I was thinking about gear ratios a little this weekend while testing out my new GTUS differential. I know my speedometer is correct since I have the correct gear for the tranny cable. But what if I didn't? Well, I know that the speedometer will be off a little, but so will the odometer right? Then I thought; what if I changed the overall diameter of my tires? The odometer would be off also, but by how much? Would it be worth worrying about? This is not so much a concern for resale value, but more for maintenance and gas mileage.

 

You worry too much..lol #1, you have the correct speedo gear, which means with stock diameter wheels both your odometer and speedo are as correct as factory. If you change tire diameter it will affect both odom and speedo. Bigger dia wheels = showing slower indicated speed and miles will be added to your odom less. Smaller diameter wheels = going slower then indicated speed and miles will rack up.

~Mike.....

 

Hehehe, yeah I know I worry too much. And I do realize I have the correct gear. I was more interested in HOW MUCH the wheel diameter and gear ratios effect the odometer. Like if I went with 225/50-15s will my odometer be off by 10 miles for every 1000 or 100 miles for every 1000. I know there is a little math problem I could do, but I'm a pretty lazy guy and was hoping someone else already had done this equation. I'm thinking that if you swapped rear gears without the correct speedo gear and went with a shorter tire, you would be racking up a good bit more miles on the odometer than the car actually traveled.

 

I would say gear ratio-going from a 4.1 to a 4.3=1.0488% greater. Therefore for every mile the odometer would register 1.0488 miles, or for every 1000 1048.80 miles. Speedometer would also be off 1.0488%. If at actual 60 mph you'd see 62.928 mph. Speedometer/odometer would also vary according to the % of tire size, either greater than or less than, the size currently on the vehicle.
Hope that's right!

 

It's a straight percentage comparison, there's no speacial equation. If you replace a 4.1:1 diff with a 4.3:1 one and don't replace the speedo gear the speedo and odo will read 5% high. If you originally had a 3.9:1 diff the error would be 10%. Same with your tyres. If you fit tyres that have a rolling radius 5% greater than stock then the speed/odo will read 5% low.

Turbonut, check your maths. You were close...