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How to determine stock size tire, will the vin help?

Jorge6.5

Jorge6.5

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Upstate New York
I want to put Oem size tires on my truck, 1990 k1500 short bed standard cab with 5speed and 305 v8. Reason being, I want best fuel economy and to accurately determine fuel mileage. I currently have 265s but they are doggy with the 3.42 gears. I always thought 245s were stock size but after some recent searching, I have found that 225s were a possible size. So I'm not sure on what tires to buy.

Will my vin number be able to tell me what the truck came with from factory?

Thanks
 
My "95" 2500 Suburban has a sticker on the drivers side door. Mine shows 245s as original so I wouldn´t be suprized if yours was 225.
 
Most likely was 245/75/16's. I believe the 225's came with 2.73 gears in 2 wheel drive trucks.
 
Unfortunately my door sticker is gone.

I was leaning toward the 245s due to the fact it's a 4x4 model. Actually been considering 215/85r16, same height as the 245/75r16 but not as wide. I figured these will help fuel mileage further.

Will the vin number have tell size info?
 
I think up to 90 could've had 225 tires, even on 4X4. Not sure where you could tell...

Sent from Behind the Pine Curtain
 
After a bunch of searching, I looked up my vin number. That showed up as having LT225/75r16 tires but to be sure, I checked the rpo codes. XHN also verified that it's LT225/75r16 tires. Checked the owners manual too but it didn't say for sure, just had an example that also said LT225/75r16

I'm gonna go with the 225s to help maximize my MPGs and maybe be able to hold it in 5 gear

Thanks again
 
3.42 and they suck with all these hills around here (the catskill mountains) I checked the size of the 225s, 29.5inches

It's a commuter vehicle. I don't do any off-roading anymore. Truck is only a swb too so they shouldn't look too small
 
I had 3.42's and 245/75's w/ a 305/4l60, not a power house but when running properly 20mpg was doable. Held gear at 70 but 60-65 it would down shift cruising bout 1700 rpm.

I honestly dont think that tire change will make a difference. Maybe the 215/85 idea but I wouldnt go down in height.

Have you checked speed via GPS?
 
I can hardly keep it in 5th gear, just not enough power especially with the 265 mud tires I put on it. The truck runs good too, everything is up to par on the motor and pretty much the wholefront end is all new. I think going to 225s from 265s will help it's around a 2" height difference. The 215s would be ok but I'm not sure I want to go that narrow and they are pretty much the same height as the 245s.

If I had 3.73 or 4.10 gears, I wouldn't mind going with 245s or just staying with 265s but in a highway or a/t style tire.
 
Something don't sound right. My uncle had an 85 K5 BLAZER with a carbed 305, 700R4, 3.08 gears, and turned 33" mud tires with no problems at 70. He even towed with it without much trouble. It was no rocket, but held speed just fine chugging along.
 
I dropped close to 300 rpm at 60 mph going from 225/85/16 to 235/85/16

You should see about that in rpm increase.
 
It shouldn't have too much trouble as is if all is well. The engine or tbi must be weak or something is wrong somewhere.

source unknown
 
I'm thinking the engine is getting weak but I mean it runs good for 236k miles. It would have no problem holding 70mph in 5th but the roads I drive are 55mph with hills. It goes great in 4th. I dont like lugging motors and won't do it. Perhaps the gear ratios in high gear are different from auto to standard?

I'm planning on doing the ultimate tbi mods along with some other change to the intake side. Might help boost power to where I need it
 
The auto has a .70:1 OD, and the manual has a .73:1 OD. So the auto lugs down lower. The shorter tires will help, and the 265's aren't doing it any favors.
 
In retrospect there will be a smaller rpm difference with yours, the taller gearing will make less difference.
NVW said:
I dropped close to 300 rpm at 60 mph going from 225/85/16 to 235/85/16

You should see about that in rpm increase.
Click to expand...
 
I finally got around to swapping the tires to lt225 75 16. I went with new hercules courser ltr. Seems like the truck has more power, going up hills in 4th takes very little throttle now and I can actually use 5th gear without going 60mph plus. I haven't driven the truck much since the tire swap to calculate MPGs. Should have some results in the next week or so.
 
mpg's may go down on highway driving, but you will have more power. I went from 225's to 235's and noticed the opposite.
 
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