tires

[79gmcc15]

hey guys, got a couple questions about tires. I HAVE to buy 2 new rear tires for my 79 and the ones I have now are 30 x 9.50 R15 LT's and I looked at the tag on the door and that says that the truck needs LR 78 x 15 C's is that the same? do they make the latter anymore? one reson is that Ive always suspected that my spedo was off, and I noticed that my odmiter is also off, because Ive checked it useing the mile markers and when it came to the same number on the od its ether been befor the mile marker or after it by a few 100 feet. and sometimes notices it run fast or slower. but this was going between 60-70 so I dont know if that makes a diference. Ive been told that the 30 x 9.50 's are a harder size to find. is this true? if so what is a good size to put on the back. I cant aford alot. around $100 per tire is about max. I've been quoted for a used pair of tires that in realy good shape that are a lill biger for 80 out the door. so that seems best for right now. its ok to me if I get biger in the back for now. I happen to like the rake stance anyway lol. thats what it is now I think. thanks for your help!

[Palf70Step]

In today's tire size your truck probably would have come with 225-75x15 as the standard tire. Most would run 235-75x15 as that is what the standard tire for today is. If you want a regular tire to replace your current tire a 255-75 will be about the same size as your current tire. That is about a 2" height difference (taller)which means your speed is off maybe 3-5 MPH (faster)at Highway speeds.

[79gmcc15]

would it realy efect the speed diference if they go on the back? I also have I think 3.73 gears and a 350 trany, what would that do to performance?

[eagle23]

Originally Posted by 79gmcc15

would it realy efect the speed diference if they go on the back? I also have I think 3.73 gears and a 350 trany, what would that do to performance?

PFJI, I checked with a website on tire heights, and your old L78's are (per them) equal to 235/75x15's. That tire is 28.9 inches tall; it's also a very common, readily available (read: cheap ) size. If you want to go 70 series, use the 255/70x15. Beyond that, "consult your tire expert"

Being on the back will make all the difference on the speedo readings, since that's where the speedometer gets its information (back of the tranny). The taller tire that you're running now will slow acceleration (compared to the original size) and also throw off the speedo/odometer readings. You can mentally compensate for that to avoid tickets And keep in mind our speedometers are 70's accurate combined with just being old

One more bit of minutia: that taller tire would have made it appear your gas mileage got worse, since the taller tire is making fewer revs per mile, making the odometer take longer to clock a "mile". Your mileage isn't worse, and may be a bit better for the engine turning slower. Using the more correct size will give you a better idea of actual gas mileage.

One caveat: I talking sizeable tire height differences, not a half-inch or so. If you go from the 30 x 9.50's you now have to 29's the difference would likely be quite noticeable, both for acceleration and accuracy.

Hope this helps.

Brian

[8t5Silverado]

30x9.5's aren't hard to find.

My truck came with 225/75-15's and I plan on installing 31x10.5's. I'll just figure to multiply my speed by 31/28.29.

[Apache]

Or just figure up your tire size (which ever you decide on and buy) and your gear ratio, and then buy another gear for the speedometer. That should make it more accurate, but if you dont want to do that just remember that you will probally be going 5mph difference.

[piecesparts]

Man you guys make it hard. To figure out if your speedometer is off just put the truck at 60 MPH and drive a couple of miles per some mile markers. At 60 MPH it should take 60 seconds, at 65 MPH it should take 55.5 seconds, at 70 MPH it should take 51.5 seconds, at 75 MPH it should take 48 seconds. Time the distance from one marker to the next and then calculate. The formula is simple 60 MPH x 60 seconds equals 3600; so for any reading other than 60 seconds divide it into 3600 and get the speed you are going. REMEMBER Not all mile markers are the same exact distance and you should be on a level area to figure this out. Taller tires will make you go faster than your speedo shows and shorter tires will make your speedo faster than you are going.