Tire Size Conversion
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Laughing Gas
Joined: Jan 2004
Posts: 813
Likes: 5
From: Meade, KS
Tire Size Conversion
Hey all-
I know that this question doesn't REALLY fit in this forum. To tell you the truth I'm not asking for a Ford Truck, though it is for a Merc. And this is my FTE home, so I'm asking y'all anyway.
Here's the deal, I recently acquired a new project that needs new tires. The problem I've run into is that the tires I currently have are R78-15. I'm not at all familiar with this sizing. I'm familiar with the "255/75/16" style sizing, and don't know how to convert the old sizes to the new sizes. Can anyone help me with this?
Thanks!
I know that this question doesn't REALLY fit in this forum. To tell you the truth I'm not asking for a Ford Truck, though it is for a Merc. And this is my FTE home, so I'm asking y'all anyway.
Here's the deal, I recently acquired a new project that needs new tires. The problem I've run into is that the tires I currently have are R78-15. I'm not at all familiar with this sizing. I'm familiar with the "255/75/16" style sizing, and don't know how to convert the old sizes to the new sizes. Can anyone help me with this?
Thanks!
Tuned
Joined: Oct 2002
Posts: 370
Likes: 0
From: Southern Connecticut
Here is a couple charts and also a decent article. Unfortunately i dont see an R78-15. It almost sounds like the "R" means radial.
http://www.tirerack.com/tires/tirete...sionchart.html
http://www.jags.org/TechInfo/2001/05...redescript.htm
http://www.tireguides.com/tip5.html
hopefully someone else could shed some more light on this.
Mark
http://www.tirerack.com/tires/tirete...sionchart.html
http://www.jags.org/TechInfo/2001/05...redescript.htm
http://www.tireguides.com/tip5.html
hopefully someone else could shed some more light on this.
Mark
Post Fiend
Joined: Jun 2001
Posts: 7,493
Likes: 7
From: Erie, pa
Thread Starter
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Laughing Gas
Joined: Jan 2004
Posts: 813
Likes: 5
From: Meade, KS
Thanks for your input. When I get a chance I'll look for there to be a letter in front of the R. I don't remember one, but it looks like there should have been one.
Thanks again!
Thanks again!
Fleet Owner
Joined: Jul 2004
Posts: 27,288
Likes: 1,053
From: NM
Originally Posted by bobj49f2
None of these charts give a conversion for stock 16" tires. Can I just substitute "16" for "15"?
In my experience trying to calculate rolling diameter from the nameplate size is not accurate. I would like to see a database of common tires' rolling diameters. I am looking to go up in diameter to reduce RPM's, and would like to be able to tell how much improvement I'll see ahead of spending the bucks.
Hotshot
Joined: Jan 2001
Posts: 16,874
Likes: 3,114
From: SE Wisc. (the Rust Belt)
I went to Tires Plus the other day to ask for a price for 6.50X16 tires, or their modern equivalent, for my panel. The guys at the counter acted like I just landed a UFO in their parking lot. One of the guys must have been in his '50s so it was just some young kid. I didn't have the truck with me, I just gave them the number on the side wall. They didn't have any way to cross reference the size.
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Laughing Gas
Joined: Oct 2003
Posts: 890
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From: New York
Originally Posted by bobj49f2
I went to Tires Plus the other day to ask for a price for 6.50X16 tires, or their modern equivalent, for my panel. The guys at the counter acted like I just landed a UFO in their parking lot. One of the guys must have been in his '50s so it was just some young kid. I didn't have the truck with me, I just gave them the number on the side wall. They didn't have any way to cross reference the size.
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Hotshot
Joined: Jan 2001
Posts: 16,874
Likes: 3,114
From: SE Wisc. (the Rust Belt)
My neighbor told me his son-in-law is running wider, modern tires on his stock 16" rims. I haven't had a chance to check them out yet. I just want something to stick on the stock rims, I don't care is they're a little wide as long as there's no clearance problems. I don't want to go the Coker route=big $$$
Senior User
Joined: Jul 2004
Posts: 216
Likes: 0
From: North Branch, MI
Here's what I found
I just had the same question with the tires on the 78 F150 I have. The old tires are L78-15's what I found was they can be replaced by 235/75-15's. Here is the link to the web site I found the info. The chart does not show an R78-15
http://www.vintagecarconnection.com/vintage_tire_size_conversion_chart.htm
Hope this helps.
http://www.vintagecarconnection.com/vintage_tire_size_conversion_chart.htm
Hope this helps.
Last edited by sgtrwg; Aug 28, 2006 at 08:28 PM.
Fleet Mechanic
Joined: Mar 2005
Posts: 1,511
Likes: 1
From: peyton, colorado
Check the sidewall again, there will be another letter in front of the "r". It's probably a LR7815, which converts to p235/75r15, a very popular 1/2 ton pu size. HR7815 is the same as p225/75r15, GR78 the same as p215/75r15. As far as 16" conversions, a 225/75r16 is a reasonable replacement for the 650-16 original, either in a "P" metric for 1/2 ton, or load range d or e (8 or 10 ply) for the 3/4 ton. No, it's not the exact replacement, but it's a good compromise when price and availability are a factor. Btw, bobj, I'm not surprised that guy at your tire store had no clue. Most guys on the counter stop learning their products after about a month, and don't care to become professional in their trade. Then they call me....Pm me if you need more info.
Thread Starter
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Laughing Gas
Joined: Jan 2004
Posts: 813
Likes: 5
From: Meade, KS
Alright--here's the report. My uncle went out and looked at the tires for me, and all there is on the rear tires is R/78/15. On the front tires: one is P/205/75R15 the other is P215/75B15. So I've got three radials, two R/78/15, and one 205/75/15, and a bias ply 215/75/15. Now I'm really lost!
This rig is stock except for the rubber, which apparently isn't sure what size it should be. Two of the tires are wasted (the fronts) so I suppose I can replace them with something similar in size to what's on there now, but the rears will need to be replaced (they hold air, but are old and worn). For now I just want to get it rolling, but as the project moves along I'm going to want tires that are the right size so that the speedo will be correct (as much as a speedo can be correct, that is).
Anybody know of a way to figure out the original tire size requirement for a given vehicle?
Thanks all!
James
This rig is stock except for the rubber, which apparently isn't sure what size it should be. Two of the tires are wasted (the fronts) so I suppose I can replace them with something similar in size to what's on there now, but the rears will need to be replaced (they hold air, but are old and worn). For now I just want to get it rolling, but as the project moves along I'm going to want tires that are the right size so that the speedo will be correct (as much as a speedo can be correct, that is).
Anybody know of a way to figure out the original tire size requirement for a given vehicle?
Thanks all!
James
Roast em' if you got 'em
Joined: Mar 2005
Posts: 21,976
Likes: 9,807
From: Rio Rancho, NM
James,
I'm not sure what tires translate over to the R/78/15's but I do know that P205/75/15's are going to be a bit small, P215' are a bit better. I would suggest something in either a P225 or a P235
Thats just my opinion
Bobby
I'm not sure what tires translate over to the R/78/15's but I do know that P205/75/15's are going to be a bit small, P215' are a bit better. I would suggest something in either a P225 or a P235
Thats just my opinion
Bobby
