Optimal Tire Diameter

[BC10]

Hi Everyone,
What diameter tire will yield optimal performance on my truck???
After searching and reading through many of the topics based on rear differential gear ratio and tire sizes I am still stumped, this may be a result of me being a rookie in regards to rebuilding vehicles. I also tried to use the online calculators but am still stumped.

I have a 1968 short bed c10. According to the previous owner, the engine and tranny (TH400) was transplanted from a 1969 Camaro z28. The original rear differential was a 3.08, which I have since replaced with a 3.42 (much more responsive now).

I am doing a little of city and highway driving, and some light hauling. I recently purchased 15x8 stock rally wheels that will replace the current aftermarket 15x8s.

So my question is what diameter tires would give me optimal performance (power and mpg) if I am cruising at about 65-70 mph on the highway?

I am leaning towards 255/70/15, a diameter of 29", would this be ideal?
Any input is much appreciated. I am very grateful for the wealth of knowledge from members on these forums, and hope to contribute during my rebuild. Thanks!

[motorcritter]

There are a few apps that you can get to determine tire diameter. A 29" tall tire would be pretty optimal for all-around use. That would be that 255/70R15 or a 235/75R15. It comes down to what size rims you're running and what kind of ride, handling and economy (tire life & fuel mileage) you want to have. These trucks are pretty much a blank canvas, and it can be tailored as you see fit. This forum is probably the best one I've seen on the 'net- and I'm on a whole lot of them. Any question you got, someone's got the answer- or a bunch of answers from a bunch of folks. Originally, most of the 2WD trucks ran tires about 2" shorter than the ones I listed- OE size was G78-15, roughly equivalent to a 205/75R15. I would never run something that small on one of these trucks. I've found that a 235/75R15XL gives good performance all around, just my experience.

[BC10]

Originally Posted by motorcritter

There are a few apps that you can get to determine tire diameter. A 29" tall tire would be pretty optimal for all-around use. That would be that 255/70R15 or a 235/75R15. It comes down to what size rims you're running and what kind of ride, handling and economy (tire life & fuel mileage) you want to have. These trucks are pretty much a blank canvas, and it can be tailored as you see fit. This forum is probably the best one I've seen on the 'net- and I'm on a whole lot of them. Any question you got, someone's got the answer- or a bunch of answers from a bunch of folks. Originally, most of the 2WD trucks ran tires about 2" shorter than the ones I listed- OE size was G78-15, roughly equivalent to a 205/75R15. I would never run something that small on one of these trucks. I've found that a 235/75R15XL gives good performance all around, just my experience.

Thanks for the reply! Currently my truck has 235/60/15, a diameter of 26.1". I feel that that they are to small. What is the rear differential gear ratio and engine on your truck?

[tommys72]

I have 235/75/15's on mine. ..and I have no complaints about them at all. Good ride and seems to do ok on gas. .. well as good as possible for old trucks

[Already Gone]

I have 255/70 R15's on 15 x 8 rally rims on my stock height truck and I do lots of compliments about the stance. The 255's work super with an 8 inch rim.

[cool68]

easy to use calculator here: project33.com

[BC10]

Originally Posted by cool68

easy to use calculator here: project33.com

Just played around with this calculator.
On a Th400 what "gear" am I in at 70mph on an automatic? Is it 2nd?

[cool68]

3rd

[Lees68GMC]

I'm running 27" tall 275-60-15 BFGs, 3:42 gears, removing a Turbo 400 and installing a 700R4 trans. 700 has lower 1st gear plus 4th over-drive for some mileage. We'll see how it works

[Already Gone]

Originally Posted by Lees68GMC

I'm running 27" tall 275-60-15 BFGs, 3:42 gears, removing a Turbo 400 and installing a 700R4 trans. 700 has lower 1st gear plus 4th over-drive for some mileage. We'll see how it works

That combo should work super for you.

[BC10]

Originally Posted by tommys72

I have 235/75/15's on mine. ..and I have no complaints about them at all. Good ride and seems to do ok on gas. .. well as good as possible for old trucks

Originally Posted by Already Gone

I have 255/70 R15's on 15 x 8 rally rims on my stock height truck and I do lots of compliments about the stance. The 255's work super with an 8 inch rim.

Originally Posted by cool68

3rd

The calculator says that the power glide has no 3rd gear when you enter it in to calculate rpms... ???

