The Stovebolt Site Forums The Shop Area Engine Shop 3.55:1 pinion gear and speedometer reading error

I'm in the process of replacing my original pinion gear with the newer pinion gear set (ratio 3.55:1). Because the ratio is different I should be able to achieve better high-speed performance at a lower RPM. However, what will happen to my speedometer and odometer readings? I suspect these will not be accurate. Does anybody know of a "quick fix" to synchronize the instruments with the new gear ratio?

You can buy a mechanical adapter from a variety of places. It costs between $70-$100. If you are interested, I'll look for links and post them.

I have used an ($100) adapter and my speedometer is within 1/2% of accurate. The adapter that I bought is tuned to match my larger diameter tires and the 3.55 gear-set. It bolts right into the transmission and then the speedometer cable bolts into the adapter.

Tim

Tim, I bought an adapter from the same guys who sold me my 3:55 gear set and it's still off by 5 mph on the speedo. I did a two way 10 mile check using the odometer and found that I indicating 8.9 miles/10 actual (at least that's what I think I remember, LOL). Anyway, the "standard" adaptor from Patrick's (for around $35) still results in my speedometer reading 4-5 miles slower than actual. I haven't checked the odometer again as I'm in a sort of bummed out holding pattern with the truck right now due to "funny" noises in the engine.

I am running 16" tires so assume that if I want to get better accuracy I'll have to run the test again, without the adapter, and find someone who can provide the correct ratio.

Here's a site to help in determining the correct adapter: Speedometershop.com

Don

Don,

I used the 10 mile calibration technique and gave the Speedometershop.com (United Speedometer Service) my numbers and they sent me the adapter within a week. I am completely satisfied (1/2% off).

The technique at the top of the page in your link suggests a 1-mile measured mile. Luckily, when you call them, they ask for a 10-mile measurement. I averaged 4 ten-mile measurements.

I saw the Jim Carter adapter right after I bought mine. Most likely I would have tried to save $$$ and bought from Jim Carter - and I would have been bummed out.

Tim

Tim,

Thanks for the info. I firmly believe that Patrick's will refund my money as they are good guys. Once I get the engine "right" again I'll run the measured 10 mile sequences again and order from http://www.Speedometershop.com

Perhaps our combined experiences will help 49TruckNut

Don

down2sea,
at what speed is your speedometer out by 5 mph? I would expect the error to be part offset and part proportional to your speed. In otherwords an error indication of 5 mph while going 25 would be the same at 50 mph.

Good question. Actually, I think it is consistently reading 5 mph low throughout the range, although I will have to check that with the GPS. I've got to get the dang thing back on the road again first and will then start fiddling with the speedo.

Don

Thanks evderybody - this is all very good information - exactly what I was looking for!

Compare the odometer with the mile markers on the freeway. If it,s not too far off you can often change the driven gear for the speedo with one with more or less teeth. In this case you will need less teeth.

The odometer reading depends on the gear ratio adapter after the 3.55 swap and is not adjustable.

The speedometer needle however is adjustable and can be fine tuned with a movable rotating arm that holds the end of the return spring.
Take off the bezel and it will be obivous.
The reading will never be accurate across it's entire range, so tune it around your mostly driven speeds.

Patrick's gear ratio adapter is dead on accurate, if you use stock tire diameter, which is almost exactly 29".

I got Patrick's adapter with both my rides and they are accurate.

Phillip told me on the phone that they had never had a problem with the adapters. It's now up to me to do the 10 mile runs again and also check the speedometer with the GPS. Lot's of good info from this thread.

Don