Tire Sizes

classsic · OTTAWA, ONT

Tire Sizes

Pete Moss · GTA

According to this chart found on the internet 235/75R15 isn't the correct replacement size for an FR78-15.

15 INCH TIRE SIZE
BR78-15 - P165/80Rl5, P175/75Rl5, P185/70Rl5
ER78-15 - P195/75Rl5, P215/65Rl5, P245/50Rl5
FR78-15 - P205/75Rl5, P215/70Rl5, P215/65Rl5, P235/60Rl5, P245/50Rl5
GR78-15 - P215/75Rl5, P215/70RI5, P235/60Rl5, P255/55Rl5, P265/50Rl5
HR78-15 - P225/75Rl5, P235/70Rl5, P255/60Rl5, P275/50R15
JR78-15 - P225/75Rl5, P235/70Rl5, P255/60Rl5
LR78-15 - P235/75Rl5, P245/70Rl5, P255/65Rl5, P265/60Rl5, P295/50Rl5

Looks like a 205/75R15 height is 27.1 inches (one of the tires the above chart shows as a replacement for FR78-15) and a 235/75R15 has a height of 29.9 inches. Other than affecting your speedometer to some extent and the fact that a shorter tire should provide slightly better acceleration (but also a higher rpm at highway speed), I see no reason you can't continue to use the tire size you presently have if you're happy with the way the truck acts now (assuming you've been operating it for a while and are just looking to replace the tires).

Pretty much your call as I see it. Let's see if you get some other opinions smile

427CARL · CLINTON, ONT

classsic wrote:
I'm going to buy four new tires for my 1981 Chev pickup. The brochure says it came with FR78-15R tires??????? I am now running 235 R75 15. Do you think this is a good replacement size?
Thanks

99.9 % of those trucks were upgraded to P235.75R-15 . if your truck is running well now stay with them . (2 cents)

123pugsy · MARKHAM, ONT

Consider small tires in a large wheel well when making your decision.
I like them a bit wider myself. They look good on such a large vehicle.

fraso · FORT ERIE, ONT

If it doesn't rub the fenders or springs and your odometer is accurate, I would say that it's a good replacement size.

Barry's Tire Tech shows the FR78 tire as having an overall diameter of 26.94". I calculate that the P235/75R15 tire has an overall diameter of 27.88", which would cause your odometer to record 6.7% less miles. If the speedometer was calibrated for the FR78 tires, it should show 57.9 mph if you were travelling at 62.1 mph (100 km/h). In a 15" tire with a 235 mm width, you would be better off with a 65-series tire (ie, P235/65R15).

I would get a GPS with a trip odometer and compare the GPS odometer with your truck's odometer. Take the ratio of 2 trip distances and multiply by your current tire diameter to get the overall correct diameter you need and this would take into account the tire's rolling diameter. That is:

correct diameter = (Odometer distance / GPS distance) x 27.88"

Alternatively, since the highway speed limit here in Ontario is 100 km/h, you might want your speedometer to read 60 mph at 100 km/h. You would take the above calculated correct diameter and then adjust it according to the speed ratios. Obviously, this only works if your speedometer is accurate.

adjusted diameter = correct diameter x (62.1/60)

If you know the axle ratio and your speedometer pinion ratio, you can calculate exactly what size tire you need.

See Tire Upgrades

66-67-88Chevy · BRANT COUNTY, ONT

I can verify, because my truck is slightly newer at 1985, that 205/75R15 is the factory size in the door sticker.

However, 235/75R15 looks 100% better, the stock size is way to small.

Im running now a 265/75R15 front and rear and it looks like it should IMO.

classsic · OTTAWA, ONT

Thanks for the info....off to buy the 235's