Changing the Speedometer

[Dhardt]

So I just bought an 88 ranger. It doesnt have stock tires on it and the speedometer is around 11% slow. I read the article about changing the gears but currently it has a white gear(17 teeth) and going by the article 11% means i should get a 15. Is there a way to adjust the speedometer itself to account for the difference?

 

[JOLENE_THE_RANGER]

i dont believe so.. changing gears is super easy and cheap!

 

[Dhardt]

The issue is I cant find a size smaller than 16 teeth.

 

[adsm08]

No adjustment is possible at the speedo head. That gear is how you change it.

It does cause problems when you try to use a gear and tire combo that never existed from the factory.

 

[Shran]

I would try the 16 tooth and see where you're at. Keep in mind that it's a percentage, and if you get it down to say, 5% difference with the 16 tooth gear, you may only be a couple MPH difference off at highway speeds and not much at all around town.

What size tires are you running and what gears are in your truck? I've swapped speedo gears and have run many, many tire sizes and gear ratios and have never run into this issue.

 

[RonD]

There are two gears, the Driven gear is the one you can easily change, then there is the Worm gear, the Drive gear, on the output shaft of the transmission or transfer case

Changing the worm gear is usually not all that hard

1988 Ranger, manual or automatic?

2WD or 4x4?

2WD auto will be A4LD and I think the worm gear was machined on the output shaft so can't be changed

If you want to spend the money, $100+, you can get ratio adapters that attach to the transmission end of speedo cable, they use the Ford driven gears so can be adapted to tire changes, but they have an internal gear set the increases or decreases rpm ratios in situations like yours
Examples here: https://www.ebay.com/bhp/speedometer-ratio-adapter

You will not find +/- 11%, but that's not what you want, the ratio adapter you want puts you back into the Ford Driven gear ranges

 

[JOLENE_THE_RANGER]

this worked for me when i installed my c4. not sure if you tried it already
http://www.tciauto.com/tc/speedometer-gear-calculator/

 

[Dirtman]

I know not the answer you want but in my mustang the speedo is borderline useless because of the gear ratios and tires I'm running. When I take it for a cruise I just use my phone with a GPS speedo app.

 

[adsm08]

I would try the 16 tooth and see where you're at. Keep in mind that it's a percentage, and if you get it down to say, 5% difference with the 16 tooth gear, you may only be a couple MPH difference off at highway speeds and not much at all around town.

What size tires are you running and what gears are in your truck? I've swapped speedo gears and have run many, many tire sizes and gear ratios and have never run into this issue.

The 16 tooth is going to be about 7% error.

 

[rusty ol ranger]

Doesnt jegs or summit sell different speedo gears? Seems like i ran across them years ago when i put 33s on a 79 F150.

 

[Dhardt]

The truck is a 4x4 manual. The tires are p235/75r15 and currently it has the white 17 tooth gear.

 

[adsm08]

What gears are in the diff?

 

[Dhardt]

Not sure how to check that. Honestly I'm rather new at this working on my own vehicle thing.

 

[adsm08]

There should be a tag on the diff with the numbers.

 

[JOLENE_THE_RANGER]

if theres no tag on the diff. get a buddy and spin both rear tires and count how many time times the driveshaft rotates per one full rotation of rear tires. the number of driveshaft rotations will give you an approximate gear ratio.
crude example: if the driveshaft spins 3 and a half times to one full tire rotation you have a 3.50 gear ratio.