Community Forums

wdocter · 08-10-2018, 12:46 PM

If I change the rear end gears from 3.42 to 3.73 my speedometer will be off. Is the speedometer something that can be reprogrammed so it is again accurate? If so who might do that? the dealer?
Thanks

cfowler55 · 08-10-2018, 01:14 PM

The speedo will be inaccurate. Programming is a possibility. Depends on vehicle.

Old-Biscuit · 08-10-2018, 01:29 PM

What truck and why do YOU want to swap gears?

YES speedo will be OFF

3:42 to 3:73---speedo 60 MPH indicated.....55 MPH actual speed

tuffr2 · 08-10-2018, 02:24 PM

Swapping gears is not easy. Yes, the speedo will be off.

Cooperhawk · 08-10-2018, 02:27 PM

I changed the rear end on our Airstream 345 Turbo diesel a few years back. It was built in 1985 when speed limits were 55 and to do 70 meant really revving the engine up to the limit.

I can't remember the ratios but it allowed the Isuzu Diesel engine to operate at 65 mph at a good rpm. It worked out well.

Yes the speedometer was off, but I just got a Garmin and that gave me the right mph.

Still have the old rear end in the shop. It is out of a P30 Chevy chassis.

Mekanic · 08-10-2018, 02:38 PM

Quote:

Originally Posted by wdocter

If I change the rear end gears from 3.42 to 3.73 my speedometer will be off. Is the speedometer something that can be reprogrammed so it is again accurate? If so who might do that? the dealer?
Thanks

Usually going from a 342 to a 373 You will be disappointing with the results compared to the money/effort involved.

Have you thought about a shorter tire? This will lower the overall ratio.

Cumminsfan · 08-10-2018, 03:46 PM

My son just swapped gears in his Ram 1500. He couldn't get the dealer to reprogram it. He had to buy a tuner. Just more $$$. I'd call around 1st before I dropped $$$ on a gear change. IMO 3.42 to 3.73 is only a 10% reduction. I'd go straight to 4.10's.

Dropthejacks · 08-10-2018, 04:14 PM

The dealer can do it. I changed tire size and they reprogrammed it to adjust for the change. I have no doubt they can change it for a rear end swap to. Run by and ask them before you commit. A performance shop could also do it too I would think.

60sumtin · 08-10-2018, 05:03 PM

Quote:

Originally Posted by wdocter

If I change the rear end gears from 3.42 to 3.73 my speedometer will be off. Is the speedometer something that can be reprogrammed so it is again accurate? If so who might do that? the dealer?
Thanks

I did that on my '98 GMC. It keeps the RPMs up to give you more a bit more torque but as expected it also decreases the gas milage while not towing. Contact a dealer and ask them if the speedo can be re-programed. I take it your truck is a GM?

lcboy · 08-10-2018, 07:52 PM

I had the 323 gears swapped out for 373 because the guy said going one size would hardly be noticeable. It made a big difference in power. Raised about 200 rpms at 65 mph. Lost about 1 mpg at 65 mph. I had a hypertech to reprogram speedo.

SmokeyWren · 08-10-2018, 08:00 PM

Quote:

Originally Posted by wdocter

If I change the rear end gears from 3.42 to 3.73 my speedometer will be off.

Yes.

Here's the math:
3.73 minus 3.42 = 0.31
0.31 divided by 3.42 = 0.906, or rounded = 9.1%

3.73 is "shorter legs" than 3.42, so if "the computer" is not corrected, it will think you have driven 9.1% farther than you actually drove. Your speedo (and trip meter) will be slow by 9.1%. So an indicated 70 MPH will be an actual 63.6 MPH.

Quote:

Is the speedometer something that can be reprogrammed so it is again accurate? If so who might do that? the dealer?

The dealer's service department should be able to make the speedo accurate for any axle ratio that is available on that make/model vehicle. Most competent speed shops will have the scanner and software to do it too.

CAVEAT! Replacing the ring gear and pinion is not something you want to DIY unless you are already an expert at overhauling differentials. It requires very exacting measurements and tolerances and special tools.

If your tow vehicle is a 4x4, you must replace the ring gear and pinion in both the front and rear differentials.

As Cumminsfan noted, that 9.1% change may not be what you want. 3.42 to 4.10 is 19.9% change. IOW, Engine RPM of 2,200 with 3.42 will be 2,638 with 4.10 axle at the same corrected speed.

L.C.Gray · 08-11-2018, 04:55 AM

Depending on your tow vehicle, just lock out the top gear and you'll get some of the same effect.

My truck has the 3.73 gearing rather than the 4.30 gearing it's available with. I lock out 6th gear and it tows much better on the highway without constantly hunting gears. I have yet to do the math but I'd be willing to bet you a big red sody pop that the ratios of 5th/3.73 and 6th/4.30 work out to be be close to the same. Gear changes are expensive, locking out top gear is free.

minytrker · 08-11-2018, 06:37 PM

I program vehicle ecm's and calibrate speedometers. A dealership can only calibrate it if that gear ratio or tire size was an option in that vehicle usually. We do programming for a lot dealerships because they dont have the software or capabilities we do. The abs computer has to be programmer also alot of times depending on how big of tires you changed.

mrgrayaz · 08-20-2018, 04:08 PM

Most modern vehicles calculate vehicle speed based off ABS sensors in the wheel hubs - gearing is irrelevant....change gears all you want, as long as you keep the tires the same size the speedo stays correct.