Speedometer Calibration

What is the best way (most efficient and cost friendly) to calibrate your speedometer for larger tires. The stock tires on my 1996 Ranger 4.0 V6/manual transmission were 195/70 r14 and now they are 235/75 r15 (the previous owner did this but did nothing to the speedometer to adjust it). I checked with some shops in California and they want $200 to $300 to do what seems like an inexpensive task. Superlift sells a kit for the do-it-yourselfer but that costs almost $200 itself. Does anyone know of a cheaper and better way to get this done?

Thanks,

John

Reply to
Tornad01
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Lacking any better alternatives, you could try finding one of those "Your speed is" setups on the road. Then drive by at several different speeds and record both what your speedo read and what the sign said. Plot all those points on a graph and you now have a conversion factor to use. (I would drive by it at each 5 or 10 MPH marker on your speedo up to the speed limit at that rig because I don't know if the difference on the speedo is linear or not) If you are really ambitious, after plotting the conversion graph you could then use that to re-label your speedometer by putting paper or some labels over it. Naturally this method requires no special equipment or changes to the internals of the vehicle.

-D

Reply to
Derrick 'dman' Hudson

Time your speedometer with a constant speed and a watch will be more convenient. If you can't find a time/speed table say so and I'll post one. But all you really need it to know the speed at higher speeds anyway drive at 60mph on the speedometer and see how long it takes to go a mile.

On my '75 F150 the odometer was right on the money when new with the dinky little cars tires that came on it. the speedometer was off a little reading high. With my 235/85-16s the odometer is now off

13%.

I re-adjusted the return spring on the speedometer so that it just barily returns to zero. That changed the speedometer from being ~10 mph off at 60 mph to being only ~5 mph off at 60 mph.

Back in '78 I asked a ford dealer's parts department about new set of nylon speedometer/transmission gears to make the correction (warner t-18) but they didn't have a setup that would correct it all the way... ...close but not "right". So I decided that if it's going to be "wrong" anyway, it might as well be wrong "stock", at least it's consistant. :)

I don't know anything about a '96 Ranger, and whether it has a set of nylon tranmission/speedometer gears or not. :/

Alvin in AZ (was H78-15s changed to 235/85-16s with a 3.25 rear) ps- stock 16" wheels back then were split rim with tubes pps- 16.5s were all the rage, didn't want them SOBs either ;)

Reply to
alvinj

do-it-yourselfer

When your speedo reads 60 mph, you are actually going 70.01272669424116 mph. So to be safe, just figure that for every 10 mph an your speedo add another 2 mph. 10 = 12, 20 = 24, 30 = 36, etc

Reply to
Tyrone

I used a pocket GPS to find my true speed. Programmable Speedometer/Odometer Modules (PSOM) are used on most 1992 through 1998 Ford trucks. You would need to have it reprogrammed, but I'm not sure of the method used. You might have to go to a speedo shop, or a dealer. Being it's a 96, I don't think a simple speedo plastic gear change will work, as in the older trucks. My truck runs appx 10 mph faster than it shows at highway speeds. I need to change my gear...But mines a 68 truck. :) Makes it easier..Maybe do a search on (PSOM) snd see if you get any info...MK

Reply to
Mark Keith

I checked with some shops in California and they want $200 to $300 to do what seems like an inexpensive task.

what shops? dealers? or just "shops"....

I am not sure about your year model, but the later models can be re-programmed via the vehicles PCM, I had mine reprogrammed at a dealer for $49 bucks out the door... they just hook their machine to your truck, and punch in a new number for the larger diameter tire...(each tire has a revolutions per mile number from the manufacturer)

Reply to
Harley4Sale

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