19.5 Tire Size - The 1947 - Present Chevrolet & GMC Truck Message Board Network

soxplayer · 06-06-2009, 03:58 PM

My wife's 67 C20 is an old Forest Service Stepside with ultility boxes. The GVWR is 7500 and according to the owners manual it came equipped with 8-19.5 6 ply front tires and 8-19.5 10 ply tires in the rear. Right now it is on 7.50-16 LT on demountable 8 lug rims. Does anybody now how to determine the diameter of the original tires (i.e. 78,80 or 90 series). The differential is a 4.56:1 and the shorter tires really limit highway travel. I am considereing 235/85 16 on newer tubeless rims but I really want to know how close these taller tires are to the original diameter in order to lower the highway revs to an appropriate level.

Cyclone44 · 06-06-2009, 04:14 PM

Welcome from Knoxville Tn.....I will move this where it should get more responses....

DREAMS OF A 67 · 06-06-2009, 04:16 PM

FROM KNOXVILLE TN!!!

**Stocker** · 06-06-2009, 05:45 PM

Welcome from NorCal! I tried a couple different tire size comparison sites, but neither lists your original size...

**john** · 06-06-2009, 06:30 PM

Heres a site that shows 34.6 diameter for a 8-19.5 10 ply. And welcome to the best truck site.
http://www.sylmar.ca/autoville/tires/truck-609029.jpg

BLAZERMAN · 06-06-2009, 07:12 PM

I'm running 8.00 19.5 12 ply tires on my 1 ton and they measure close to 35" tall. Thats about the tallest tire available for the 19.5 rims and they are hard to find.

factorystock · 06-06-2009, 08:26 PM

The formula: RPM = MPH x AXLE RATIO x 336 / TIRE DIAMETER. For maximum highway speed without changing the whole rear axle, change out the 4.56 pumpkin with a 4.10 pumkin and use the period correct 19.5" wheels. Tire sizes sometimes vary between 1"-2" from different manufactures. Most 8-19.5 tires are 33" to 35" tall.

soxplayer · 11-08-2011, 01:32 AM

Just thought I would dredge this up and give a clear answer. The truck actually has a 5.13:1 axle ratio. The 235/85/16 tires give perfect speedometer and odometer readings so I suspected they are slightly bigger than stock. I eventually found the original spare and it is shorter than the new tires. Long story short: 65 MPG = 3800 RPM = 14 MPG out of a 250 six.

**special-K** · 11-08-2011, 08:14 AM

Yeah,those 19.5s were a bit short on the sidewall height,just like 16.5s. They were a lower profile diameter. You could probably go to 255/85s for a bit lower RPMs. But,that rear could change right out with a 4.10 with minor work. Just un-bolt the driveshaft and slip the axle shafts out and you can remove the entire differential from the front.

06-06-2009, 03:58 PM	#1
soxplayer Registered User Join Date: Jun 2009 Location: Forest City IA Posts: 56	19.5 Tire Size My wife's 67 C20 is an old Forest Service Stepside with ultility boxes. The GVWR is 7500 and according to the owners manual it came equipped with 8-19.5 6 ply front tires and 8-19.5 10 ply tires in the rear. Right now it is on 7.50-16 LT on demountable 8 lug rims. Does anybody now how to determine the diameter of the original tires (i.e. 78,80 or 90 series). The differential is a 4.56:1 and the shorter tires really limit highway travel. I am considereing 235/85 16 on newer tubeless rims but I really want to know how close these taller tires are to the original diameter in order to lower the highway revs to an appropriate level.

06-06-2009, 04:14 PM	#2
Cyclone44 Step Brother Join Date: Sep 2007 Location: K-town Posts: 4,029	Re: 19.5 Tire Size Welcome from Knoxville Tn.....I will move this where it should get more responses....

06-06-2009, 04:16 PM	#3
DREAMS OF A 67 Step Brother Join Date: Sep 2007 Location: knoxville TN Posts: 1,950	Re: 19.5 Tire Size FROM KNOXVILLE TN!!! __________________ Joe

06-06-2009, 05:45 PM	#4
Stocker 20' Daredevil (Ret) Join Date: May 2003 Location: Jefferson State Posts: 13,544	Re: 19.5 Tire Size Welcome from NorCal! I tried a couple different tire size comparison sites, but neither lists your original size... __________________ - Mike - 1972 K20 LWB 350/350/205 RIP El Jay

06-06-2009, 06:30 PM	#5
john member #16 Join Date: Dec 1999 Location: Yuba City Ca. Posts: 3,618	Re: 19.5 Tire Size Heres a site that shows 34.6 diameter for a 8-19.5 10 ply. And welcome to the best truck site. http://www.sylmar.ca/autoville/tires/truck-609029.jpg __________________ john http://67-72chevytrucks.com/vboard/showgroups.php board vendors http://www.flickr.com/photos/1745471...7602865984234/ Last edited by john; 06-06-2009 at 06:32 PM.

06-06-2009, 07:12 PM	#6
BLAZERMAN Currently Blazerless Join Date: Apr 2000 Location: St. Joseph Mo. Posts: 4,826	Re: 19.5 Tire Size I'm running 8.00 19.5 12 ply tires on my 1 ton and they measure close to 35" tall. Thats about the tallest tire available for the 19.5 rims and they are hard to find. __________________ 1972 C/30 133" W.B. C&C 1970 GMC 3500 157" W.B. single wheel C&C

06-06-2009, 08:26 PM	#7
factorystock Registered User Join Date: Feb 2007 Location: west coast Posts: 3,357	Re: 19.5 Tire Size The formula: RPM = MPH x AXLE RATIO x 336 / TIRE DIAMETER. For maximum highway speed without changing the whole rear axle, change out the 4.56 pumpkin with a 4.10 pumkin and use the period correct 19.5" wheels. Tire sizes sometimes vary between 1"-2" from different manufactures. Most 8-19.5 tires are 33" to 35" tall.

11-08-2011, 01:32 AM	#8
soxplayer Registered User Join Date: Jun 2009 Location: Forest City IA Posts: 56	Re: 19.5 Tire Size Just thought I would dredge this up and give a clear answer. The truck actually has a 5.13:1 axle ratio. The 235/85/16 tires give perfect speedometer and odometer readings so I suspected they are slightly bigger than stock. I eventually found the original spare and it is shorter than the new tires. Long story short: 65 MPG = 3800 RPM = 14 MPG out of a 250 six.

11-08-2011, 08:14 AM	#9
special-K Special Order Join Date: Nov 2004 Location: Mt Airy, MD Posts: 85,862	Re: 19.5 Tire Size Yeah,those 19.5s were a bit short on the sidewall height,just like 16.5s. They were a lower profile diameter. You could probably go to 255/85s for a bit lower RPMs. But,that rear could change right out with a 4.10 with minor work. Just un-bolt the driveshaft and slip the axle shafts out and you can remove the entire differential from the front. __________________ "BUILDING A BETTER WAY TO SERVE THE USA"......67/72......"The New Breed" GMC '67 C1500 Wideside Super Custom SWB: 327/M22/3.42 posi.........."The '67" (project) GMC '72 K2500 Wideside Sierra Custom Camper: 350/TH350/4.10 Power-Lok..."The '72" (rolling) Tim "Don't call me a redneck. I'm a rough cut country gentleman" R.I.P. ~ East Side Low Life ~ El Jay ~ 72BLUZ ~ Fasteddie69 ~ Ron586 ~ 67ChevyRedneck ~ Grumpy Old Man ~



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