Tire size

[HighSierra79]

So, I've got my old 86 gmc truck sitting in the barn, just itchin for warm weather to break. Needs some new rubber, just feel uneasy at 70 with 14 yr old tires. Anyway, thinking about 275/60/15 on a gm 15x8, factory stance. Worried about bulge look. Anybody ever ran this setup?

 

[North Idaho Wolfman]

IMHO that's a good tire size, a little fatter than stock (235/75R15) but not over the top.
But I will caution you that if you change from the factory size then you will be changing the speedometer calibration.

 

[Daren Todd]

Smaller tire diameter, and the speedo reads faster then your actually going. Bigger tire size and speedo reads lower then your actually going. Around 35 miles an hour they both will read fairly accurate. But what we found on a f550 flat bed when we switched from low profile tires was at 50 mph the truck was actually going 60. Had a coworker get a ticket in a work zone right after the swap

Just have your wife follow you and double check the speedometer after the swap so you know what it's reading at compared to actual speed

 

[skeets]

That's what I run on my Silverado, no problems cept they are 16's

 

[D2Cat]

"Just have your wife follow you and double check the speedometer after the swap so you know what it's reading at compared to actual speed."

If one is un-wifed could one use a GPS?

 

[Daren Todd]

"Just have your wife follow you and double check the speedometer after the swap so you know what it's reading at compared to actual speed."

If one is un-wifed could one use a GPS?

True, or a buddy

 

[85Hokie]

"Just have your wife follow you and double check the speedometer after the swap so you know what it's reading at compared to actual speed."

If one is un-wifed could one use a GPS?

I think it wooooullld be muchhhh easy to use the gps.......somehow the gps doesnt need to understand the how and the whys......

 

[Tomcat]

I thought GPS was for location, not speed.

 

[Daren Todd]

I thought GPS was for location, not speed.

It shows speed also on the display. The one in my work truck gets pretty screwy so I don't trust it. Had it showing I was doing 30mph when I had just let off the brakes at a stop light. And have had it showing 10 mph when I was doing 75 down the interstate

 

[Tooljunkie]

I use an app on my iphone HUD. It works well, can reverse screen and throw phone on dash and it displays on windshield at night. Handy if speedo is off.

 

[nzzshl]

That speedo differential issue is a non-event. A transmission shop will be able to quickly provide an appropriate gear for the speed that will bring you back into the window of +- 2 mph. The gears are plastic (always have been). Just unbolt the speedometer cable adapter on the side of the transmission and separate it from the transmission case. Takes about 10 minutes. They didn't integrate electronic speed sensors onto GM truck until 1988. That's when they installed RWAL (Rear Wheel Anti-Lock) brakes. Digital sensors became the norm at that time to talk to the controller softwares. Anyway, good luck with it. Any problems, rattle me........I check the site almost daily, sometimes multiple times for new threads.

 

[85Hokie]

I use an app on my iphone HUD. It works well, can reverse screen and throw phone on dash and it displays on windshield at night. Handy if speedo is off.

I have that same app, very neat !!! I find most GPS's are far more accurate than the speedos......GPS cares not about tires nor gears.....straight out point to point and speed from one to the other.

 

[Tooljunkie]

I think we are overthinking this. 235/75r15 vs 275/60r15 is likely 10% or less difference in size. 265/75r15 is 31" tall give or take depending on tire manufacturer. Would be best to compare tire size by manufacturer.

 

[tiredguy]

Should be perfect since the 235/75R15 & 275/60R15 are the same height.
Actually on the 8 inch wide factory rally wheels is to wide for a 235 even
though they came from the factory that way, and with the 275 the tread will be flat like it should and the sidewalls just right too. My 79 Chevy Short box
4x4 is the same way very common on both 2 and 4 wheel drives.
Glad you're smarter than a tire realizing it's probably time due to age, and
when you pull those 235's off look at the area around the face of the bead
against the rim and I bet you'll see signs of chafing/cracking at the top edge
too. That's where the real problem with age is a critical problem and for sure
you don't want to mess up such a sweet old ride.
Al