Taller Tires Needed

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SleeperDart505

SleeperDart505

Stock wheels, Stock Paint, 505" Stroker, Full Cage
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Ringgold, GA
I have 295/50R15 BFGoodrich raised white letter tires on the rear of my 1968 Dart. These are approximately 26.5" tall. They fit very well. The car has mini tubs. Not sure the width of the wheels. I love the look. But with the 3.73 gear I'd like to go to a taller tire to keep from running at 4000 RPM on the Intersate. From what I can find BFGoodrich doesn't make a 295/60R15 or a 295/70R15.. I thought a 60 or 70 would be tall enough to help but no luck so far. Any ideas?

How many RPM's would a 28" tire turn if the 26.5" tall tire turns 4000 RPM? Would a 28" tall tire be enough? Or should I look for a 29" or 30" tire? Anybody want to share the math formula so I can do this myself in the future? Thanks!

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The speed at a given rpm is directly proportional to tire height. A 10% increase in height means a 10% increase in speed at a given rpm, or a 10% decrease in rpm at a given speed. On the other hand, with your current specs you should be cruising 3400 at 70, not hitting 4k until around 83.

A 29” tire should put you just over 3k at 70.
 
Mickey Thompson et streets has taller sizing in wide tires.
275/60/15 in a radial T/A is 28 and is about the biggest tallest they make unless they still make a 255/70/15, but those are getting on the skinny side .
 
IF you have room...... 315/60x15. ET street pro.
If that's too wide, et street ss, 295/65x15.
Both are around 30" tall. 295 needs 8"-10" rim. My 315s are on twelve.
The place where those trees will be close in the front of the wheelwell opening. You can space the front spring eye back for more clearance.
My savoy has the rearend moved back an inch.

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remytherat said:
The speed at a given rpm is directly proportional to tire height. A 10% increase in height means a 10% increase in speed at a given rpm, or a 10% decrease in rpm at a given speed. On the other hand, with your current specs you should be cruising 3400 at 70, not hitting 4k until around 83.

A 29” tire should put you just over 3k at 70.
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My speedometer cable isn't working. All I had was a tachometer. I was in the slow lane and EVERYBODY was passing me on I-75 so I assumed I was doing about 70 and they were doing 75-80 in the other two lanes. Regardless I was doing 4000 RPM and was getting passed by EVERYONE so I know I need to lower my RPM's because I can't drive on the interstate at 4000 RPM's. But I need to be able to run at least as fast as I was going (whatever that was) or else I'll be getting run over. :BangHead:
 
33IMP said:
IF you have room...... 315/60x15. ET street pro.
If that's too wide, et street ss, 295/65x15.
Both are around 30" tall. 295 needs 8"-10" rim. My 315s are on twelve.
The place where those trees will be close in the front of the wheelwell opening. You can space the front spring eye back for more clearance.
My savoy has the rearend moved back an inch.

View attachment 1716197267
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The 295/65x15 would fit. I was hoping to find a regular street tire to stay with the "sleeper" theme. But at least a 30" tall tire is what I need for sure.
 
.my opinion only, with a 505, you should have at least a drag radial. The ss is not as obviously a drag tire than the et R or pro.
 
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My runner with a factory 4.10 Dana and 28"tires is 3700 at 70 mph.
With no speedo, you might have been going faster than you think..... or, it doesn't have 3.73.... or, the converter has a bunch of slip in it.
 
33IMP said:
My runner with a factory 4.10 Dana and 28"tires is 3700 at 70 mph.
With no speedo, you might have been going faster than you think..... or, it doesn't have 3.73.... or, the converter has a bunch of slip in it.
Click to expand...
this is my guess.

the maths don't pen out or everybody there is a lead footed speed demon.
 
SleeperDart505 said:
The 295/65x15 would fit. I was hoping to find a regular street tire to stay with the "sleeper" theme. But at least a 30" tall tire is what I need for sure.
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265/70x15, 29 1/2+ tall, 10" section, 7 to 9 inch rim. Basically an suv tire. (My opinion, basically worthless with a 505 hooked to em) Nobody makes a 275/70 any more
 
If you want to fiddle with estimating your tire size, RPM, and speed, try this calculator. I like it a lot. If your transmission isn't one of those on the selection list you can put in your own gear ratios (and rear end ratios too) manually, so it will work with any combination you like.

The calculator says 26.5" tire, 3.73 gears, 4000 RPM gives you 85 MPH. I think @33IMP is right, the math doesn't pencil out - you likely have a bunch more gear than 3.73's. You might want to get someone to pace you at whatever speed, then check your RPM. From that we can figure out a decent guess what your gears are.

Speed/Tire/RPM calculator
 
halfafish said:
If you want to fiddle with estimating your tire size, RPM, and speed, try this calculator. I like it a lot. If your transmission isn't one of those on the selection list you can put in your own gear ratios (and rear end ratios too) manually, so it will work with any combination you like.

The calculator says 26.5" tire, 3.73 gears, 4000 RPM gives you 85 MPH. I think @33IMP is right, the math doesn't pencil out - you likely have a bunch more gear than 3.73's. You might want to get someone to pace you at whatever speed, then check your RPM. From that we can figure out a decent guess what your gears are.

