Tire size and Speedometer question?

[Ray Cobia]

If I take the G-78-15 tires off my 72, and put 255/70/15 tires on, what will it do to my speedometer reading?

[Longhorn Man]

How tall are the G-78-15's? And how tall are the 255/70's?
(I know I never did figure out the old measurments out)
If the tire your installing is taller, it will slow down the speedo, and likewise, if your new tire is smaller, it will speed up your speedo reading.
Normally an inch or two isn't enough to worry about it.

[72CustomCamper]

Ray, just do the math.

Don't know what the heck a G-78-15 is but the other size works out like this:

255/70/R15

255=width in mm of the tread area
70=the tire wall height as a factor of width (70% of 255mm)
R15=Diameter of the wheel which the tire was meant for

Knowing this, just calculate the circumference or diameter of the tire and compare it to your originals. If it is bigger, like longhornmail said, than you will cover more distance per revolution and therefore you will be going faster than your speedometer indicates.

This stuff is useful when you want to go to a wider tire but retain the original overall diameter...

[adamls]

Here goes, some math from an engineer.


Tire Diameter = (2*Section Width*Aspect Ratio)/2540 + Rim Diameter, example 255/70/R17
Tire Diameter=(2*255*70)/2540+17=31.06"

Actual MPH=(new tire diameter/old tire diameter)*Gauge Reading

I too do not know what G-78-15 is so I cannot figure this out for you. But assuming your old tire is a 28" inch tire then when your gauge is reading 70 your actual MPH is (31.06/28)*70=77.6 MPH

[ChevLoRay]

G78's, huh? Wow. I thought those had all dry-rotted from age. I mean, I sold and mounted those when radials were fabric-belted and true steel radials were a luxury tire. You need to get out more, man.

I have 255-70R15's on my truck. You may recall a comment I made in the post about Hotrod70C10's delimna. My truck indicates a speed of 30 mph, when I am actually going 34 mph. The stock final drive on my truck, according to the data I have, is that it should be a 3.07 gear. I thought they all came with 3.73's. However, with no tach, I have nothing else to go on. The data plate in the glovebox doesn't indicate an optional rear gear from the factory, in spite of the 2000-lb rear spring option on the SWB truck I have.

Oh, I still have a G78-14 Dunlop white stripe AND an F70-14 Polyglas GT! Why? Well, they're mounted on a pair of Mickey Thompson Medallion wheels. I never could get another pair to match them, so I never used them. They're the GM 5 on 4.75 bolt pattern, i.e., standard Chevy pattern for many years....I took them off of a '69 Camaro, in '74.

I think the G78's were the chronological replacements for the original 8.25-15's. F's replaced the 7.75's. H's replaced the 8.50's. P-series tires are metric, as in P225's, etc.

[mrein3]

G78s are approximately equal to 225-70r15. You youngsters prolly never bought a tire with a letter size.

[70c10]

From www.tirerack.com new tire diameter divided by original tire diameter= blank. Multiply this number by indicated speed = actual speed with new tires!

[Ray Cobia]

ChevLoRay--Thank you for a very knowledgeable post. You just proved a point for me, that tire size (height) can affect speedometer readings. Someone told me that I sounded silly by saying that. This was a hypothetical question. The original equipment tire on my 72 was G-78s. It now has 225/70/15 Goodyear Eagles, with 8" Rallys. Man, I still get out ever now and then, look at my list of "RIDES" below. Thanks again!!

[Hotrod70C10]

So.. Chevloray your truck actually reads 4 mph slower than you are actually going? If that is true...Then I was wrong I would have been going right at 40 to 41. I was wrong if what you are sayin is correct. By the looks of those old tires they look taller than the 255/75/15's. By this the officer was right and I was going that fast. I wouldn't have thought anything of it if she would have told me that she stopped me for speeding in the first place. I just felt that she bumped up my speed for letting her know my truck was emmisions exempt.

[rusty ol' 69']

Ok, Ray you were right I was wrong. Your the man. Big guy. king of the truck world. By the way, this is common knowledge and your avoiding the obvious so your right. Your the man!!! Dude

[Suicide-D]

here's a couple of sites that might interest you. The second one has some cool calculators

1.Tires

2.Gears,Rpm,Mph,Tire Height...etc....

[Ray Cobia]

Thanks Dirt Dobber, Both of those are great!!

[Pardue'sCustoms]

G78 man it been a while since i saw thwm