original tires Goodyear 185 ?75 R15
stated pressure 20psi F 24psi R
new tires Goodyear 185 65 R15
What are the pressures for these tires?
The tire pressures would be the same.
Tester
Use the same pressures you used in the original tires. You haven’t changed the size enough to make any difference in pressure.
Both of the above answers are absolutely wrong - unless you like what happened a few years ago with Ford and Firestone.
What you want to do is match the load carrying capacity of the original tires. I am not at my reference books (and won’t be for about a week), but the tire size you quoted as the original size doesn’t seem right - but let’s go with it for a moment and I’ll post back later when I can get at my reference books.
So ASSUMING that the original tire size was 185/75R15 at 20 / 24, then in order to carry the same load, a 185/65R15 needs to have … well, can’t find any load tables on the internet for a 185/75R15, so I guess I need my reference books to do this part!
I just did a look up at Coker Tire - a company that supplies period correct tires for vintage vehicles. Their website said the original tire size for a 1974 Triumph TR6 was 185R15 - which makes complete sense to me.
If this is true, then a 185/65R15 needs to use - and I’m guessing because I don’t have my reference books in front of me - about 25 psi / 29 psi.
I’ll post back later to update this info!
I want to say that the proper sized tire would be a 185/80/R15, I don’t have my TR6 shop manual with me at the moment. We have 205/70/R15’s on our TR6. And the overal diameter is pretty close. 185/65/R15’s would be significantly smaller than stock.
OK, I am back where my reference books are!
According to Tires Guides, a 1974 Triumph TR6 originally came with 185R15’s mounted on 4 1/2" rims, inflated to 20 psi front / 24 psi rear.
A 185/65R15 has an allowable rim width range of 5" to 6 1/2", and in order to get the same load carrying capacity that a 185R15 gets at 20 / 24 psi, it has to have 24 / 28 psi.
First, the stock rim (4 1/2") is too narrow for the 185/65R15’s. This will tend to make the tire arch across the tread face and be prone to follow the ruts and grooves in the pavement as well as being sensitive to the wind. The car will feel (and be) unstable.
The minimum recommended pressure for passenger radial tires is 26 psi - and since it is also recommended that the pressure split be maintained front to rear, the overall package cames to 26 / 30 psi. This will further aggravate the arching affect.
Unfortunately, there aren’t any modern tires that will fit a 15 X 4 1/2" rim.
This now becomes a difficult decision. Do you use a modern tire that isn’t quite right for the situation - or go to Coker Tire and buy properly fitting, period correct - but expensive - tires?
Same footprint…so should be the same pressure.
How about wider rims?
Then he can set aside the original rims until he sells the car.
Thanks a lot for your extra effort. I thought this would be easy.
I don’t drive it much. I will use 24/28 psi and see what happens.
thanks again,
Ron