[BC10]

Originally Posted by Lees68GMC

I'm running 27" tall 275-60-15 BFGs, 3:42 gears, removing a Turbo 400 and installing a 700R4 trans. 700 has lower 1st gear plus 4th over-drive for some mileage. We'll see how it works

Beautiful, what is the stance on her?

[cool68]

Originally Posted by BC10

The calculator says that the power glide has no 3rd gear when you enter it in to calculate rpms... ???

***at top of page said trans is a 400 = 3 spd. powerglide = 2 spd

[Already Gone]

Originally Posted by BC10

Beautiful, what is the stance on her?

He has a picture of his truck in post #9.

[Z10]

Originally Posted by BC10

The calculator says that the power glide has no 3rd gear when you enter it in to calculate rpms... ???

That's because power glides are 2 speeds.

[WIDESIDE72]

There is no magic diameter. This is more a matter of compromise. A short tire will give better acceleration, since it yields a lower final drive ratio, but the same tire wont return the best mileage. In my experience, you wont see much noticeable difference in performance and mileage in 235/255 tires. Personally I am running 275/60r15 because I like the look of a filled out wheel well.

BTW, 67-69 z/28's only came with 4 speed manual transmissions. Im surprised the previois owner didn't tell you it had the "Corvette engine" we have all heard so much of. :lol

[Z10]

Originally Posted by WIDESIDE72

There is no magic diameter. This is more a matter of compromise. A short tire will give better acceleration, since it yields a lower final drive ratio, but the same tire wont return the best mileage. In my experience, you wont see much noticeable difference in performance and mileage in 235/255 tires. Personally I am running 275/60r15 because I like the look of a filled out wheel well.

BTW, 67-69 z/28's only came with 4 speed manual transmissions. Im surprised the previois owner didn't tell you it had the "Corvette engine" we have all heard so much of. :lol

And the 67-69 Z28's came standard with 3.73 gears.

[motorcritter]

Wow, I don't check the forum for a while and the crowd shows up! As to my rear axle ratio, the '70 C10 has 3.73, I haven't bothered to check the '71 Cheyenne yet (after 2years of driving it), but I get poor mileage, no matter what. So, the C10 will get late model injection, the Cheyenne is a 402 Sultan's Favorite (and left all original @ 54K original miles) and the '68 GMC doesn't have an engine yet.

[BC10]

Originally Posted by WIDESIDE72

There is no magic diameter. This is more a matter of compromise. A short tire will give better acceleration, since it yields a lower final drive ratio, but the same tire wont return the best mileage. In my experience, you wont see much noticeable difference in performance and mileage in 235/255 tires. Personally I am running 275/60r15 because I like the look of a filled out wheel well.

BTW, 67-69 z/28's only came with 4 speed manual transmissions. Im surprised the previois owner didn't tell you it had the "Corvette engine" we have all heard so much of. :lol

My bad memory, it was originally a 1979 Camaro engine (cant remember if it was a z28 or not) and not a 1969. Thanks for the clarification. Sorry about the confusion, still new at all of this and just learning.

[Jimjific]

I have 255/70 R15's on 15 x 8 rally rims on my stock height truck and I do lots of compliments about the stance. The 255's work super with an 8 inch rim.

I am looking to do the same exact setup. One question, did it mess your speedometer to go to that size tire? If so, did you change your speedo gear? I guess that is two questions.

Thanks,
Jim

[BC10]

Originally Posted by WIDESIDE72

There is no magic diameter. This is more a matter of compromise. A short tire will give better acceleration, since it yields a lower final drive ratio, but the same tire wont return the best mileage. In my experience, you wont see much noticeable difference in performance and mileage in 235/255 tires. Personally I am running 275/60r15 because I like the look of a filled out wheel well.

BTW, 67-69 z/28's only came with 4 speed manual transmissions. Im surprised the previois owner didn't tell you it had the "Corvette engine" we have all heard so much of. :lol

Wide side, how well does the width of the 275 fit under your fender, and what is the clearance? Is your truck lowered?

[WIDESIDE72]

Truck is dropped 3/4. A picture is worh a thousand words.

[WIDESIDE72]

No rubbing bottoming out or other issues. She does mark her territory, though!

[WIDESIDE72]

Una mas

[Already Gone]

Originally Posted by Jimjific

I am looking to do the same exact setup. One question, did it mess your speedometer to go to that size tire? If so, did you change your speedo gear? I guess that is two questions.

Thanks,
Jim

I've checked my speedo against my GPS and with the speedo at 95MPH the GPS was showing 93 MPH and that is close enough for me.