Changing tire size


#1

thinking about changing tire size from a 205/75/15 to a 235/75 /15 on a 1981 chevy pickup just wondering if this is something that can be done A mechanic told me that it might cause some problems but was not sure . See I have a blazer that i no longer need that has 4 new tires that i want to keep and put on my 1981 pickup that needs tires


#2

Well a 205 to a 235 isnt that much difference but try fitting the 235 on your truck(great truck btw)and turning the steering wheel to see if you have enough clearence


#3

almost forgot to tell you to fit that tire in the FRONT lol (obvious i know but just wanted to remind you)and if that blazer is a 73-87 its wheelbase should match your puckup


#4

On an older truck…I would wonder about (and have checked) the strain on steering and suspension components of non approved sizes. If everything is sound it’s worth a try. No ABS, 4 wd etc. , but slight difference in speedo…see POWERDOG to check difference as a %. I have seem older trucks with larger tires with the wheels snapped off on the side of the road more than once.


#5

It may cause some wheel rub when turning…but it may not. Sinc you already have the tires…try it…If there’s rub then take them off.


#6

I have seem older trucks with larger tires with the wheels snapped off on the side of the road more than once.

If you radically change the wheel/tire size, you can mess up the geometry of the loading on your wheel bearings and cause failure. For a 30mm increase in width, I wouldn’t worry about that. The tire diameter will be 2 x 30mm x 75% larger, so the speedometer will read a bit low. And of course, a bigger tire could well bump into something during turning or suspension travel. Does the owner’s manual say anything about permitted tire sizes? Are you planning to put the new tires on the existing steel wheels – will they fit?


#7

Not only do you have to be careful about rubbing, but you also have to be careful about the width of the rim. In this case, the stock rim (6") is at the lower range of what is acceptable for a P235/75R15.

Needless to say, the diameter of the tire is going to affect the speedometer (and odometer).

Google “Tire Calculator” and you’ll find lots of calculators that will aid in determining how much change there will be.

And lastly:

Be careful of old tires. They may appear OK, but inside the rubber has deteriorated and could cause the tire to fail. There have been many bulletins issued - by tire manufacturers and car manufacturers - and the bulletins seem to focus on different things. My take is that in hot climates (AZ, CA, NM, TX, NV, and FL), 6 years is the limit, where in cold climates (MN, ND, MT, WI, etc.) 10 years is the limit and states in between are …ah… in between.


#8

They will fit fine, although your rear end ratio will be effectively changed, so it may seem a little more sluggish. The speedometer/odometer will be a little off as well, but you can get a different speedometer gear to fix that pretty easily. Overall it’s not that big of a deal.


#9

One other thing not mentioned yet is offset. My 84 S-15 rims were negative offset. If the other rims are positive or no offset then this could cause handling issues. Can also push the tires outside the fenders.


#10

Remember too that braking may be affected as well as accel. I’ll stand by my comment about the added strain on front end components…not enough of a tire size difference to be sure, but truck is old and components should be in good shape BEFORE I’d make any change. If your just trying to run the clunker into the ground, make sure a failure occurs in a managable situation. Suspention components at 60 mph on a crowded freeway aren’t that forgiving. I’ve had a wheel fall of in ther clunker driving years, luckily at low speed. That was exciting.


#11

Others have made excellent points. www.carbibles.com has a calculator so you can see how it will affect your speedo.

One point I’d like to add is that it’s almost a 10% increase in tire width. Pickups are light in the rear end anyway, and it may just be enough of an increase to cause hydroplaning in wet weather. I’d go easy until you find out for sure. You don’t want to spin the truck sround on the exit ramp in a rainstorm.


#12

Good point however 235/75/R15 tires were very very common in trucks of that era, I would wager that the truck is question probably has the standard tire size and that 235/75/R15’s were an optional size. I don’t think the OP has anything worry about suspension-wise. Trucks of that era were fairly overbuilt. My first vehicle ever was a 74’ Ford F-100, my dad still has it and still uses it.