The Stovebolt Site Forums The Shop Area General Truck Talk Speedometer ratio increaser

I have a dilemma where I just finished a 1955 1st series Chevy 3100 pickup restoration and the original speedometer is reading 44% slow. I bought a straight-through speedo ratio adapter (+44%)and it would work, except that I can't use a straight-through adapter because the vacuum brake booster is right in the way. I need a 90 degree ratio adapter, but no one seems to make one. The tallest adapter one company makes in a 90 degree is a +36%. I tried using a 90 degree 1:1 geared adapter in line with the straight through +44% adapter and it broke the link shaft in the 90 degree adapter. Later I found out that you can't run the two together because it puts too much torque on the 90 degree adapter and will always break the shaft. Question: Does anyone know of a company that can make a +44% 90 degree ratio adapter that will fit this truck?

What in the world do you do to a '55 1st 3100 that could
possibly throw off the speedometer by almost 50%???
3-speed or 4-speed? Monster tires? Different rear axle?
and why would you want to go 75 or 80 mph in a Stovebolt?

I know that doesn't solve your problem but your question leads
to a lot of head scratching if you are doing a restoration.
Such as, a restored Stovebolt does not have a vacuum brake
booster, and wouldn't reach those speeds.
It sounds like you're building a hot rod, right, rather than a\ restoration.

Denny Graham
Sandwich, IL

There is a guy in Florida that makes correctors. Do not have the number handy but he is the back of a lot of hotrod/automotive magazines. He might be able to point you in the right direction. 44% wow! Something is way custom ??

"Restoration" was a general term. A "mild custom" would be a more appropriate term. It basically looks stock on the outside, but my customer wanted the modern conveniences of power steering and power brakes. In addition, he wanted taller rear end gears (3.38:1) with the stock 4-speed transmission. I'm not sure why there is such a disparity in the speedo reading (probably the rear end gears), as the wheels and tires are not that much different than the originals. The power brake booster is the unit that Classic Performance Products sells that mounts right on the stock master cylinder bracket on the frame. Problem is the booster is too close to where the speedo cable attaches to the transmission. The problem can be solved by moving the master cylinder and booster assembly further rearward, but this is a major project that I would rather not perform unless there is an alternate solution.

This might be helpful: http://www.4lo.com/4LoCalc.htm
And this: http://www.bobsspeedometer.com/1/120/ratio_adapter.asp

Since it is a "mild custom" could you talk the customer into using a GPS speedometer? There are several being offered by both auto parts people, like Auto Meter, and GPS people like Garmin. They are dead accurate no matter the gear ratio and tire size. Just google gps car speedometer.

That's cool Evan. Looked them up and you're right, several
different makers and they've got lots of features such as nice
large display of your speed, speed alarm warning, compass,
water temperature and voltage alarm, mileage and fuel remaining.
Gives you a lot of the info that the new cars come with and
all for about fifty bucks in a small portable package.

A week or so back I put new rims and taller Michelin's on my
'50, 3600. Also swapped the 4.57 for a 4.10 right after I got
it. Took it out the other day with my daughters ten year old
TomTom GPS to check the calibration of the speedo. The GPS is
super accurate, it updates the speed immediately, I can only
imagine how accurate the new ones are.
In my case all I have to do is mentally add about five mph
to the speedo reading to be withing the posted speed limit.
Only problem with her old GPS is the speed is given in tiny
unlighted numbers down in the corner of the display.
Of course that method of correction wouldn't work so well if
you've doubled the speed of the vehicle.
Thinking of taking a look at one of those new ones for my
truck. Could use a mag base or suction cup so I could tuck
it away in the glove box at a show or cruise night.

Denny Graham
Sandwich, IL

The problem is that the customer wants the speedo to look original, so the GPS speedo is not on the table right now. I will check with Bob's Speedometer to see if they have a good solution to this issue and go from there. I appreciate all of your input!