tire size for a correct speedometer reading

[OldGMJon]

I have a set of 245 75R 16 tires on my C10. My speedometer says I am doing 42 MPH but I am really going 50 MPH.
I saw on one thread that some one riding 75's was saying it was throwing off his speedo.
Assuming it is tire and rim size causing my incorrect reading, I picked up a set of 15" nubbies.
I have searched out a hundred threads but can't seem to find anyone stating what size tire would make my speedometer show a closer to correct reading.
15" rims seem to have a limited options but was thinking maybe 235 70R may do. Anybody got the info?

[jtrichard]

IF you like the mileage/power and look of the tire size you are running you can use one of these it will bring your speedo real close http://www.ebay.com/itm/AC-Delco-Ste...b671b5&vxp=mtr

[bhc41051]

I think you might be putting the cart before the horse on this one; why don't you call a speedometer shop. I have no experience with this place. But they're kinda sorta close to you. https://plus.google.com/103811379957...04/about?hl=en

[markeb01]

I'm not sure changing tire diameters is the best approach to speedo accuracy this many years after production. Too many thing may have been changed over the life of the truck. The ratio adapter noted above may help, but it might not be the ratio change you need.

A speedometer shop would be the best bet as they can simply change the plastic driven gear in your transmission to one that gets you very close to a correct reading:

[OldGMJon]

All good advice thanks for responding...Sacramento is about a four hour drive from here though.

Originally Posted by jtrichard

IF you like the mileage/power and look of the tire size you are running you can use one of these it will bring your speedo real close http://www.ebay.com/itm/AC-Delco-Ste...b671b5&vxp=mtr

Originally Posted by bhc41051

I think you might be putting the cart before the horse on this one; why don't you call a speedometer shop. I have no experience with this place. But they're kinda sorta close to you. https://plus.google.com/103811379957...04/about?hl=en

Originally Posted by markeb01

I'm not sure changing tire diameters is the best approach to speedo accuracy this many years after production. Too many thing may have been changed over the life of the truck. The ratio adapter noted above may help, but it might not be the ratio change you need.

A speedometer shop would be the best bet as they can simply change the plastic driven gear in your transmission to one that gets you very close to a correct reading:

[Hugh in NC]

Originally Posted by markeb01

I'm not sure changing tire diameters is the best approach to speedo accuracy this many years after production. Too many thing may have been changed over the life of the truck. The ratio adapter noted above may help, but it might not be the ratio change you need.

A speedometer shop would be the best bet as they can simply change the plastic driven gear in your transmission to one that gets you very close to a correct reading:

I went thru this with me truck recently. My speedometer was 5 mph slow. I changed the plastic gear by 2 teeth. If I am remembering right, I had the blue gear which is 19 teeth and had to change to the brown gear with 17 teeth. Lower number of teeth speeds up the speedometer gauge. This is the gear that is on the end of the speedometer cable where it goes into the transmission. Gears are available on line if you do not have a local speedometer or transmission shop.

Hugh

[slorio]

OldGMJon:
I am in the middle of this problem as well. A couple of thoughts:

* There are two sizes to these speedo gears. The ones readily available are .78" in diameter. My tranny uses a large gear that is .87" in diameter. You will need to pull out your gear and measure it to see what the diameter is. Then you will need to count the number of teeth on the one you have. The general rule is to get a 5% change in the speedo, you need to change this gear by one tooth. One tooth smaller makes it go faster on the speedo. So 42mph/50 mph = 84%. That would mean that you want a gear 3 teeth smaller than what you have to speed up your speedo 15%. Apparently, the factory generally checked the speedo's at 60 mph, but your 50mph check should be close.

* If you have the .87" gear, you will have problems finding a gear smaller than 22 teeth. Apparently, in that size they are no longer available. They do come up occasionally on E bay, but they are used gears and command a premium price if you can find them. The speedo shops recommend that you can pull off your tailshaft housing and replace the gear on the tranny tailshaft so that you can use the smaller "driven" gear that is shown in the pictures. Otherwise the external ratio adapter that one of the other folks referenced is a solution as well.

I hope this helps. This website was the best that I could find to sort this issue out:
http://chevelleengineer.home.comcast...eer/speedo.htm

I hope this helps.
Regards,
Steve
New Orleans

[joedoh]

a 235/70/15 would not show the correct speed.

the constant here is engine rpm, so a gearing calculator is needed, a tire size comparator will not do. I used the john g burns calc, he has had it online for so many years and it is an invaluable resource I keep going back to.

anyway, plugging in a 245/75/16 tire and a 1:1 4th gear and using just an arbitrary 3.73 rear gear (its not important to the calculation), I get roughly 1700 rpm to display 50mph (this may not be your rpm, but that also is not important to the calculation). keeping everything else constant (any economists in our bunch?) I change the tire size to 235/70/15 @1700 rpm and...


