Speedometer/tire size issue

[TexasTeal95]

I have a question about an issue I was having with my speedo and couldn't find what I was looking for using the search and google. I have a 95 c1500 2wd with the 4l60e with RC 4" lift spindles and 2.5" blocks out back. To go along with the lift I had some BFG A/T's 32x11.50R15 installed and my speedo reads correctly without installing DIP switches on the VSSB behind the glove box. How is this possible? I always had a feeling that the speedo wasn't reading correctly when the truck had the stock size tires. Also after installing the bigger tires the transmission doesn't seem to shift at the correct times at low speeds like when making turns onto neighborhood streets and accelerating after the turn to speed up. It feels as if the truck doesn't have enough gear so you have to give it a little more throttle to make it downshift if that makes sense. Gears are stock 3.42's. Anybody know why my speedo reads correctly with 32's?

I want to add a Detroit True-trac carrier and some 3.73 or 3.90 gears but I need to resolve this issue first.

 

[mr98z71]

Well, installing bigger tires always changes the way your speedo will read. How do you know it is accurate now? are you using a gps app on your phone? Your computer is programmed to perform a 2-3 shift at 35 Mph at 70% throttle, 40 Mph at 76% throttle so on so forth (those were just random numbers by the way but serve the same purpose for this example). now that you have bigger tires, your trans might think that you are going 20 mph when really you are traveling at 28 mph due to the bigger circumference of the tire. the same happens for downshifting but backwards. Do you tow with your truck? if so you may want to step up to 4.10's, and if you have a 10 bolt currently, scrap it and built a 14 Bolt semi float, it will bolt right up and be 6 lug.

 

[TexasTeal95]

Well, installing bigger tires always changes the way your speedo will read. How do you know it is accurate now? are you using a gps app on your phone? Your computer is programmed to perform a 2-3 shift at 35 Mph at 70% throttle, 40 Mph at 76% throttle so on so forth (those were just random numbers by the way but serve the same purpose for this example). now that you have bigger tires, your trans might think that you are going 20 mph when really you are traveling at 28 mph due to the bigger circumference of the tire. the same happens for downshifting but backwards. Do you tow with your truck? if so you may want to step up to 4.10's, and if you have a 10 bolt currently, scrap it and built a 14 Bolt semi float, it will bolt right up and be 6 lug.

As far as I know, my speedo reads correctly because there are several speed limit signs around my area that have digital read outs of your speed as you drive towards them. They all correspond with the reading on my speedo gauge. I will give gps a try though and see what that says.

I haven't towed with my truck yet but I plan on getting a travel trailer. Swapping out rear ends isn't in the cards right now or at least not until I get another vehicle to act as my daily driver.

 

[dcp1992]

I have a 96 Yukon 2wd and when it was stock with stock tire size my speedometer was reading over then what I was actually going. So I was going slower than what the speedo was showing. I did the 4" spindles, coil spacers, rear block with 3500 leaf springs and I got some cheap worn out 35's and then my speedo was about 7% under what I was going. I now have a rear end with 3.73s and 33.5" tires and my speedo is pretty spot on now.

So you speedometer being correct with 32s does not seem too far fetched since mine was off stock with stock tires. I do agree with mr98z71 though, the trans will shift a little funny if the speedo is actually off but it will also shift funny just because of the bigger tires and keeping the 3.42. More weight and circumference of the tire requires more torque so it will feel different.

 

[TexasTeal95]

I have a 96 Yukon 2wd and when it was stock with stock tire size my speedometer was reading over then what I was actually going. So I was going slower than what the speedo was showing. I did the 4" spindles, coil spacers, rear block with 3500 leaf springs and I got some cheap worn out 35's and then my speedo was about 7% under what I was going. I now have a rear end with 3.73s and 33.5" tires and my speedo is pretty spot on now.

So you speedometer being correct with 32s does not seem too far fetched since mine was off stock with stock tires. I do agree with mr98z71 though, the trans will shift a little funny if the speedo is actually off but it will also shift funny just because of the bigger tires and keeping the 3.42. More weight and circumference of the tire requires more torque so it will feel different.

Now that you mention that, I think I remember noticing that the speedo seemed like it was reading too fast before I installed the lift and 32's on my truck over a year ago. What you said about the transmission shifting makes sense as well. Now I am wondering how much the speedo will be off once I put a different ring and pinion set in the rear end. If its spot on with the the 32's and 3.42 gearing, I wonder how much faster the speedo will read with a 3.73 gear set. Also considering 3.90's or possibly 4.10's?