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GM K5 Blazer / K-Trucks Regear Chart (1973-1991)
Factory transmissions, transfer cases and axle ratios for the GM K5 Blazer / K-Trucks — with the two tables every regear decision runs on: cruise RPM at 70 mph and crawl ratio, for every ratio you can actually buy. All data verified against manufacturer specs (sources at the bottom of the page).
Factory presets
| Trim / preset | Transmission | Transfer case | Axle | Factory tire | Crawl | RPM @ 70 |
|---|---|---|---|---|---|---|
| SM465 + NP208 + 3.73 (K5/K10) | SM465 4-speed manual | NP208 part-time (chain) | 3.73 | 235/75R15 (28.9 in) | 63.8:1 | 3,037 |
| TH350 + NP205 + 3.08 (early K5) | TH350 3-speed automatic | NP205 part-time (gear drive) | 3.08 | 235/75R15 (28.9 in) | 15.2:1 | 2,508 |
| 700R4 + NP241 + 3.42 (late K5/K1500, 1988-91) | 700R4 4-speed automatic | NP241 part-time (chain, 1988-91) | 3.42 | LT265/75R16 (31.0 in) | 28.5:1 | 1,817 |
Crawl and RPM computed from the factory ratios listed — load any preset into My Garage and the calculators use it everywhere.
Cruise RPM at 70 mph — SM465 4-speed manual, top gear 1.00
| Tire | 3.08 | 3.42 | 3.73 | 4.10 | 4.56 | 4.88 | 5.13 | 5.38 |
|---|---|---|---|---|---|---|---|---|
| 29 in | 2,499 | 2,775 | 3,026 | 3,327 | 3,700 | 3,959 | 4,162 | 4,365 |
| 31 in | 2,338 | 2,596 | 2,831 | 3,112 | 3,461 | 3,704 | 3,894 | 4,083 |
| 33 in | 2,196 | 2,439 | 2,660 | 2,923 | 3,251 | 3,480 | 3,658 | 3,836 |
| 35 in | 2,071 | 2,299 | 2,508 | 2,756 | 3,066 | 3,281 | 3,449 | 3,617 |
| 37 in | 1,959 | 2,175 | 2,372 | 2,607 | 2,900 | 3,103 | 3,262 | 3,421 |
Crawl ratio — first gear 6.55, by transfer case and axle
| Transfer case | 3.08 | 3.42 | 3.73 | 4.10 | 4.56 | 4.88 | 5.13 | 5.38 |
|---|---|---|---|---|---|---|---|---|
| NP208 part-time (chain) (2.61:1) | 52.7 | 58.5 | 63.8 | 70.1 | 78.0 | 83.4 | 87.7 | 92.0 |
| NP241 part-time (chain, 1988-91) (2.72:1) | 54.9 | 60.9 | 66.5 | 73.0 | 81.2 | 86.9 | 91.4 | 95.9 |
| NP205 part-time (gear drive) (1.96:1) | 39.5 | 43.9 | 47.9 | 52.6 | 58.5 | 62.6 | 65.9 | 69.1 |
Axles under this platform
- GM 10-bolt (8.5-in) Carrier break: 2.73 (Series 2 carrier for 2.56 & down; Series 3 carrier for 2.73 & up — 8.5/8.6-in version). Applies to the 8.5-in truck 10-bolt (K5/K10). The 7.5-in car 10-bolt is a different, weaker axle and is not covered here.
- GM 10-bolt (8.5-in) / 12-bolt early Carrier break: 2.73 (Series 2 carrier for 2.56 & down; Series 3 carrier for 2.73 & up — 8.5/8.6-in version). Applies to the 8.5-in truck 10-bolt (K5/K10). The 7.5-in car 10-bolt is a different, weaker axle and is not covered here.
- Dana 44 (73-76) / GM 10-bolt later Carrier break: 3.92 (3.73 & down use one carrier; 3.92 & up the other — confirmed by Yukon carrier listings). Non-Rubicon TJ rear is 30-spline; Rubicon TJ is 32-spline rear with air/electric locker variants by year. Practical ratios 4.56-5.13 for 35-37s.
- GM 12-bolt / 10-bolt Carrier break: 2.73 (Series 2 carrier for 2.56 & down; Series 3 carrier for 2.73 & up — 8.5/8.6-in version). Applies to the 8.5-in truck 10-bolt (K5/K10). The 7.5-in car 10-bolt is a different, weaker axle and is not covered here.
Era spans Dana 44 front (to ~1976), GM 10-bolt front/rear, and 12-bolt rear on early trucks. SM465 1st (6.55) does the crawling work, so classic GM regears are usually about highway RPM and tire size, not crawl.
Deciding? Run your exact combo in the regear comparison, then let the $39 Decision System turn it into a parts list, a budget and a shop-ready build sheet.
Sources
- GM SM465 published ratio table via Wikipedia/GM literature (6.55/3.58/1.70/1.00, reverse 6.09)
- Advance Adapters SM465 listing (corroborates 6.55-family, lists variant figures)
- NP208/NP241/NP205 published low-range summaries