speedo fix after big tires??

[blazin]

i have a problem.. i dont know how to fix my speedo. seem there is no place around anymore (albert canada) that does old school speedo recalibration. im told to change the speed gear in the transfer case. but to what? i have a 700r4, 205 transer and 4.56 gears. thoughts?

[milled]

i think napa makes a reducer that attaches to the outside of the t case... If you have a speed odometor shop around you give them a call, those people deal with this stuff all the time.

[Hoods69BadBowTie]

Originally Posted by milled

i think napa makes a reducer that attaches to the outside of the t case... If you have a speed odometor shop around you give them a call, those people deal with this stuff all the time.

Yea x2 I have heard about the reducers but never dealt with them myself.

[blazin]

Originally Posted by milled

i think napa makes a reducer that attaches to the outside of the t case... If you have a speed odometor shop around you give them a call, those people deal with this stuff all the time.

all the speedo shops have closed down. everything is electronic and the deals do that stuff.

[OrrieG]

Working from memory but when I put the TH350 & 3.74 rearend in my 59 the speedo was off. I ran it for 5 miles at 60 mph speedo reading on the freeway and timed how long it took. should have taken 5 minutes or 300 seconds. I divided the time it took in seconds by 300 and came up with the percentage over or under. I then pulled the plastic gear out of the tranny and took it down to the local Chevy dealership. He had a chart that listed the different colors and difference it changed the output. I got the one that changed it close to what I had calculated installed it and rechecked. Speedo was within 5 seconds for 5 miles so I was happy. This was in 85 so who knows if the current counter guys can help and if GM still makes the different gears. NAPA might have access to the same information too.

When I changed tires to a different diameter it changed again, I just did the test run and just mentally adjusted. I do the same for my Chevelle, I run short tires on mags in the summer, stock tall tires in the winter. Difference is 4 mph at 65.

[special-K]

Originally Posted by blazin

i have a problem.. i dont know how to fix my speedo. seem there is no place around anymore (albert canada) that does old school speedo recalibration. im told to change the speed gear in the transfer case. but to what? i have a 700r4, 205 transer and 4.56 gears. thoughts?

What diameter tires?I here what you are saying.It`s not enough to know you can get reducers at a parts store.You need to know how much you are off first.I found that with 37" tires my speedo was 10% slow.My odometer ran .9 for every mile by the marker`too.This was a 3/4 that would have had 235/85s (7.50-16s) from factory.The first night in town with my new big tires I got a ticket for speeding (figures).I told them I just put the tires on and hadn`t had a chance to figure my speedo difference yet.They offered to go to a road with no traffic and clock me driving by at 25 and they`d tell me what their gun read.So I drove by at close to 40 .They didn`t understand why it was off more than 15 mph,but they let me go.I had a chance to get an accurate reading but was more interested in getting let off on the ticket .I love telling that story! Man,they were stumped.This was in`77 before anyone around my area had tires that big.

[blazin]

Originally Posted by special-K

What diameter tires?I here what you are saying.It`s not enough to know you can get reducers at a parts store.You need to know how much you are off first.I found that with 37" tires my speedo was 10% slow.My odometer ran .9 for every mile by the marker`too.This was a 3/4 that would have had 235/85s (7.50-16s) from factory.The first night in town with my new big tires I got a ticket for speeding (figures).I told them I just put the tires on and hadn`t had a chance to figure my speedo difference yet.They offered to go to a road with no traffic and clock me driving by at 25 and they`d tell me what their gun read.So I drove by at close to 40 .They didn`t understand why it was off more than 15 mph,but they let me go.I had a chance to get an accurate reading but was more interested in getting let off on the ticket .I love telling that story! Man,they were stumped.This was in`77 before anyone around my area had tires that big.

haha great story.. i might have to use that one someday and do the same you did i have 35 inch tires, 700r4 tranny, 205 transfer case, 4.56 gears. one thing i also have a problem with is the length of my speedo cable. the 700r4 is 2 1/2 inchs longer then the 350 that was in there. there was a shop here in town that made cables, hade a dyno in there shop for setup after big tires and gears or whatever. they closed and i cant find anyone that knows about it.

[OrrieG]

The cable I pulled out of my 74 donor was a two piece, main section from the speedo out, then a section about 2' long from the end near the trans to the trans. It was a factory auto so I assumed that it was a factory extension for the 4wd option. Obvioiusly more than the 2.5" you need to make up but an option.

[blazin]

Originally Posted by OrrieG

The cable I pulled out of my 74 donor was a two piece, main section from the speedo out, then a section about 2' long from the end near the trans to the trans. It was a factory auto so I assumed that it was a factory extension for the 4wd option. Obvioiusly more than the 2.5" you need to make up but an option.

i ordered a 104 inch cable today, im told it will work. it will be a little to long, but better then no cable now if i could just figure out the speedo gear i need, i'd b set.

[1985-GMC]

This place should have everything you need. http://www.transmissioncenter.net/sp...n_______va.htm

[msgross]

I'd borrow someone with a GPS and check the difference. That's a good website for reference too...