OEM Tire size??

[Michael Wood]

I have the 88' R30 dually and the speedometer is off.

When going 50-60 mph it is reading about 10 mph slower than what I am actually doing.

The only thing I can think of is it may have a different size tire than stock.

Does anyone know the original size tires?

 

[DanMcG]

Not sure if they still did it in 88, but there's a data tag on the driver door of my 85 that has the tire and wheel size from the factory

 

[ajd89]

My 89 crew tag calls for 215/85 16s

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[PrairieDrifter]

It could have had a gear change at some point in its life?

 

[Keith Seymore]

I have the 88' R30 dually and the speedometer is off.

When going 50-60 mph it is reading about 10 mph slower than what I am actually doing.

The only thing I can think of is it may have a different size tire than stock.

Does anyone know the original size tires?

The generic info packet from the GM Heritage Center archive shows two options:

An LT215/85R16 tubeless radial or a 7.50-16 bias ply tube type tire

https://www.gmheritagecenter.com/do...its/Chevrolet-Trucks/1987-Chevrolet-Truck.pdf

(I chose the '87 kit to make sure I was getting the R/V truck; if you choose the kit for 1988 or 1989 it is GMT400 data).

Do you have your SPID label in the glovebox? It will show the tire RPO and the rear axle RPO.

One other thought is perhaps your combination required an external speedo adapter that someone has removed. (My '87 R10 had a speedo adapter).

K

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[Michael Wood]

Thanks for the info, I do not have any door stickers on the truck for reference.

The glove box door is in one of the toolboxes with a broken hinge (which I have a replacement) so I will go dig that out and look for a sticker.

It could have had a gear change at some point I guess, I have no way of knowing.

I do know it came factory with a TH400 and now has a SM465, top gear on both should be 1:1 right? So that shouldn't make a difference at highway speeds.

The rear has 235/85-16's on it now

 

[Michael Wood]

Ok, checked the loose glove box door and found the sticker still there.

It says originally it came with 7.50-16 and after looking up that in metric sizes and comparing it to the 235/85-16 that is on it I should only be experiencing a 1-2 mph difference at 60 mph.

So tire size isn't the issue I guess. It also says that it came with 4:10 rear gears so tomorrow I will check out what is actually in it.

Of course, since it came with a TH400 originally... I don't know what donor truck the SM465 came out of or what final drive gear it had and depending on that would dictate
which speedometer gear it has I guess.

Is there a way to figure out which gear I need to buy for the SM465 to go along with the 4:10 gears to get the right MPH?

There has to be a calculator out there somewhere....I'll start looking.

 

[CSFJ]

I would put my money on the rear axle being regeared or changed, before I would suspect someone changed the final drive in the trans. Not saying it's impossible, but I've never heard of a sm465 with anthing but a 1:1 final.

 

[Michael Wood]

OMFG! Now I have a headache trying to find this info!

I found everything I need to figure this out I think EXCEPT how to know what your drive gear size is in the transmission.

It seems to me the only way to tell is to take the trans apart and visually confirm which drive gear you have? Seems like a PIA.

Am I missing something here? Is there was way to know which drive gear I have

I would put my money on the rear axle being regeared or changed, before I would suspect someone changed the final drive in the trans. Not saying it's impossible, but I've never heard of a sm465 with anthing but a 1:1 final.

I agree. I wasn't saying or meaning to say that the gearing in the 465 was changed (by final drive I meant the differential maybe my terminology was wrong)

My truck originally had a Th400 with 4:10's in the diff......
They got the 465 out of a different truck somewhere in the past with who knows WHAT diff gears.

My point was the TH400 probably had a different speedo gear than the SM465 does so when they put the SM465 in my truck it threw the speedo off.

UNLESS.......the Auto's and manuals had the same speedo gear, if so then that leaves the differential I guess, maybe I have 3:73 or something now.

I will check the gearing later I am up to my neck in other work right now.

 

[CSFJ]

OMFG! Now I have a headache trying to find this info!

I found everything I need to figure this out I think EXCEPT how to know what your drive gear size is in the transmission.

It seems to me the only way to tell is to take the trans apart and visually confirm which drive gear you have? Seems like a PIA.

Am I missing something here? Is there was way to know which drive gear I have


I agree. I wasn't saying or meaning to say that the gearing in the 465 was changed (by final drive I meant the differential maybe my terminology was wrong)

My truck originally had a Th400 with 4:10's in the diff......
They got the 465 out of a different truck somewhere in the past with who knows WHAT diff gears.

My point was the TH400 probably had a different speedo gear than the SM465 does so when they put the SM465 in my truck it threw the speedo off.

UNLESS.......the Auto's and manuals had the same speedo gear, if so then that leaves the differential I guess, maybe I have 3:73 or something now.

I will check the gearing later I am up to my neck in other work right now.

Lol, ok, now I get what you're asking. For some reason my mind went to actual gearing, not speedo gearing. Yeah, you'll probably have to pull the speedo drive adapter to see what you have installed atm.

 

[Matt69olds]

https://www.tciauto.com/speedometer-gear-calculator

Maybe this will help?

 

[Michael Wood]

https://www.tciauto.com/speedometer-gear-calculator

Maybe this will help?

I looked at TCI's site already and unless I misunderstood something all that stuff only helps IF you know your speedo drive gear size.

I guess I will have to pull the gear out of the trans and see.........OR just live with the 10 mph discrepancy at highway speeds.

It's not my daily driver, it's ONLY for hauling construction debris to the dump or large objects home.

Just went down to Phoenix 2 days ago and bought & brought home a new fridge, stove and a all steel 8' picnic table.....still had room to spare...lol

 

[Matt69olds]

Depending on what rearend gears when in the donor transmission truck, you may need to swap both the drive and driven gear. Find out what drive gear you have now, or just enter a tooth count. Keep playing around with tooth count until you get as close to a whole number, without a decimal. In other words, say you have a 15 tooth drive, and the calculator says you need a driven with 38.5 teeth. You can’t have half a tooth, to get the speedo accurate you would need a ratio adapter. Hope that makes sense.

 

[Michael Wood]

Yep. I guess I am at the point to pull the speedo gear and check it.

 

[PrairieDrifter]

I’d bet all these little factors are throwing it off a little bit on their own then it adds up to that 10mph. The transmission swap would be the biggest factor I think, then mix it with slightly different sized tires and possibly incorrect speedo parts it all adds up to a decent sized discrepancy