I’ve had this truck for about 10 years and the PO install the speedometer adapter. It wasn’t until just recently that I realized my speedometer is reading 10 mph faster than I’m actually driving. I removed the adapter and now speedometer is only reading three or 4 mph over what I’m actually driving. The truck has 33 x 10.5 BFGs so shouldn’t the mph actually read slower than what I’m actually going? The truck also has the H55F transition….not sure if this makes a difference. Is there a way to adjust this to true mph?
Speedometer adapter issue (1 Viewer)
- Thread starter Jakey77
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Do you know what ratio the axle gearing is? Stock is 3.7. Easy way to check would be to count the number of times the driveshaft spins with one tire revolution.
- Thread starter
- #3
I don’t know…how would I calculate this. Get rear wheels off ground…one rotation to how many driveshaft spins equals what? I don’t remember the Po saying anything about gear ratio change but it doesn’t make sense that I’m reading 3 mph faster than I’m actually going with 33” tires
Less than 4 revolutions of the driveshaft with one tire revolution would equate to stock gearing (3.7). With what you’re describing I’d guess you have 4.11’s. It wouldn’t be dead on, but would be close to actual speed. Another indication would be how first gear behaves. It would be pretty short with 4.11’s. I have 3.7 gearing (also 33x10.5x15) and I don’t have to shift out of first gear “immediately” when moving on flat ground.
Only other way to actually check would be to pull the rear differential cover and look for stamped markings or count the teeth on the ring and pinion gear. 3.7 ratio is 37 on the ring gear and 10 on the pinion gear. 4.11 is 37 on the ring gear and 9 on the pinion gear.
Only other way to actually check would be to pull the rear differential cover and look for stamped markings or count the teeth on the ring and pinion gear. 3.7 ratio is 37 on the ring gear and 10 on the pinion gear. 4.11 is 37 on the ring gear and 9 on the pinion gear.
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- Thread starter
- #5
Oh yeah…I’m pretty much have to shift out of 1st immediately. What is the benefit of 4.11 vs 3.7?
4.11’s with 33” tires is essentially the same as 29” tires and 3.7’s. My rig is great for highway driving, although when I had it in Colorado I wished I had 4.11’s. More “torque” for going up grades and better engine braking in gear for going downhill. Generally speaking the higher the ratio the more torque you have with lower top speed. The lower the ratio, less torque, higher top speed.
- Thread starter
- #7
Thanks for the info…this answered my question! The PO must have not known he had 4.11 and put the adapter on…it has been. Reading 10pm faster for at least 40k miles
