Help me get new tires! - The 1947 - Present Chevrolet & GMC Truck Message Board Network

JRANGER · 01-30-2012, 09:48 PM

Well my tires are worn out and since im getting my Tax Return I figure I might as well get rubber for the truck in case the roads get icy...That is what i bought her for in the first place anyway...

So that being said....

I have 255/70R15's on the truck....Can I go bigger without messing up the speedo? I have a 21 gear speedometer gear and ell all I know about the rear end is a couple #s on the axel.... which are 1249578 R which was on the bottom.. On the 0 FI680...all though the 0 look like a flat head almost haha

Im wanting a good tire for winter as this truck isnt going to be driven on the regular

gchemist · 01-30-2012, 10:31 PM

You can only go wider. Taller will change the speedo. You can change the tire ratio by going different size rim.
Check out this nifty calculator.
http://www.miata.net/garage/tirecalc.html

You can change the driven gear to compensate.
Find the correct drive and driven gears with this calculator.
http://www.tciauto.com/tc/speedo-info

JRANGER · 01-30-2012, 10:43 PM

Do you know the stock rear gear size? Stock tire size? Wondering if the guy I bought it changed out the tires and rims for not standard size...Gear is rusty and looks stock

Restrorob · 01-30-2012, 10:56 PM

A 78 C10 came with 225/75-15's, I would assume GMC's did as well. Look on the inside of your glovebox door, Likely would be on the option/build list.

There's no telling what's been changed if anything.....

JRANGER · 01-30-2012, 11:06 PM

Shows me nothing...closet thing on my build list L78-15/B TBLS HWY BL and i have no clue what that is lol

Restrorob · 01-30-2012, 11:10 PM

L78-15 converts over to a 235/75R15 size, they're 28.88 dia. and your 255's are 29.06.....

JRANGER · 01-30-2012, 11:12 PM

ahhh so my tires are wrong.....and my speedo is prob wrong which would explain why my mom said i was going 80 hen i drove the truck home when i thought i was going 70/75

billybobjimmyjoejack · 01-30-2012, 11:16 PM

I ordered my 1978 C10 in October of 1977 and took possession of it on Jan. 10th of 1978. I still have the truck and it has one of the original tires as a spare L78-15 is the size that came with the truck. The gear ratio was 3:07 and it is a BIG 10.

Restrorob · 01-30-2012, 11:18 PM

Yep..... The tires on the back of my 97 are smaller than stock and I don't know what else was changed, so my speedo is about 12 miles off.

JRANGER · 01-30-2012, 11:21 PM

hmm weird..Based on the information with the calcs my base gear should be a driven gear of 44 but i think i remember it being green which is 42...... Based on that and the 29" diameter I should be using a 20 tooth gear?? Sound right to yall?

Or based on the tire calc i could put on 265/75 15s

Restrorob · 01-30-2012, 11:31 PM

Sorry, Never used that calc. before.....

01-30-2012, 09:48 PM	#1
JRANGER Registered User Join Date: Nov 2011 Location: Nashville TN Posts: 3,595	Help me get new tires! Well my tires are worn out and since im getting my Tax Return I figure I might as well get rubber for the truck in case the roads get icy...That is what i bought her for in the first place anyway... So that being said.... I have 255/70R15's on the truck....Can I go bigger without messing up the speedo? I have a 21 gear speedometer gear and ell all I know about the rear end is a couple #s on the axel.... which are 1249578 R which was on the bottom.. On the 0 FI680...all though the 0 look like a flat head almost haha Im wanting a good tire for winter as this truck isnt going to be driven on the regular __________________ PLEASE HELP WITH PROJECT UNDERHAULIN http://67-72chevytrucks.com/vboard/showthread.php?t=306320 Gemini - 1978 GMC C15 High Sierra - 305 c/e - Given to Mother Electra - 1967 Mustang Coupe - 302/w 3 speed Build Chyna - 1968 Chevy C20 Long Stepper

01-30-2012, 10:31 PM	#2
gchemist BAD BOW-Silverado XST Join Date: May 2000 Location: Senior Member from Austin, TX Posts: 6,431	Re: Help me get new tires! You can only go wider. Taller will change the speedo. You can change the tire ratio by going different size rim. Check out this nifty calculator. http://www.miata.net/garage/tirecalc.html You can change the driven gear to compensate. Find the correct drive and driven gears with this calculator. http://www.tciauto.com/tc/speedo-info __________________ Gerardo a.k.a. Mad Chemist Silverado XST videos

01-30-2012, 10:43 PM	#3
JRANGER Registered User Join Date: Nov 2011 Location: Nashville TN Posts: 3,595	Re: Help me get new tires! Do you know the stock rear gear size? Stock tire size? Wondering if the guy I bought it changed out the tires and rims for not standard size...Gear is rusty and looks stock __________________ PLEASE HELP WITH PROJECT UNDERHAULIN http://67-72chevytrucks.com/vboard/showthread.php?t=306320 Gemini - 1978 GMC C15 High Sierra - 305 c/e - Given to Mother Electra - 1967 Mustang Coupe - 302/w 3 speed Build Chyna - 1968 Chevy C20 Long Stepper