Speed/Tire/RPM calculator
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Yep. He might have TOLD you it had 3.73s, cause he might have been afraid you wouldn't buy it with 4.56s, LOL.
Edit: 26.5 tall tire, torqueflite with 5% converter slip, and 4.30 gear...... 70 mph at 4008 rpm.
If you really have 3.73s, and 5% slip.... 29.5 tall tire would be 3.123 rpm at 70mph
2nd edit: checking gear ratio is EASY if you have a sure grip. Jack up both rear wheels, neutral, turn the tire one revolution, and count the driveshaft turns.
 
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33IMP said:
My runner with a factory 4.10 Dana and 28"tires is 3700 at 70 mph.
With no speedo, you might have been going faster than you think..... or, it doesn't have 3.73.... or, the converter has a bunch of slip in it.
Click to expand...
Possible I was going faster than I think. However I was being passed by EVERYBODY (including tractor trailers) and I was in the slow lane. I'd like to be able to keep up and right now that looks like about 4000 RPM for me. (I need to fix the speedo cable)

As far as the converter goes the seller said it was a Hughes transmission and a Hughes 2500 RPM converter. With the 505" motor it foot brakes easily to 2800 and might even do 3000 if I tried. I think this is about right for this car, don't you?

I think a taller tire would help. But I'd like to find a tire that looks like a muscle car and not like a drag race car if you know what I mean. :lol:
 
SleeperDart505 said:
Who makes the SS? Mickey Thompson? Hoosier?
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Those are Mickeys. (And at least 4 or 5 other companies also make "drag radials").
E.T. Street SS have a somewhat like normal tread, but not much depth, and like all drag radials, will wear out quickly daily driven.
E.T. Street R have minimal tread, a more aggressive compound, more race tire than the SS.
E.T. Street Pro have NO tread, only a divot here and there, and are pure race tires for 4 second 1/8 mile drag racers. (I'm not anywhere NEAR fast, but I bought the pros, cause I needed a big radial to fill the wheelwells of my Opel.)
 
Toyo, Nitto, M&H, Hoosier also make drag radials.
Hoosier makes a tire called Quick Time, that looks quite a bit like a regular street tire, but they are a bias tire with a bit of wrinkle wall sidewall. (The quick time PRO is in reality a full on drag slick.... with a couple shallow grooves, but it is street legal.)
 
33IMP said:
Those are Mickeys. (And at least 4 or 5 other companies also make "drag radials").
E.T. Street SS have a somewhat like normal tread, but not much depth, and like all drag radials, will wear out quickly daily driven.
E.T. Street R have minimal tread, a more aggressive compound, more race tire than the SS.
E.T. Street Pro have NO tread, only a divot here and there, and are pure race tires for 4 second 1/8 mile drag racers. (I'm not anywhere NEAR fast, but I bought the pros, cause I needed a big radial to fill the wheelwells of my Opel.)
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Thanks for the help! I've had street cars before and I've had a Super Gas '67 Camaro before but never had a car like this where I was trying to do both. :p
 
33IMP said:
Yep. He might have TOLD you it had 3.73s, cause he might have been afraid you wouldn't buy it with 4.56s, LOL.
Edit: 26.5 tall tire, torqueflite with 5% converter slip, and 4.30 gear...... 70 mph at 4008 rpm.
If you really have 3.73s, and 5% slip.... 29.5 tall tire would be 3.123 rpm at 70mph
2nd edit: checking gear ratio is EASY if you have a sure grip. Jack up both rear wheels, neutral, turn the tire one revolution, and count the driveshaft turns.
Click to expand...
OK. I jacked up the car today and verified that I have 3.73 gears. The driveshaft went around 3 3/4 turns to 1 full turn for the tire. Right on the money. 3.73 gears. That's good to get out of the way. It's one less variable to worry about. Now.........I gotta get that speedometer hooked up!! The cable is there under the dash. I gotta crawl up under the car to see if it's hooked up to the 727. :BangHead:
 
when you install the speedo cable it might be a good time to make sure you've got the right speedo pinion for the tire.
3.73 w/ 26" is a 38 (with a 27" is 36)

here's a wild theory though... is your tach the type that has multiple cyl settings and maybe it got bumped to 4 or 6 cyl and you're getting a 4K reading when you're really turning something less than that?

because, again, the math has me scratching my head. even at 3K you'd be clipping darn near 65mph
 
junkyardhero said:
when you install the speedo cable it might be a good time to make sure you've got the right speedo pinion for the tire.
3.73 w/ 26" is a 38 (with a 27" is 36)

here's a wild theory though... is your tach the type that has multiple cyl settings and maybe it got bumped to 4 or 6 cyl and you're getting a 4K reading when you're really turning something less than that?

because, again, the math has me scratching my head. even at 3K you'd be clipping darn near 65mph
Click to expand...
What pinion would it be for a 30" tall tire? A 30 pinion? That's what height tire I think I'm going to go with. Where can I buy these speedometer pinions??
 
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