46mph. almost 47. so the tires didnt make enough difference, in fact not even going to an ultra small 205/75/15 makes your speedometer read correctly, still almost 45.

since your speedo is off about 16%, thats a tough one to swallow. my best advice would be to try the speedometer gear swap like the guys above suggest.

but my REAL advice would be to just get a paint marker pen and use the gps speedometer in your phone (or your daughters phone, or even your granddaughters phone, no judgements!) and put some dots on the speedometer lens to represent your REAL speeds... you could even pen the numbers in. if you wanted it to be super clean take it to a vinyl graphics shop, have them scan your dots, design and print out and install a new sticker for the lens. you can even have them use translucent vinyl and it will let light through at night. I am a work smarter not harder guy and would probably throw myself a party for not having to crawl under the truck, even though that may be easier.

good luck to you!

[OldGMJon]

Wow! great info here. I appreciate your help. I really had no idea about the gears listed above and thought tire size was the main driver here. I am in the process of a swb build and really wanted the information for that rig thinking the right tires would end any issues before they start. Now I see it is more complicated than I thought. I think I will use Joe's paint marker technique on the old rust bucket whose speedo is so far off for now. Thanks again, jon

QUOTE=Hugh in NC;7122272]I went thru this with me truck recently. My speedometer was 5 mph slow. I changed the plastic gear by 2 teeth. If I am remembering right, I had the blue gear which is 19 teeth and had to change to the brown gear with 17 teeth. Lower number of teeth speeds up the speedometer gauge. This is the gear that is on the end of the speedometer cable where it goes into the transmission. Gears are available on line if you do not have a local speedometer or transmission shop.

Hugh[/QUOTE]

Originally Posted by slorio

OldGMJon:
I am in the middle of this problem as well. A couple of thoughts:

* There are two sizes to these speedo gears. The ones readily available are .78" in diameter. My tranny uses a large gear that is .87" in diameter. You will need to pull out your gear and measure it to see what the diameter is. Then you will need to count the number of teeth on the one you have. The general rule is to get a 5% change in the speedo, you need to change this gear by one tooth. One tooth smaller makes it go faster on the speedo. So 42mph/50 mph = 84%. That would mean that you want a gear 3 teeth smaller than what you have to speed up your speedo 15%. Apparently, the factory generally checked the speedo's at 60 mph, but your 50mph check should be close.

* If you have the .87" gear, you will have problems finding a gear smaller than 22 teeth. Apparently, in that size they are no longer available. They do come up occasionally on E bay, but they are used gears and command a premium price if you can find them. The speedo shops recommend that you can pull off your tailshaft housing and replace the gear on the tranny tailshaft so that you can use the smaller "driven" gear that is shown in the pictures. Otherwise the external ratio adapter that one of the other folks referenced is a solution as well.

I hope this helps. This website was the best that I could find to sort this issue out:
http://chevelleengineer.home.comcast...eer/speedo.htm

I hope this helps.
Regards,
Steve
New Orleans

Originally Posted by joedoh

a 235/70/15 would not show the correct speed.

the constant here is engine rpm, so a gearing calculator is needed, a tire size comparator will not do. I used the john g burns calc, he has had it online for so many years and it is an invaluable resource I keep going back to.

anyway, plugging in a 245/75/16 tire and a 1:1 4th gear and using just an arbitrary 3.73 rear gear (its not important to the calculation), I get roughly 1700 rpm to display 50mph (this may not be your rpm, but that also is not important to the calculation). keeping everything else constant (any economists in our bunch?) I change the tire size to 235/70/15 @1700 rpm and...


46mph. almost 47. so the tires didnt make enough difference, in fact not even going to an ultra small 205/75/15 makes your speedometer read correctly, still almost 45.

since your speedo is off about 16%, thats a tough one to swallow. my best advice would be to try the speedometer gear swap like the guys above suggest.

but my REAL advice would be to just get a paint marker pen and use the gps speedometer in your phone (or your daughters phone, or even your granddaughters phone, no judgements!) and put some dots on the speedometer lens to represent your REAL speeds... you could even pen the numbers in. if you wanted it to be super clean take it to a vinyl graphics shop, have them scan your dots, design and print out and install a new sticker for the lens. you can even have them use translucent vinyl and it will let light through at night. I am a work smarter not harder guy and would probably throw myself a party for not having to crawl under the truck, even though that may be easier.

good luck to you!

[Big Fenders]

Originally Posted by markeb01

I'm not sure changing tire diameters is the best approach to speedo accuracy this many years after production. Too many thing may have been changed over the life of the truck. The ratio adapter noted above may help, but it might not be the ratio change you need.

A speedometer shop would be the best bet as they can simply change the plastic driven gear in your transmission to one that gets you very close to a correct reading:

What I did was get and mount a GPS on my windshield showing speed and then doing as Mark suggested using different gears [teeth count] till I zeroed it in. BTW my trans is a TH350
Bob

[LostMy65]

Saved for future reference.