What is my actual drive ratio with a tire and gear change?

[85K10SB]

Hey all, I'm a new member and this may be a newbie question but here goes.
I have an 85 K10 with a 700R4 + NP208 that WAS originally equipped with the small 28" tires and 3.08 gears. When I acquired the truck the previous owner had replaced the stock tires with 33's. The gearing was so terrible that I couldn't maintain speed on a flat road on OD, and naturally the tranny was running a little on the warm side because it was turning so slow(1100rpm@60). So, I replaced the 3.08's with 4.11's to try and bring the RPM's back to where they were supposed to be. I am now turning 2100 at 60mph, which feels about right. Now my only complaint is the speedo being off. Does any one know what my actual final drive might be. I'm think it's around a 3.73, but that's just a guess. Jus looking for a starting point to get one of those speedo cable ratio reducers to correct the speedo. Sorry for the long winded post, just trying to give some background in case it's needed. Thanks in advance for any responses.

JB

[b454rat]

check out 4lo.com, they have calculators to figure out final drives with different gearing and tires...

[85K10SB]

Thanks b454rat! I've used some online calculators but never had much luck.
Checked it out using proper ratios and it says I'm at a 4.68:1 final drive ratio. I'm a little confused. Wouldn't the larger tires take away some gearing? It makes sense by how my speedo reads but that Tcase was set up for 308's so it would read fast with just a gear change to 4.11's. Thought the tire size would bring it back down though. Does this make sense to more experienced wrenchers out there?

[imjeff]

The current final drive number doesn't care what gears or tires used to be on your truck. Don't overthink it. When you say "it makes sense how my speedo reads" just stop there!

[85K10SB]

You're absolutely right I was way over thinking things. I went out and measured my tires and found that they actually only 30" tall! I input that tire size in the calculator and it came out to 4.11. I also found a chart for speedo gears for different tire and gear combos on the website below. I am not soliciting for this company just thought it might be useful for others.

http://www.transmissioncenter.net/sp...n_______va.htm

[OrrieG]

Did not read the whole post on my phone, and just hit reply when I got to my computer. You can still use this to dial it in:

Calculators are just estimates. Get a friend and head out to the freeway. Hold the truck at 60 for five mile markers while friend keeps track of the time. Take the time and divide by 5. If speed is right on you will get 1. If not, calculate the percentage over or under. Then do a search and find a site that lists the different percentages for the colored gears in your trans, or you will know what you need to get for the ratio changer.

[85K10SB]

Thanks OrrieG! I will definitely use the method you described once the roads clear off up here in snow country.

[clauffer73]

You could try downloading an speed app on your phone. It should be pretty close.