metal22lica

hmm..i just finished checking out the Gear Page and i have G294 which is a 8.0" gear with 4.10 ratio and a 4 Pinion, Open Diff. so then i was checking at the bottom where it said that 4.10 ratio should be used with 225/70R-15 tires, but i have 31's on. so i was wondering if this would affect my speedo and gas milage? something effects my speedo b/c when i drive my moms car to school its 110 miles there and back, and when i drive the 4R its only 100. thanks guys.

jx94148

225/70R/15s are 28" 10% smaller than 31s. Your spedo and odometer are 10% off. When it says 70 you are really going 77.

Cebby

I might be missing something here, but I don't think the axle gearing has anything to do with the speedo inaccuracies. The speedo gear/adjustment is for the diameter of the tires.

Dr. Zhivago

Cebby, the Axle Ratio has everything to do with Speedo/Odometer inaccuracies. Actually, it's a matter of the Final Drive Ratio including tire diameters. But below is a simple explanation of why it matters.

If the Axle is making more (or less) turns per mile than when it left the factory, then the Driveshaft and subsequently the Transmission is making more turns per mile and therefore the Speedometer Gear and/or the VSS (Vehicle Speed Sensor) is sending the wrong information to the Odo/Speedo.

The leg bone's connected to the knee bone, the knee bone's connected to the thigh bone, the thigh bone's connected to the hip bone, etc. etc. etc.

Hope this helps.
Dr. Z

Cebby

I guess I never understood it that way. So, you are saying that if I have two identical vehicles with the same size tires and same speedo gear, that have different axle gearing, that I will get different speeds at the speedo?:confused:

Dr. Zhivago

Exactly. It's because the Ratio of "Turns Per Mile" of the Axle (and subsequent parts) will be different between the two vehicles. The transmissions won't be turning the same speed, which is what the Speedo Gear (and VSS) is driven by.

Hope this helps.
Dr. Z

Cebby

This is quite timely, considering I've been having a few issues with my speed sensor lately.

Consider me enlightened...

jx94148

The spedometer connects to the transmition so any changes to diff ratios or tire size throw the spedo and odometer off unless you do the math and get the right combination.

metal22lica

so then i see where the 10% thing comes into effect with the 100/110 mile thing. hmm..so when im on the highway going 80 mph..im really going 88...now wonder i fly by everyone. so then if i have to fill up every 150 miles...the odo should really say 165?