01-30-2012, 10:56 PM	#4
Restrorob The One And Only !!! Join Date: Jan 2007 Location: Hole in the woods Florida Posts: 4,567	Re: Help me get new tires! A 78 C10 came with 225/75-15's, I would assume GMC's did as well. Look on the inside of your glovebox door, Likely would be on the option/build list. There's no telling what's been changed if anything..... __________________ 93 Jeep XJ 88 Buick La Sabra Toys; 88 TA-GTA in the works.... 97 GMC Ext. Cab 502 Crate 5 Spd......Sucks Gas But Haulz Azz ! The primary function of an Engineer is to make it difficult for the Fabricator and impossible for the Mechanic. *"Why go out preserved when you can go out beat up, worn out, sliding in sideways screaming, Holy Sh!t What A Ride" !!!*

01-30-2012, 11:06 PM	#5
JRANGER Registered User Join Date: Nov 2011 Location: Nashville TN Posts: 3,595	Re: Help me get new tires! Shows me nothing...closet thing on my build list L78-15/B TBLS HWY BL and i have no clue what that is lol __________________ PLEASE HELP WITH PROJECT UNDERHAULIN http://67-72chevytrucks.com/vboard/showthread.php?t=306320 Gemini - 1978 GMC C15 High Sierra - 305 c/e - Given to Mother Electra - 1967 Mustang Coupe - 302/w 3 speed Build Chyna - 1968 Chevy C20 Long Stepper

01-30-2012, 11:10 PM	#6
Restrorob The One And Only !!! Join Date: Jan 2007 Location: Hole in the woods Florida Posts: 4,567	Re: Help me get new tires! L78-15 converts over to a 235/75R15 size, they're 28.88 dia. and your 255's are 29.06..... __________________ 93 Jeep XJ 88 Buick La Sabra Toys; 88 TA-GTA in the works.... 97 GMC Ext. Cab 502 Crate 5 Spd......Sucks Gas But Haulz Azz ! The primary function of an Engineer is to make it difficult for the Fabricator and impossible for the Mechanic. *"Why go out preserved when you can go out beat up, worn out, sliding in sideways screaming, Holy Sh!t What A Ride" !!!* Last edited by Restrorob; 01-30-2012 at 11:15 PM.

01-30-2012, 11:12 PM	#7
JRANGER Registered User Join Date: Nov 2011 Location: Nashville TN Posts: 3,595	Re: Help me get new tires! ahhh so my tires are wrong.....and my speedo is prob wrong which would explain why my mom said i was going 80 hen i drove the truck home when i thought i was going 70/75 __________________ PLEASE HELP WITH PROJECT UNDERHAULIN http://67-72chevytrucks.com/vboard/showthread.php?t=306320 Gemini - 1978 GMC C15 High Sierra - 305 c/e - Given to Mother Electra - 1967 Mustang Coupe - 302/w 3 speed Build Chyna - 1968 Chevy C20 Long Stepper

01-30-2012, 11:16 PM	#8
billybobjimmyjoejack Registered User Join Date: Jan 2012 Location: Portland, OR Posts: 15	Re: Help me get new tires! I ordered my 1978 C10 in October of 1977 and took possession of it on Jan. 10th of 1978. I still have the truck and it has one of the original tires as a spare L78-15 is the size that came with the truck. The gear ratio was 3:07 and it is a BIG 10.

01-30-2012, 11:18 PM	#9
Restrorob The One And Only !!! Join Date: Jan 2007 Location: Hole in the woods Florida Posts: 4,567	Re: Help me get new tires! Yep..... The tires on the back of my 97 are smaller than stock and I don't know what else was changed, so my speedo is about 12 miles off. __________________ 93 Jeep XJ 88 Buick La Sabra Toys; 88 TA-GTA in the works.... 97 GMC Ext. Cab 502 Crate 5 Spd......Sucks Gas But Haulz Azz ! The primary function of an Engineer is to make it difficult for the Fabricator and impossible for the Mechanic. *"Why go out preserved when you can go out beat up, worn out, sliding in sideways screaming, Holy Sh!t What A Ride" !!!*

01-30-2012, 11:21 PM	#10
JRANGER Registered User Join Date: Nov 2011 Location: Nashville TN Posts: 3,595	Re: Help me get new tires! hmm weird..Based on the information with the calcs my base gear should be a driven gear of 44 but i think i remember it being green which is 42...... Based on that and the 29" diameter I should be using a 20 tooth gear?? Sound right to yall? Or based on the tire calc i could put on 265/75 15s __________________ PLEASE HELP WITH PROJECT UNDERHAULIN http://67-72chevytrucks.com/vboard/showthread.php?t=306320 Gemini - 1978 GMC C15 High Sierra - 305 c/e - Given to Mother Electra - 1967 Mustang Coupe - 302/w 3 speed Build Chyna - 1968 Chevy C20 Long Stepper

01-30-2012, 11:31 PM	#11
Restrorob The One And Only !!! Join Date: Jan 2007 Location: Hole in the woods Florida Posts: 4,567	Re: Help me get new tires! Sorry, Never used that calc. before..... __________________ 93 Jeep XJ 88 Buick La Sabra Toys; 88 TA-GTA in the works.... 97 GMC Ext. Cab 502 Crate 5 Spd......Sucks Gas But Haulz Azz ! The primary function of an Engineer is to make it difficult for the Fabricator and impossible for the Mechanic. *"Why go out preserved when you can go out beat up, worn out, sliding in sideways screaming, Holy Sh!t What A Ride" !!!*

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