2wd speedo

[RedD250]

I just got the FarFromStock 2" leveling kit put on, and now my truck sits up really nice, almost like a stock 4x4 truck. Need some bigger tires!

I know the back is sagging, our wonderful postal service lost my blocks. Been missing for 3 weeks, and the shipper can finally file a claim today, but it still looks great. I had the 7226s springs before, and this sits up way higher than those did.

Anyway, now the stock tires look a bit ridiculous, and sit way too far inboard, so I'm gonna get a set of 17" wheels and some 33s. Has anybody had any luck getting the speedometer to read correctly with taller tires on a 2wd? I know that the D series used a different setup than the W, something is missing, a gear ratio reducer or something, so I'm not sure what to get to fix it for the taller tires.

 

[Mikey-KE7LBB]

You'll need a different speed ratio adapter. Once you figure how far off the speedometer is by percentage, you simply replace your existing adapter. https://transmissioncenter.net/shop/54ra2-gm-dodge-ratio-adapter/

Do a Google search for 'speed ratio adapter' and there are several options out there.

 

[RedD250]

Cool, thanks. Looks like I'll only be off by 2.4% with if I go with 285/70R17s, so I may be out of luck. Smallest they got is 11%. I suppose I'll just have to deal with it. It'll only be off by about 1 MPH anyway haha. Over the next 30 years though, that will add up to a significant difference in actual mileage vs recorded mileage.

 

[Mikey-KE7LBB]

Calculating the percentage you need is done by removing the adapter you have, and mounting the VSS directly to the trans. While using a GPS, drive at 50mph and take note of what the speedometer is reading. The difference you see is half of the total error percentage. Divide the actual speed into whatever your speedometer was reading to get your final percentage.

When I switched from the stock 215's to 235's on my '92 W-350, I came up with a difference of 1.2, which is 20%, so I'm betting yours would be closer to the 38% one they have.

 

[RedD250]

Oh, I didn't know that. I just used a generic tire size calculator to compare a 235/85r16 to a 285/70r17 and looked at the speed difference chart, which indicated I'd be going 2.4% faster than indicated. I'll just wait til I get the tires on there and compare to a GPS and do the math. Thank you, I had looked on google to see if there was a formula and didn't find anything.

 

[Mikey-KE7LBB]

You could also calculate by distance if you wish. Drive 10 miles (according to your odometer) and compare to actual distance recorded by GPS. In my case, 10 miles on odometer = 12 miles actual 12÷10=1.2 which is a 20% difference.

Clear as mud?

 

[RedD250]

Yeah, I got it lol. I may do it by mileage instead of speed since my odometer is dead on accurate GPS verified currently. My speedo reads slightly faster than I'm actually going. Don't see how that's possible since they both get their signal from the VSS, but that's how it is. Makes no sense. Went for a 250 mile drive, and the odometer was within a tenth of a mile of the GPS tracking, but speedo was off progressively more as speed increased. Anywhere from 1-5 mph. Before I replaced the VSS, both were dead on accurate, but my overdrive wouldn't work. Gotta love these vintage vehicle systems lol.

 

[AnonEd]

My truck is the same...odo accurate, speedo optimistic. But I've driven new stuff that isn't an y better, including a car with 3 clocks (plus sat radio and nav) and none of the 3 showed the same time.