Originally Posted by TrdSurgie
Gear ratio has nothing to do with your speedometer. Going to a 33 inch tire actually gets the speedometer closer to correct than what it is stock. Use a gps to compair.
Not really sure where this thinking is from... (yeah, I know it's an old post, but deserves attention).
I have worked in Automotive Electronics for over 25 years. Specifically in Factory Speedometer repair and troubleshooting. I cannot think of a vehicle on the road, that the above statement applie to, that is less than 5 years old, and I'm not certain of the later models, as some may be using GPS for speed readings... but None for all the years I worked in it, were independent of gear ratios in both the transmission and Rearend. (in vehicles with rear wheel drive for sure)
I used to actually do Speedometer accuracy tests on a Dyno and the Speed and the Odo are both affected by all of the final drive gearing. Back in the day, our adaptations required a small gear box called a ratio adapter. Nowadays, with all the electronic speedos (which I was dealing with on their initial application in consumer vehicles), the same still applies, yet you adapt it using electronic signal modification and correction. In GM (where most of my dealings were), The DRAC (Digital Ratio Adapter circuits) were first introduced in a separate box and mounted remotely from the EM and Instrument Cluster. WE got the gear ratio from the glove box tag (or wherever it was located) and did a Tire roll-out for circumference, and had charts that gave us the circuit modifications for THAT application, to make it read correctly. This had to be done anytime there was a tire Size change, and if someone changed their rear end gears, we had them supply the Gear Ratio. All of that applies to both Speedo and Odo on ALL vehicles. The odometer uses the speedometer to determine how much to add to the mileage... There is no disconnect at all. There cannot be unless you are modifying the circuitry in the instrumentation and ECM Separately. The only thing Many of the Adapters and correctors provide, truly, is a Speed signal modification, based on what you input. It corrects both readings, by modifying ONE signal set. That set is ONLY the SPEED SIGNAL inputs. On the more Schmancy ones, it can also change the shift points per RPM/Speed and other things, but the stuff that talks to your speedo is still the same info that controls your ODO. I believe the Hypertech does the Trans info as well from what I understand of it.
I'm an old guy... I see tons of things done on these trucks (Tacomas) that are not very well understood. Some of them I can ignore simply because I know it doesn't matter much, and it really doesn't affect many people. But the Speedo info like the quoted one above, and some of the wheel change info, can be very misleading and get some undesirable results.