's still similar (to yours @ 3.75) on the interstate -but, I too love the 'snap' of the semi-steeper gears, so they're keepers. Problem is; you go to 3.25 and numerically lower, and she gets poochier and poochier, until it runs/moves like a 1975 Ltd Brougham with a smog-laden 351M. You are quickly reminded of what kind of heft you are launching.

If you have money to burn, why not go O/D (overdrive)? If not that, then (as I plan on doing) finding the best of both worlds is in the right wheel/tire combo.

Speaking of; I'd love to hear about those Mad Dog headers! I DO have the ground clearance and have talked to Mark on the phone about them --wonder if you have pics to share? Did you get yours ceramic coated, and if so, how'd they fit back together after that? (when I called him he told me he didn't offer any coatings, thus couldn't tell me.)

ps: If you wanna true-up that speedo, get us a color (plus, you can squint and see the teeth number on it), and anyone here can get you in the ballpark.
 

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To enjoy both worlds, you are going to have to go to OD, whether AUTO or STD (IMO).
 

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To enjoy both worlds, you are going to have to go to OD, whether AUTO or STD (IMO).

Yep what I was thinking also. Tire sizing to gear is almost like p^%$$ing in the wind. Yes you can get 'bargains' from C/L sometimes, but the first time for tire replacement you may wish you had stayed with a more "conventional" tire size.

If you arent in for swapping to an O/D trans, I would think about saving up for a Gear Vendors O/D unit....

(what I am going to try to do eventually, as I dont like the way the T-5's/ TKO's shift and love the T/L and love the gear I am pulling now, but with todays gas prices and all the upped speed limits..... )

M2C.
 

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Discussion Starter #12
My '64 4spd Gal runs 3.50s, and I can tell ya it's still similar (to yours @ 3.75) on the interstate -but, I too love the 'snap' of the semi-steeper gears, so they're keepers. Problem is; you go to 3.25 and numerically lower, and she gets poochier and poochier, until it runs/moves like a 1975 Ltd Brougham with a smog-laden 351M. You are quickly reminded of what kind of heft you are launching.

If you have money to burn, why not go O/D (overdrive)? If not that, then (as I plan on doing) finding the best of both worlds is in the right wheel/tire combo.

Speaking of; I'd love to hear about those Mad Dog headers! I DO have the ground clearance and have talked to Mark on the phone about them --wonder if you have pics to share? Did you get yours ceramic coated, and if so, how'd they fit back together after that? (when I called him he told me he didn't offer any coatings, thus couldn't tell me.)

ps: If you wanna true-up that speedo, get us a color (plus, you can squint and see the teeth number on it), and anyone here can get you in the ballpark.
Norm, love my Maddog headers. I did replace my tired front springs and I think I have enough clearance now but to be safe I want some more. They fit perfectly! I just used VHT header paint found at Oreilly's or any car parts house. I was very impressed with the fit and quality, I would buy them again. Ill take some pics today. When I drag race I just unhook my exhaust at the collector connection, swing the pipe out and I have wide open header and it sounds sweet! Also I have a 390, the OEM starter fits really close to one of the header pipes, I decided to spend 125 bucks and got a mini starter and have a lot of clearance now.

On my speedo, I had called a place that sells the gears and even a gizmo that reduces at the back of the speedo in the dash. Something about the C6 I have hey could not fix it with a gear, I'd have to do the reducer.
 

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On my speedo, I had called a place that sells the gears and even a gizmo that reduces at the back of the speedo in the dash. Something about the C6 I have hey could not fix it with a gear, I'd have to do the reducer.

What's the place? I read 10 mph over and need a gizmo too.
 

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On my speedo, I had called a place that sells the gears and even a gizmo that reduces at the back of the speedo in the dash. Something about the C6 I have hey could not fix it with a gear, I'd have to do the reducer.
Hey Tex,

Excellent on the MD headers, will look fwd to your pics! On the speedo fix, not sure if you have room back behind speedo head for this style, here's one on eBay: Stewart Warner SPEEDOMETER GEAR REDUCER - 1.3636 - NEW! | eBay

.... also research to make sure that ratio will get you in 'range' (a gear between 16~21 teeth will then work).

Sadly yes, lots of C-6s were mated to "moon" geared land yachts, making the jump to 3.75 out of drive gear's range (the gear on the tailshaft, which in many apps is part of shaft)

Norm
 

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Before my Gal became a 500XL, I used to run 15" wheels with 1972 LTD caps.

The larger wheels/tires looked better than the 14's with stock caps, but when I went full XL, I went back to 14's with 500XL caps.

Wish I could run my 3:50's, 15" rims, and 15" 500XL caps (which don't exist).

The slightly larger tire would also help a bit on the NJ Garden State Parkway.

Oh well, I can wish.
 

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Don't confuse wheel diameter with overall diameter of the tire, which is measurement you are most concerned with. Tire sites like Tire Rack will show specs of some tires that will include overall diameter, revolutions per mile, and others tid bits. With the overall diameter of the tire, axle ratio and trans final drive ratio, you can calculate the engine RPM at any speed to see what difference each tire size will make for you.
 

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Try HERE Calculate till yer tired.... (no pun intended);)
 

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Here's a spreadsheet I put together a while back for dreaming about this stuff. If you know your tire height or rev/mile and the calc doesn't show it right just fudge the wheel size or tire series to make it come out the way you want. It also calculates the speedometer gears if you know what drive gear is in your tranny and on the right side of the speedo calc is what you'll get with a reducer.
 

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65er I must be missin supthin.... ;)
 
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